Passive Solar Design Overview - Part 5: Distribution, Ventilation, and Cooling

This is a guest post from Will Stewart, a Systems Engineer in the energy industry.

In this final article in the passive solar design overview series (see Parts 1, 2, 3, and 4), we will cover the techniques used to avoid hot and cold spots in a passive solar building, how to provide fresh air, and how to provide cooling (in many situations).

Wind directed HRV cowlings at BedZed
The first aspect we must address is the needs of the building occupants. While we may strive for ideal conditions as often as possible, a zero net energy home might not be required by its owners to always fit the mold of a typical power-intensive HVAC design. To understand how we need to distribute the heat from the building's thermal masses, we need to understand how the human body interacts with the conditioned space, and how to define comfort.


In simple terms, the human body is considered to have obtained thermal comfort when a body’s heat loss equals its heat gain.

More specifically, the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) devised a simple chart showing the upper and lower bounds of temperature and humidity by for what it defines as human comfort zones, taking into consideration differing clothing levels by season.

ASHRAE Comfort Zones

Other factors are also important, such as metabolic rate, clothing insulation, radiant temperature of other masses or heat sources, and air speed.

The body exchanges, in a typical scenario [17]:
  • 62% of it's body heat via radiation,
  • 15% by evaporation,
  • 10% by convection,
  • 10% by respiration and
  • 3% by conduction.

Note that during the US Energy Crisis in the 1970's, Jimmy Carter sat in front of a fire with a sweater on, asking Americans to set their thermostats to 65 degrees F in the winter time, outside of the ASHRAE comfort zone. As we experience other such shortfalls in supply in the future, expect the BAU comfort zones to be questioned and expanded to fit the situation at hand.


Capturing and storing solar thermal energy is not enough; how do we ensure its dispersal when and where we need it? There are two types of heat transfer related to passive solar heat distribution; convection and radiation.

  • Convection:

    Simply put, warm air rises. Any thermal storage element that is warmer than the surrounding air will heat the air closest to it, causing that air to rise. There are extensive calculations necessary to determine convection heat transfer with precision, but for our purposes, an approximation will suffice;

        Q = h A (Ts – Ta)

                Q = heat transferred
                h = convection parameter (approximations assuming laminar flow)
                     Horizontal surface: h = 0.27 (ΔT / A) 0.25
                     Vertical surface: h = 0.29 (ΔT / A) 0.25
                A = area
                Ts = surface temperature
                Ta = ambient (indoor) temperature

  • Radiation:
    If a thermal storage element is warmer than nearby objects within line of sight, the element will transfer heat in the infrared wavelengths. The energy emitted via radiation from an object warmed by the sun (i.e., thermal storage wall, masonry floor) can be calculated with this formula;

        Q = ε σ A T4

                ε = surface emissivity (See Part 2 of this series and additional values in this list).
                σ = Stephan Boltzmann constant, 5.7 x 10-8 W/m2/K4
                T = absolute temperature K (°C + 272)

Direct Gain:

Part of our answer here depends upon the depth of the heated space, and the location and conductivity of the thermal mass; if a house is too deep (north to south), then the polar-facing side can be cooler than the area warmed by the thermal mass. The heat is convected up from the warmed masonry floor, allowing one to walk about in stocking feet in the winter.

Heat transfer from direct gain thermal mass (Graphic courtesy of

Interior design of direct gain buildings that utilize thermal mass can take advantage of the depth the winter sun can penetrate into the occupied space, which reaches the maximum depth on December 21st. Rooms or portions of larger rooms on the polar-facing side of the building where the sun does not directly heat the thermal mass are candidates for utility rooms, closets, bathrooms, and other rooms where occupancy is infrequent and/or of short duration.

Direct Gain heat transfer (Graphic courtesy of RecycleWorks)

Indirect Gain:

Trombe walls distribute heat in two manners;

  • By convection of warmed air back into the top of the room during the daytime, and
  • by thermal radiation from the trombe wall mass, primarily at night

In designs that take advantage of trombe walls, we clearly see that during the night, objects close to the trombe wall will be warmer than those further away. Such considerations play a major role in the interior design and living space layout of homes utilizing this technique. Unless a storage mass with a period of at least one day is factored in (see Part 3), trombe walls would not be suitable to office or other work spaces that are only occupied during the day.

Trombe wall heat distribution

Isolated Gain:

Isolated gain designs are very similar to Trombe walls with respect to heat (or cooling) distribution.

Isolated Gain


An occupied space requires a minimal amount of circulation and fresh air to avoid the sense of staleness. During pleasant weather, the most obvious approach would be to simply open the windows (except for offices without operable windows). During heating (and cooling) weather, there are several ways to accomplish bringing in fresh air, including;

Passive Solar Pre-Heat

Fresh air can be brought in through a passive solar panel on sunny days via the thermosiphon effect (i.e., warm air rises). To avoid unwanted infiltration, the opening should be sealed during non-sunny hours. Fans may be used to control the flow rate.Very cold locations and/or those with marginal solar resources may not be viable candidates for this approach. Air that is expelled from the house is normally at the desired temperature, and the heat energy in that air is lost to the outside.

Passive Solar Pre-heater

Heat recovery ventilator (HRV)

Heat recovery ventilators are (normally) powered ventilators and are considered active technology when such, though are often employed in passive solar buildings and are a key element in Passive Houses. High efficiency is important, and they are often only turned on a scheduled basis, when CO2 levels reach a threshold, or when indoor humidity is excessive. HRVs exchange the heat energy from stale warm inside air with fresh cold outside air, warming the incoming fresh outside air with outgoing warm inside air. Confused? Just take look at the picture below, and note that HRVs capture the indoor heat energy before it is exhausted to the outside;
Heat Recovery Ventilator

A new non-powered twist to HRVs was made at the London Beddington Zero Energy Development (dubbed "BedZed"). Wind is used as the motivating force, steering a rotating wind vane ventilator on the roof. The fresh air intake is on the windward side, and the stale air exhaust is on the leeward side. While the wind speed determines the overall air change rate, manual damper controls can be employed to moderate the ventilation rate on high wind days.

Wind directed HRV cowlings at BedZed

Energy recovery ventilator (ERV)

These are very similar to HRVs, except that they also account for differences in humidity, capturing latent heat energy that might otherwise be lost. The moisture capture is often implemented through the use of a rotating dessicant wheel, which absorbs the humidity from one air stream, and releases it into the other air stream as the wheel turns. This helps to maintain humidity levels in winter time, and often keeps humidity out in summer, though high humidity climates can often overwhelm the dessicant wheel capacity.

Energy Recovery Ventilator


There are a number of approaches and variations concerning summer ventilation in passive solar design; in this overview, we will briefly examine two techniques that are applicable in low to moderate humidity environments.

Solar Chimney

Air rises when it is warmed. Some designs incorporate an elevated component in a building, often as a cupola or a stairway with an extended height ceiling. The solar chimney is a collecting point for warm air, which it dissipates through its vents or a high window. One step further is the use of a vertical passageway from different floors to the elevated compontent, so that the warmer air from each floor can rise through up in a chimney effect. Taking it yet one step further, painting a vertical exhaust element black would absorb the sun's energy, heating the air further, and creating a stronger updraft.

Solar Chimney (Graphic courtesy of

Night Flush

In areas where cooling season nighttime temperatures are lower than approximately 65F (for at least some portion of the season) and humidity levels are low to moderate, a night flush can be employed by opening windows and exposing internal thermal mass to lower ambient temperatures, reducing the temperature of the thermal mass. This has the effect of rejecting heat accumulated during the previous day. When morning comes, the windows are closed when the ambient temperature rises above that of the thermal mass, with the thermal mass helping to moderate the temperature swing throughout the day (if enough thermal mass has be utilized). One drawback of this approach in residences is the potential for sleeping occupants to be warm upon repose, then chilled during the early morning hours. Commercial office and retail buildings, on the other hand, do not have this issue.

A number of commercial buildings have been designed and constructed utilizing the night flush approach, some examples of which include;

US Federal Building, San Francisco (night flush windows open)

1. David Kent Ballast, Architect's Handbook of Formulas, Tables, and Mathematical Calculations, Prentice Hall, 1988
2. Kissock, J, Internal Heat Gains and Design Heating & Cooling Loads, University of Dayton Lecture
3. Michael J. Crosbie, The Passive Solar Design and Construction Handbook, John Wiley and Sons, 1998
4. John Little, Randall Thomas, Design with Energy: The Conservation and Use of Energy in Buildings, Cambridge University Press, 1984
5. Passive Solar Heating and Cooling, Arizona Solar Center
6. Jeff Vail, Annualized Geo-Solar,
7. K. Darkwa *, J.-S. Kim, Dynamics of energy storage in phase change drywall systems, Wiley, 2005
8 Jo Darkwa, Mathematical Modelling and Simulation of Phase Change Drywalls for Heating Application in a Passive Solar Building, AIAA, 2005
9. Warszawski, Abraham, Industrialized and Automated Building Systems, Taylor & Francis, 1999
10. US Department of Defense, Passive Solar Buildings, Unified Facilities Criteria, UFC 3-440-03N, 2004
11. F. Bruckmeyer, The Equivalent Brick Wall, Gesunheitsingenieur, 63(6), 1942, pg 61-65
12. J. Douglas Balcomb, Passive Solar Buildings, MIT Press, 1988
13. M. Hoffman, M. Feldman, Calculation of the Thermal Responses of Buildings by the Total Time Constant Method, Building and Environment, Vol 16, No. 2, pg 71-85, 1981
14. Givoni, Baruch, Climate Considerations in Building and Urban Design, John Wiley and Sons, 1998 pg. 115-147
15. Hoseggen, Rasmus, Dynamic use of the building structure - energy performance and thermal environment, Norwegian University of Science and Technology, 2008
16. Bruce Haglund, Kurt Rathmann, Thermal Mass in Solar and Energy-Conserving Buildings (.pdf), University of Idaho
17. D. Baggs, Thermal Mass & its Role in Building Comfort and Energy Efficiency, EcoSpecifier Technical Guides

currently my favorite subject .. 103 deg. today

The issue is "to cool thermal mass at night" during the summer.

That's some pretty cool stuff, and the design is pretty hot too ;-)

It's right up my alley as I expand my energy efficiency and alternative energy portfolio.
This post certainly gets a thumbs up and a permanent book mark from me. Thanks!

I appreciate an article on passive solar that at least mentions cooling. So often passive solar articles talk only about heating. I once ordered two books on passive solar - only to realize that they were both written by Canadians who never even considered that cooling might be an issue. Nothing against Canadians - but the subject of designing for hot humid climates needs a lot more thought.


Hey Texas_Engineer, I happen to be in South Florida, do you have any knowledge you'd like to share?

I'm already involved in the Solar PV business but I'm certainly looking for more information all the time. Cooling is, as you might guess, is high on the list of must have amenities down here where we live :-)

Cooling and dehumidification.  You can have fairly high temperatures if the air is dry, but even 65°F feels sticky at 100% humidity.  You can also re-evaporate water into dry air to cool it.  Up here in Michigan, I would not need A/C if I could keep the humidity under 20% on the hottest days.

I have a little bit of information on LiBr characteristics but not enough to do a design.  I think it would be very interesting to build a system which does dehumidification, DHW heating and perhaps runs a low-pressure vapor engine (running on evolved water vapor using a vacuum condenser) just to see what elements work.

I think it would be very interesting to build a system which does dehumidification, DHW heating and perhaps runs a low-pressure vapor engine (running on evolved water vapor using a vacuum condenser) just to see what elements work.

Interesting idea indeed. Yes, I have already been thinking along the lines of dehumidification as it is. I know that air temps even in the high 70's if dry can feel very comfortable. That with an efficient ceiling fan I'm sure would be quite tolerable.

As it is, even today was quite sticky here in South Florida and it probably will be another two months before the air outside gets to where I like it and I can start opening my windows at night.

FMagyar, I live near the coast at the north end of Palm Beach County. My house was built in the late 60s. Twelve years ago I needed some roof work done, and the roofer talked me in to getting rid of the original concrete tile and replacing it with shingle. Bad idea, my AC/electric usage went up.

In the last few years I have (finally) begun thinking about reducing energy consumption, learning about peak resources, learning about renewables. I found some publications from the Florida Solar Energy Council, after reading these, my take on it is that basically all the new Florida home construction for (at least) the last 50 years has ignored the sensible house-cooling ideas which were evolved by the Florida settlers years ago, and all of the possible improvements from modern technology are ignored if they cost more, houses are built as cheap as possible. Here are some examples:
a. tree shading. Drive around one of those "planned communities", everybody has a big open grassy yard, no trees
b. roof overhang. Old Florida houses had more overhang to shade the windows. Duhhh.
c. efficient natural ventilation. Forget it, everything here is now airconditioned.
d. efficient AC. How about this great idea, let's put the ducts up in the attic instead of down in the conditioned airspace. Slap forehead, yet that's 99% of the houses.

I contacted FSEC to ask some specific questions about roofs and attics, again they have done the studies and built instrumented test houses. More papers to read. One of the things I learned was that those old concrete tile roofs kept the attics cooler than the cheap shingles. Maybe that's why all those old houses...yeah.

I have a tiny electronics company, I think maybe I should be doing something, making some product, associated with reduced energy consumption and/or renewables. My blackboard is covered with scribbles but so far I haven't reached a product focus. On my own I have been fooling around, errr, performing some R&D experiments with some solar stuff. I installed a couple panels on my house roof to run some attic fans, I am pulling in air at one end and pulling it out at the other end. Next I am building a solar tracker to tilt the panels, follow the sun. I have looked at swamp coolers and misters, but they are so inefficient here (too much humidity) that they can't be used for living space cooling. However, I think that attic mister fans could offer some benefit.

I am at the point where I need to get more serious. I signed up for the FSEC Photovoltaic Systems course, its a hands-on week long course in Cocoa Beach. I got in to the October class. It is hard to get in to this course, online registration sells out about 15 minutes after signup begins.

Scooperman, drop me a line my email is in my profile and if you are intersted I can give you some information about the PV design and installation course that the company I work with gives in Boca. We do one a month it's geared mostly towards contractors and has a hands on component as well.

As for what you say about construction techniques in Florida over the last few decades all I can say is "Yeah I Know".

For those who don't already know, FSEC is a well-respected R&D center loaded with valuable information and workshops;

You would likely also benefit from the Florida Green Building Council workshops and newsletters. See a list of incentives available in Florida;

Here's two more that I like.

This one is nationwide:

This one is Florida specific

I have a little bit of information on LiBr characteristics but not enough to do a design. I think it would be very interesting to build a system which does dehumidification, DHW heating and perhaps runs a low-pressure vapor engine (running on evolved water vapor using a vacuum condenser) just to see what elements work.

I've been ruminating on this some time myself, and with other similar comments, can only mean that such an approach is viable and in sore need of a simple workable solution.

Perhaps this information may help you:

George thanks for the link I do appreciate it. I actually have two siblings who live in Germany my sister's brother in law knows the founder of a large solar company that employs similar technology in Germany and they do market in the US. However the rub lies in the fact that this technology works much better in the cooler dryer northern climes. The issue we have in places like South Florida is the double whammy of very high heat and very high humidity. So while this technology works I think that it may need some tweaking if it is to be successfully implemented here. I will take another look at it and keep an open mind.

Here is another way for using less energy for cooling; making ice during the night.

That's interesting. Tks

not passive

I'm not a passive purist per per se, I'm willing to look at ideas that are mechanical and electrical that can be run off of solar heat for example a stirling engine or say a highly efficient DC compressor that can be run on a KW of PV panels. Obviously we are talking about off the shelf concepts that might be able to be adapted to non fossil fuel energy sources that can then be coupled with passive solar design for the best possible bang for the buck.

I think something much cheaper than a hi-tech Stirling engine
is a steam Cyclone power generator:

hi-tech Stirling engine

George, there is not much that one can call high tech about a Stirling engine.
Sure, you can complicate anything...

If there is not someting inherently high tech about stirlings why do they cost out the ying yang?

Stirlings seem to be like fusion -the power of tomorrow,forever.

Materials and lubrication.  Instead of generating heat in the working fluid, you've got to pass it through a wall; this means using a material which can conduct the heat and hold the pressures.  If any of your lubricant gets to the hot side, it cokes up and leaves you needing an engine rebuild or replacement heat exchanger.  The seals to prevent that... you get the idea.

One of our sometime-commenters says he's had some success with Beale-type Stirlings, homebuilt.  He has not given any details.

A low-tech Stirling engine would be something like the one shown in the next link:

I assume this engine is only capable of generating 0.5 HP (mechanical)
and I assume that the working fluid is unpressurized air.

One nit:  K = °C + 273.15, not 272.

Start your house design using this handy on line solar calculator:

For my home on the Alabama Gulf Coast I used the following:

*House oriented facing north with 30 inch overhand on south
*Minimal east and west facing windows placed high up under eave
*South side windows 30 inches above floor
*Insulated foundation wall around slab to use slab and fill dirt thermal mass
*Aerated autoclaved concrete walls, 12 inch plus 8 inch AAC interior walls giving 75 tons thermal mass
*Light colored metal roof

HVAC only electricity is about 600 KW-HR in JLY for a 2100 sq ft house, which is excellent for the area. I have a 16 SEER air conditioner.

600 kWhr of A/C on the Alabama Gulf Coast in July is indeed a remarkably low energy consumption. All of the measures you've taken are sound energy saving steps.

What do you normally set the thermostat at? What are your winter energy consumption levels like?

I keep the inside temperature around 76-77 F.

The construction minimizes air infiltration so my humidity load is low.

I have compact flourescent lighting and nat gas hot water.

I have a heat pump with natural gas backup. In JAN I use 300 KW of electrity for the heat pump and about 30 therms of gas.

KW 08
JAN 716
FEB 728
MAR 649
APR 531
MAY 465
JUN 865
JLY 1115
AUG 988
SEP 889
OCT 505
NOV 421
DEC 650
Annual 8522 KW HR

JAN 30
FEB 28
MAR 14
APR 11
NOV 12
DEC 19

Although I only have about 300 sq ft of window area the house has excellent interior daylight because of window placement and the wall to window distance being rather short with interior walls being about 12 feet from exterior. I can read anywhere inside without artificial light when the sun is shining. I also have an excellent view of the outdoors and a decent amount of privacy from keeping windows off the floor. The only tall windows (5'6") face north.

I have seen too many nice houses with floor to cieling windows that have to be covered with curtains to keep out solar heat.

Thanks to Will Stewart for this excellent series.

What's important is how to do retrofits, since most of us are not building new homes or offices.

From my personal experience, I can tell you that it's doable by a DIY-er, or a professional who is given a design, or is given design guidelines.

About 30 years ago I retrofitted a standard 2 X 4 studded exterior wall with a solar space heater whose design may have come from The Mother Earth News (I don't recall, since I was copying what someone else had done). It works entirely by convection once there's a temperature differential, so no electric fans are needed. This space heater is still working, though I no longer live in the house.

In a large room I cut 4" X 12" slots in the wooden floor boards between the joists, all along the wall. The cold air went into these holes where it was guided into the bottom of the solar collector which hugged the exterior wall. Near the top of the collector I cut slots in the wall, similar to the slots in the floor, through which the warm air would flow back into the room. I used thin glass for the glazing, and it did crack due to stress from expansion and contraction. But using tempered glass would probably avoid this, or plastic panels. Within the space heater I mounted several layers of grating used in concrete work, painted flat black, and mounted at an angle so that the air was forced to move past it and be heated.

Check out this website of Gary Reysa, who built a nice wall solar space heater along the same lines. I especially like his thin plastic flapper valve which prevents reverse air flow once things cool down in the afternoon. They require no human intervention, whereas on the one I built, I used 1/4" plywood covers which had to be manually opened and closed.

Build It Solar website

In a search on Google I also found this, which sounds promising, but I haven't yet seen the book.

How To Build and Benefit From A PASSIVE SOLAR Collector As A Space Heater, Second Edition, by Ralph W. Ritchie Book as PDF file: $7.95

This is space heating .... where is the space cooling ?

My post was in response to the series of articles.

However, low-energy, passive cooling as a retrofit is a hard nut to crack.

If you're building new, then there are those possibilities outlined in the links provided in the replies of others here.

[It's not space cooling, but I am brought to think of how Thomas Jefferson managed to eat ice cream during the summers back in the 18th and 19th centuries. His slaves dug a _very_ deep hole in the ground in which was stored ice which during the winter they hauled up from the frozen Rivanna River. Don't know how long this lasted, but I imagine it lasted long into the summer. Lots of energy went into that ice cream!]

Most people world-over do without air-conditioning.

And, yes, some people are going to die from the heat.

The hottest days tend to be sunny in most places. I think solar panels connected to fans, heat pumps (especially in attics), and small one-room dehumidifiers could help mitigate this. You don't have to get the room to 70 F to keep "most" people from dying of heat.

Also, there's a reason the South used to be known for elaborate patios and porches.

Also, there's a reason the South used to be known for elaborate patios and porches.

And let's not forget the high ceilings.

The intractable case for passive cooling appears to be when there are consecutive days and nights of hot, humid and windless weather. Say 30C/86F overnight minimum with 65% humidity. In that case there may be no option but mechanical assist cooling such as fans or air conditioning.

Since there will be more weather like this in a greenhouse world I wonder how energy savings can be achieved on a large scale. Within a generation millions of baby boomers will need senior care. There simply may not be say 500 watts of cooling per person in heatwaves. Could a combination of passive and efficient mechanical cooling get it down to say 50 watts per person? Perhaps we will work and sleep in reverse incubators that maintain just a cubic metre of cool dry air. No large screen plasma TVs inside your cool box.

To those who say we will need less electricity in a powered-down world of the future remember there will be new demands for electric transport, sea water desalination and the thermal comfort of an ageing population.

There simply may not be say 500 watts of cooling per person in heatwaves.

People will then die. Such has been happening for years due to social and economical differences.

Already happening,22606,26120948-2682,00.html
I'm not sure about 'death panels' for medical care but one day committees will debate how much heating and cooling society can afford to spend on the elderly.

Could a combination of passive and efficient mechanical cooling get it down to say 50 watts per person? Perhaps we will work and sleep in reverse incubators that maintain just a cubic metre of cool dry air.

The air-conditioning equivalent of a kotatsu has merit.  If you look at the price of a dehumidifier, it does not appear to be an expensive thing to make.  A 25 pint per day dehumidifier has, by my BOTE calculation, more than 300 watts of heat-moving capacity.  Keeping a small space cool and dry would take relatively little power even if there is a 120-watt heat source in it.

Yes, why is it that educated people cannot see what is in front of them.

Why heat and cool the whole house? So American in attitude, and so much waste.

It's a shocker that people are still building slab floor houses, let alone the ones that have the insulation values of a matchbox....

Yes, why is it that educated people cannot see what is in front of them.


A group of blind men touch an elephant to learn what it is like. Each one touches a different part, but only one part, such as the side or the tusk. They then compare notes on what they felt, and learn they are in complete disagreement. The story indicates that reality may be viewed differently depending upon one's perspective, and suggests that what seems an absolute truth may be relative due to the deceptive nature of half-truths.

Perhaps our own views are way too compartmentalized and we are all blind to the big picture.

BTW, There were many examples of this graphic to choose from on the internet I deliberately chose this one one because of the fact that it is the perspective of someone examining bioinformatics, I thought it would underscore the point that our own experiences and specializations narrow our focus to the exclusion of the synergistic perspective. Yet we are all facing the same grand scale issues and seem to be blind to complexity and interactions at a grander scale.

I think perhaps it is time for upgrading our concept of Education 1.0 to Education 2.0 and start understanding that 1 + 1 = 4 and not 2.

In case that sounds like it doesn't make much sense, that's a somewhat oblique refernece to Buckminster Fuller's "Synergetics, Exploration's in The Geometry of Thinking", when he examines the combined properties of two triangles when they are configured in a tetrahedral (4 triangles) form and suddenly the synergistic properties of the tetrahedron emerge.

Slabs in warm climates have value as thermal mass, provided you insulate the foundation wall.

Our White Mtn house was on an insulated slab.. the insulation is the main thing, as usual.

While not quite at the level of a kotatsu, there is a 25 SEER 9000 BTU/hr heat pump available that could be used in a selected room(s) or zone(s).

Says .. Inverter-driven DC Compressor operates at high efficiency

Now just get a DC model to run off solar panels

Maybe an American variation on Kotatsu could be to use the couches and comfy chairs as hot water storage units, which would also make a compound use of that bulky stuff.. ok, maybe just put PEX tubes in there and have them plugged in just like radiant floors, with a little thermostat dial in the armrest. Easy! Nice place to sit and crochet your new duvet!


(might also be ok to set your chilly fillets onto a toasty bidet!)

ok, maybe just put PEX tubes in there and have them plugged in just like radiant floors, with a little thermostat dial in the armrest.

If you add some LED lighting you could have some real couch potatoes...The only question is who gets to pedal on the stationary generator bike?

The maid can do that.

... or Charlton Heston.

Body heat is 400 BTU/hr at rest. At 16 BTU/watt for cooling a person would require 25 watts, plus whatever it takes for heat conduction and infiltration (including humidity) of the structure.

Warm climates have 24 hour temperature averages slightly above the comfort range. Example, JLY in Atlanta GA 77F. That's where thermal mass is useful.

Humidity control in the SE USA is 25% of cooling.

In a well insulated house the majority of heat transfer takes place through windows. Solar heat gain is 50 BTU/sq ft whenever sun is shining through. Proper window placement and shading is essential.

Blame Frank Lloyd Wright for too much glass. He is credited with bringing nature inside, summer and winter too!

Dehumidifiers are the wrong approach when it is hot because they exhaust warmer air. They are for use in cool, damp conditions.

A straight dehumidifier wouldn't do, but if you put the human between the evaporator and the condenser you've got something.

Passive and Low Energy Cooling

Passive and low energy cooling of buildings
By Baruch Givoni

Passive cooling of buildings
By M. Santamouris, D. Asimakopoulos

Natural Cooling

Good links. I would have liked to been much more expansive on cooling techniques, but the article length was already over the preferred norm.

I'll note that the earth tubes that mentioned by Givoni can be problematic in humid environs, due to the ever-present condensation in the tubes that will lead to mold, mildew, Legionaire's disease, and other unhealthy organisms. Some solutions have been to leave a wire in the tubes, which can then be used to draw a bleach-soaked rag through on a periodic basis (and another wire is pulled through at the same time for the next treatment. Others also have a drop in height from one end to the other in order to promote drainage. Still others run the earth-cooled air through an air-to-air heat exchanger so that the earth tube air does not enter the conditioned space (albeit providing no fresh air).

If the water table is high enough, it would take far less energy to pump groundwater through a fan coil than blowing air through a secondary system of earth tubes and a heat exchanger.  I have considered this myself; just using groundwater as pre-cooling for air going into a dehumidifier, and also as a secondary heat sink for the compressed refrigerant, would allow a rather small system to provide outsized dehumidification and cooling.

I agree

How about if one had a well in the middle of the structure ?

PV driven ground source heat pump for heating and cooling

You still may need to draw in air from somewhere.

One of the features of the Ground Tubes can be that the natural convection of warmer air up through the house creates a natural draft pulling air into and through the ground system. We would let ours run that way (passive) or add a small fan, by which I mean under 20 watts.. it was really a minor consideration.. but it did require that the house be fairly tight. That makes more sense as a warming heat exchanger in wintertime, I suppose, but we could see the effect in summer as well.

Sun Lizard Solar Air Conditioning

In Winter

In winter, the Sun Lizard allows you to capture the warmth of the sun into the air space where you live, work and breathe.

Sun Lizard in heating mode

The solar powered fans draw air from the building at ceiling level through an inlet vent. The air is forced through the solar heat collector sun baking on top of the roof, boosting the air temperature to as much as 50 degrees Celsius. This hot air is gently blown back through the heating outlet vent and flows back into the building at floor level, giving you free and natural warmth from the sun every day. This closed loop system is very efficient at heating your building.

In Summer

In summer, hot air is often trapped inside the building by the ceiling, roof and insulation. The Sun Lizard removes the hot air from the room so you don’t get a build up of heat in either the air or the thermal mass of the building and your furnishings. You then tap into the natural cool air of your building and can reduce the need for air-conditioning.

Sun Lizard in Cooling Mode

The hot air is extracted by solar powered fans through the inlet vent and released to the outside, reducing the build up of hot air inside the building.


I think you've shown a poor design for a solar heating system.

First off, feeding the warmest air from the room into the solar collector reduces the overall thermal gain into the room, since the collector efficiency is reduced at the higher temperatures produced. Thus, you would need a larger collector area for the same net energy produced. Using the fan to move the colder air from the bottom of the room into the collector might help, but then there's this second problem.

Secondly, running that warm air thru the collector on cold days would result in lots of condensation on the cover plates, which will cause troubles when it freezes or when there is poor drainage to deal with the water. That water must go somewhere, usually down hill. You also need to worry about the problem of night time cooling and thermosyphon of cold air back into the structure after dark, which can be minimized with dampers.

The third problem is the cooling loop, where the cooling appears to be provided by contact with the ground under the house. You do not specify how this is done, but bring in warm, moist air under a house and cooling it with ground contact is a sure recipe for condensation and mildew problems in a humid climate. Been there and done that. Placing duct work under ground with drainage is the better solution, as the moisture can be removed before the air enters the structure. Of course, digging trenches and placing plastic ducts and drains under an existing structure is a real pain in the a$$...

E. Swanson

You're right about placing the inlet lower, but at least with my solar box heater, which was running at 5F this February, (and blowing into the house at 125F) the sun keeps that thing warm enough to prevent any condensation that I could see. If there's a bit of it overnight, it's quickly evaporated once the box is heated and air is flowing again.

The drained, underground ductwork is a system I've lived with, and I wouldn't build a house without it. The dampness has to be carefully considered, but I think it's unfortunate that this issue has stunted that type of tech. We can solve that. I would even consider designing such a system with the intention of Collecting Water with it.. take the bug and make a feature out of it.

Even introducing running water to the ducts could be part of the solution, since mold needs conditions that are not too wet or too dry.

Some good basic information but very fragmented and with very wide scope for just one small post/ article. The result is quite convoluted and with inadequate info, basically jumping from subject to subject. may I suggest future posts on such matters cover one subject at a time in order to have adequate, effficient well focused information available. One subject would be "night time venitaltion" or "thermal mass for passive cooling or heating" or "stack ventilation" or "wind forsed ventilation" and so on. If we are going to refer to thermal comfort lets just do that in a more comprehesnive way and include basic info like the temp/wind differentiation/thresholds that can seriously affect thermal comfort.

Typicaly the term "passive" (heating, cooling, ventilation) is used for systems without electical / mechanical means (i.e. fans). The term "passive" denotes the appropriate use of materials, shapes, forms, tecnhiques& methods of cosntruction and so on. A typical solar hot water collector utilizes passive technology, some types used in northern countries with pumps included, are not considered passive. The "solar chimney" graphic is a typical diagram indicating stack effect and stack ventilation. A solar chimney typicaly has quite different design with smaller cross section, more glass / exposure to sun, and is insulated from the rest of the building (except at specific air intakes) otherwise you 'ld end up heating instead of cooling.

If this were a green building blog, I would have indeed delved much deeper into each of these subjects. As it is, this is already the 5th article in a series on a petroleum-focused blog, so the article was intended to be an exposure to interesting technologies and techniques, not a workshop lecture.

The discussion of what "passive" means was covered in Part 1 of the series. Powered ventilation fans were included here to provide background for very low energy techniques that are part of the design framework developed by the Passive House Institute.

On the subject of stack ventilation vs. solar chimney, the two are by no means mutually exclusive - many design approaches blend aspects of both;

The claim of some exposure does not justify such broad, convoluted info in one piece. As a building specialist I dont find how this could be very helpful for someone - especialy one not in the field - I'ld say chances are it may be confusing and not too practical. Simply too many different, diverse and quite specialised subjects on the one hand and very little space to explain even the very basics. No one would be looking for any formulas in such article, better to simply explain the principles of thermal mass and how this can be utilised in passive cooling and heating.

Your link to solar chimneys has a list but the articles are not accesible. Perhaps I didnt explain this well - Solar chimneys utilise stack effect, yet the image you post is not indicative of a solar chimney. If someone envisions building something like that and thinks it will act as a solar chimney it just wont happen. Solar chimneys are a rare breed anyway, with very few built examples globally, mostly done experimentally. High inital costs, buiding limitations and no great results - which brings me back to my point about the nature of this article.

I'm not completely sure I understand the source of your confusion. The first paragraph above the fold explains the subjects to be covered;

...we will cover the techniques used to avoid hot and cold spots in a passive solar building, how to provide fresh air, and how to provide cooling (in many situations).

I then go on to state the challenge for the first subject, covering convection and radiation at a relatively high level, but with some information for the many engineers that frequent this blog;

Capturing and storing solar thermal energy is not enough; how do we ensure its dispersal when and where we need it?

I then go on the identify the differences in convection and radiation between the main variants of solar design - direct, indirect, and isolated gain.

I then go on the cover the second subject; ventilation, covering the current and emerging low energy concepts in the field.

I then briefly discuss two variations of passive cooling, though the article is already a bit too long.

Again, if you are looking for in-depth treatment of the subject, I'd suggest reviewing the references I noted or visiting a green building blog where this information is treated in-depth. I'll regard your comments, which I believe have some merit, in the context of the other comments above from those who stated their acceptance of the article.

I will use a metaphor. As a building specialist, your attempt seems like a 1/2 hour cooking class in which you attempt to demonstrate way too many different recipes while you only go as far as reciting only some of the very basic ingredients and just a few clues for each recipe. You are better off taking one recipe at a time and presenting it approprietely.

I think you might consider regarding this as a talk about nutrition, then, and not a cooking class.

"In this final article in the passive solar design overview series.."

He's really been fairly clear about the intention of this post, and as a specialist and a professional, you might simply try to offer "Here's a link to Chimneys that better clarifies their layout and purpose.." or something.

And These are topics that I and others no doubt would like to learn about in detail.. maybe you could offer posts on some of the more specific approaches.

Well yes perhaps a talk about nutrition might be a better metaphor. It doenst matter if this is an overview - it is so fragmented and wide in scope it is misleading. Again another example, just the aspect of human thermal comfort is vast - consider that in most modern building we utilise one thermostat per room (if even) well, the palm of the human hand has several thousand thermal sensors per square inch. So mixing up formulas, thermal comfort (which involves human physiology) as well as air distribution,cooling, heating, etc. in one little article is just a very bad approach. The initial statement of "avoiding cold and hot spots" is a poor description. Cold and hot spots is not a professional term, more likely it would refer to cold bridges i.e. general building construction methods and not passive design approach.

You have a point about positive feedback but that only makes sense if there is a solid frame to lock on the various new bits & pieces. The whole point would be to consolidate information in a meaningful way. You cant even begin writing about any passive / bioclimatic applications without refering to a specific climate / microclimate and overall context (such as building uses) because you will end up providing info that is not applicable, grossly overgeneralised and quite frankly very misleading.

Passive / bioclimatic buidling design is a "holistic" kind of approach, intertwined with lots of special parameters that have to be taken into account at all times or else things go terribly wrong. Yet, inorder to begin understanding this strategy, one has to understand the various parts in the begining quite well and one way to achieve this is by focusing on a certain aspect each time.

So if this forum is going to be of any use in building design aspects it should promote articles that are more focused. To provide a basic common ground so that a more meaningful and practical exchange of information may take place.

As a building energy engineer (Colorado University M.S. Building Energy Engineering Program 1986) I found Will's overview useful. No reasonable person is going to build a structure based on a blog post, but Will gives general concepts and sources for more information.
If we could get builders and architects to read his post, it could only improve the brain-dead construction that is the norm in the US. Most people in the building industry could not define "passive solar design".
Efpalinos, I think you are far too critical of a volunteer good-faith effort. If people want more detail and focus, they need to buy books and/or hire professionals.

I tend to agree with your comments. I say that having lived in a house with a passive "sun room" type system for about 20 years. Also, I'm an engineer and helped write a book on renewable energy back in 1974, so I've been aware of the problems for some time. And, I live in a house with super insulation and a large south wall collector system.

While I think passive is a useful concept in regions with relatively mild winters, passive won't work very well at higher latitudes with below freezing temperatures and cloudy weather. That's because the losses thru the glazing at night or on cloudy days can quickly cut the inside temperatures, even with large thermal mass. If the design employs a large aperture for solar gain to compensate for the overall heat loss, the heat gain on sunny days will drive the inside temperature to uncomfortable levels.

Stewart's inclusion of a graphic with a south wall heater with a fan does not represent a passive system. That design may work if there is sufficient thermal storage which is isolated from the interior of the building such that the flow of thermal energy may be controlled. That too is part of an active system.

That passive systems are less expensive and relatively simple to implement has made them the first choice among solar advocates, many of whom are living in places such as coastal California, which experiences mild winter temperatures. That they don't work very well and require a large backup system has resulted in a generally negative impression in the public eye, I suspect. I think Stewart's article continues this overly optimistic presentation.

E. Swanson

Having built a Passive Solar home in the Maine Woods 28 years ago with my parents, I beg to differ about their applicability in places with real winters.. but in addition to balancing the amounts of sun-facing glass and thermal mass, it is also necessary to balance out the expectations of just how 'passive' everything has to be. I wouldn't dream of leaving glazing uncovered on a winter's night, just to prove a point that my house does it all and I'm cheating the principle if I have to be engaged in the process. So who cares if shutting shades and shutters at night somehow makes it 'not passive' any more.. or for that matter, if we've got some thermostat-driven fan or other automation that works within the system? There are passive systems, but they might be interacting with active ones that deserve mention.

There's such a thing as getting too hung up on those labels.. and I'm particularly wary of just how much of our modern systems are built with this same insistence that 'it takes care of itself', like some kitchen appliance that is supposed to take the burden of mixing chocolate milk off of our overwhelmed souls..

For our house up there in the White Mts, the degree of insulation, solar input, thermal mass and the underground air supply was enough to keep the pipes from freezing if we basically left it on it's own. THAT much was probably 'pure' passive.. and any amount above that temp was going to be up to us.. not that we couldn't have worked to optimize the whole thing and brought the basic 'passive' temps up higher.. but life had other demands pressing on us, and we were learning.

I don't know what you mean by 'too optimistic', unless you were getting the impression that the Post is advocating for homes that can and should be exclusively passive. Like everything we talk about here, it's surely another BB, not a silver bullet, and not meant to be taken as any kind of exclusive solution. But any Mainer who builds a house today without considering solar access and heavy insulation is going to be seen rightly as a damned fool.

...passive won't work very well at higher latitudes with below freezing temperatures and cloudy weather.

I would agree for areas that are mostly cloudy. Higher latitudes is relative; are any of these buildings referenced in Part 1 of this series in higher latitudes?

Canada, Norway, Germany, the Northern US (Maine, New Hampshire, Michigan, Wisconsin, Minnesota, North Dakota, Montana, Idaho, Washington state, etc), Scotland, the Netherlands, etc. Even the US Department of Defense has a passive solar design guide. Design approaches such as Passivhaus have achieved up to 90% reduction in energy use over traditional building methods. In areas with reasonably consistent winter insolation, well insulated passive solar buildings with sufficient thermal mass storage can approach 100% of their space heating needs with passive solar. Enhancements can be added to existing buildings, through major or minor renovations, or through simple additions.

You say it can't be done outside of areas with mild winters because, "...the losses thru the glazing at night or on cloudy days can quickly cut the inside temperatures, even with large thermal mass." Many early passive solar designs over-emphasized window area and neglected insulated shades (or other window treatments), so are poor performers. That does not mean that improved design techniques also result in poor performers. New methods, techniques, and materials have helped to enable several leaps of improvement over early passive solar designs such as you describe. The Passive House Institute now sets a minimum threshold for designs following it's framework; less than 15 kWh/(m²a) (4755 Btu/ft²/yr)

Cellular window shades are great for holding in heat in cold climates. Double layer shades are about R4. Shades with 30% light transmittance are also available. Very nice looking and not too expensive.

We have been using triple honeycomb shades for 12 years at our passive solar house, and yes, they perform wonderfully.


What do you know about the plastic films that are applied directly to windows?These products are marketed as real energy savers,supposedly cutting down quite a bit on heat gain and loss.Anybody else?
I'm thinking about trying this product .
Thanks in advance!

psI also have about five gallons per minute of nice cool gravity feed spring water.I would be grateful for any suggestions as to the best way to use this partial for cooling a single large room -somethng that doesn't require a lot of cash as the need for the cooling is temporary-maybe a couple more years.

We're using a window ac right now.

Please provide a link to the product so we can examine it. At first blush, I'd say any film applied directly to a window is not likely to make any noticeable difference. People used to attach plastic films in an offset manner along windows, though a small amount of benefit could be had if the gap was not too large (1/2" or less to discourage convection), and/or if the film slowed down infiltration from a drafty window.

I have applied heat-shrink material to windows with beautiful results.  I have also made frames which are completely wrapped in the heat-shrink, creating a double-pane insert.  These are reusable for years.

Others have suggested applying bubble wrap to windows, bubble side in.  I've not tried this but it looks quite workable to me.

Energy Star rated double pane windows are R3.3. Part of the improvement over the R2 of regular double pane glass is the coating applied to the inner glass. The coating cuts down transmission of infrared light. My guess is that the stick on films have the same type of IR transmittance.

The other part of the better R rating is the argon gas between the panes. I believe the coating does more than the argon gas.

The shrink warp films that add an air space probably add about R1, depending on the gap thickness; more if coated to reduce IR transmission. They also cut down on air infiltration which is very important in windows that were not designed to minimize infiltration.

When I built my house, starting 11 years ago, I used triple layer windows on three sides, but double layer on the south facing side. The triple layer windows had a layer of plastic film in the middle, which was coated with an IR reflecting layer. The film is made by a company called Southwall from California. The R value of these non-argon windows is relatively high (U=0.39 ==> R=2.6), but the "solar heat gain" transmission coefficient is relatively low at 0.37. Thus, the triple layer windows are not good as solar collectors, which was the reason I used the double layer ones facing toward the south. The fact that the IR coating reduces the solar heat gain implies that a larger area is required to produce any desired thermal gain. This fact is not often mentioned by folks who present the passive solar model.

The other replies to my post mention movable insulation as another approach to reducing heat loss at night, but it's difficult to see how such systems would help during the day, especially during intermittent cloud situations. Sure, those shutters might be passive in the sense that they are human powered, but moving them with some sort of motor setup controlled by sensors makes the result an active system in my view. If one is willing to make the effort, opening and closing shutters or moving curtains is not a big deal, but, the extra effort and expense must be included in the relative cost assessment. Why not go for a complete active system in the first place, instead of spending years (or decades) opening and closing shutters?

E. Swanson

One doesn't need insulating shutters for the day, unless it is cloudy and cold. Then one wouldn't open them for that day, except for whatever was desired for natural light.

I have been raising and lowering insulated shades at my house for 12 winters now and have no issues with doing so. I also cut, split, load/unload, stack outside, bring inside, and burn my own firewood. Both of those activities help keep me aware of the natural cycles of not only each day, but the seasons as well. I'm not a purist when it comes to exacting definitions of what is truly passive or not (even the Passive House Institute recommends a powered HRV), so I see no conflicts.

Going to a completely active system is a far cry from opening some shades, from a cost, installation, reliability, and maintainability perspective. My next step is to install an oversize solar hot water heater that will provide a morning 'boost' to begin the day, so one could call that addition a hybrid between and active and a passive approach. Again, I see no conflicts.

Passive houses are common in Northern Maine and Germany (cold below freezing winters home of the PassiveHaus).

The optimum investment in insulation, thermal mass, and window area/technology certainly shifts with latitude. Northern climates usually require some form of window insulation, although the newest R8 and better windows are changing this. Passive design does not mean "no backup heat required" but rather maximizing heat gain and minimizing heat loss during the heating season.

I built a first generation passive solar house in Maine with my brother in 1976, and he has been using less than half the per-square-foot heating energy as his neighbors for more than 3 decades. If we were designing the house today it would look different, but even the first generation houses outperform conventional construction.

Even if the structure is not totally Passive Cooled any and all passive cooling techniques reduce any active cooling load

My plan(shown above) is to reduce the solar heat intake on the structure (reflective roof ,shade, overhangs,insulation, trees etc.)

and then as the sun goes down get the structure to ambient temperature as quickly as possible(reflective roof etc. to night sky) and then start cooling thermal mass (earth or water) to the coolest temperature of the day (morning)

then insulate and use that thermal mass for cooling during the day ... then repeat.

in Summer...

Use Night Sky to Cool thermal mass for daytime cooling

in winter ...

Use daytime Sun to heat thermal mass for nighttime heat

Living in Irvine, it typically cools down at night ~65 degrees and warms up to 90 during the summertime. I typically leave the windows open until it is warmer outside that inside in the morning. I can't think of how much electricity this saves me. It's 1:30, 90 degrees outside, and my AC is set to turn on at 76 degrees. It hasn't turned on yet today.

Making simple lifestyle changes (yes, they do require some effort) can make a huge difference in energy usage.

Thanks for your tips in building homes. It's important that we look into saving energy before we invest in renewable ways to create it. Saving energy is a lot cheaper and more effective long term than creating it.

Every once in a while I do see something that makes me really sit up and say, WOW!!

This is cool!

I would like to bring everyones attention to Phase Change Materials.
eg candle wax. The melt temperature is chosen by the length of the carbon molecule.

These materials store and release a lot more energy than passive storage.

I don't have the time to find the link but I think that it was researched extensively by Dow chemicals.

Phase Change Material (PCM) Eutectic Solutions Used for Passive Cooling

I feel water storage is the only economically viable solution

Great topic, once again. Yes lots of info & anecdotes, but hey it's a campfire. Built a few passive solar homes, some w/ trombe walls, in the 80's outside of Denver. Incorporated many of the efficiency lessons learned from that time in every house built since (all in St.Louis). My take away after all these years is that a combination of super insulation, passive solar, & earth berming on North side & West if possible, will result in "near-zero" performance. Granted thats not possible for every site, but if you've got the option its the way to go (except in the areas that cloud up all winter). My current 1200 sq.ft. super-insulated, minimal passive solar, all elect.,normal looking house gets through the winter/heating months for bout $213. Thats mid Oct-mid Apr @ 7.5cents/kwh. & includes 1/3 of costs of a shared well & septic system w/ aerator. Course me and the pooch are real energy conscious (cat couldn't care less).

I've read and soaked in the comments above and with regards to only the cooling aspects of a system designed for hot humid areas like here in south florida it appears obvious that the lowest energy solution would be to cool a single well insulated room.
In this proposed system the energy would come from solar panels which power highly efficient freezer units to bring the temperature down of antifreeze cryogenic solution ( since water would explode the tanks ) stored in a large vacuum ( this is the only insulation solution for hot weather ) insulated storage tank ( those used commonly for liquid nitrogen ). These aren't very expensive at all in 50-100 gallon sizes. Then when the cold is needed, pump the cryogenic solution thru a heat exchanger to a regular split duct ac unit.

essentially the icebear solution but with mega insulation.

not to belate the obvious, but one of the best ways to store energy is thru a thermal differential sink and source. In a hot environment that would be by storing cold, and in a cold environment then by a heat source (think geothermal). therefore I think the use of these vacuum insulated storage tanks are the only way to store potential energy in the form cold in a hot climate like So FL. They would provide cold on demand with no intermediary ineffeciencies.

The Ice Bear manages to do with ice, and without vacuum insulation.  Since phase change is quite a bit more efficient than mere sensible heat of fluids, you might want to re-think your technological requirements.

Thanks, I forgot some of my p-chem. Of course you will gain more cooling capacity b/c of the phase change of ice to water. However, the icebear as is offers no distinct way to really save on energy used to cool a certain area compared to a conventional heat pump. It just defers the cost of electricity from prime time to non-prime time hours. The real issue here is to cool with much less energy. I am all for not insulating the heat sink, as long as you keep it indoors, then you will cool convectively albeit with some condensation. If it is kept outside without insulation, I can assure you, the heat down here will make your losses pretty huge. Vacuum is the best insulator. Those 100 gallon nitrogen storage tanks are cheap and keep their cold for weeks and weeks. It would be easy to replumb them for use with liquids as they practically are now. To reconfigure them for solid ice may be a different matter. I'll have to do the calculation sometime: Which is the better heat sink, 100 gallons of ice at -2 degrees C or 100 gallons of calcium chloride solution ( take your pick of molarity as long as it doesn't precipitate out ) at -22 degrees C ?
So the difference between -22 / -2 and the upper end would be lets say 5 deg C.

Ice storage can be beneficial in environs that are hot during the day and relatively cool at night. With cool night air, heat pump efficiency goes up dramatically, especially those with partial load capability.

If your CaCl2 solution has the same heat capacity as water, your 20°C temperature variation will only store as much cooling as the melting of 1/4 as much mass of ice.  You can put a LOT of foam insulation into that much space, and it's going to be a lot cheaper than dealing with the mechanical requirements of holding a vacuum.  Best to go with ice.

There's an idea I had a while ago, and as I'm not going to use it any time soon I'm putting it the public domain (unless someone patented it while I wasn't looking):

Build an insulated water/ice reservoir into a piece of furniture, such as the solid back of a shelving unit or bookcase.  This would be connected to an external chiller unit either via refrigerant lines (feasible with CO2 refrigerant) or a fluid loop carrying some sort of antifreeze.  This unit would be located somewhere inside the space to be cooled.  It might only be insulated by a half-inch of foam (R-7.5/inch) and thus gain a fair amount of heat, but if the heat is coming from the space to be cooled and there is no condensation, this is not a problem.  The major issue would be locating the unit properly so the weight of the water inside does not exceed the static load ratings of the structure.