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Exterior house material to maximize passive solar?

bill328 | Posted in General Questions on

I checked all over GBA and could not find any information on the differences in passive solar performance when different exterior finishing material is used. Here in Colorado we use corrugated metal, stone, stucco and wood siding mostly.

I assume that it must make a difference in how the thermal mass heats and cools. 

Does anyone have a resource or information you could direct me to?

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  1. Expert Member
    Akos | | #1

    Typically thermal mass is inside the house, the siding has nothing to do with it. Either way, it doesn't make enough of a difference in efficient construction to worry about. Just focus on getting a well sealed, insulated house with properly sized/oriented and shaded windows. Much better use of your time than trying to a high mass system to work.

  2. onslow | | #2


    Akos is right about the thermal mass belonging inside the insulation envelope. He is also right about focusing on the other points first, as well. You can, however, with careful design take advantage of thermal mass inside your well insulated envelope. If you are building where the day/night temperature swing is sufficient, it is possible to vent with open windows and drop the interior down to 67-69F without too much problem. The right interior materials can work to your benefit.

    I did not plan on having AC of any form in my build and so far we are doing fine. We do have the advantage of being at 8000' where the night temperatures are typically 30F lower than day temperatures. Sometimes more. A huge advantage we have is low humidity.

    The thermal banking we gain for each day ahead comes in the form of all 5/8 drywall and about half of total floor space being tile. One requirement is getting up earlier enough to close the windows before the sun warms things up. Another is having your design take advantage of stack flow with a high placed window(s). We have a 3rd level reading nook that serves as the high point with views in all directions, so win-win.

    Thanks to the very low humidity in our part of Colorado, we can stay comfortable even when the interior temperatures are close to 78-80. Being bald helps too. (not my wife though) I am not sure about conditions on the front range or around Denver. I suspect Denver is making itself a very potent heat island these days. Someone who lived there in the early sixties was quite surprised at the night heat on a recent visit. Be sure to consider your summer swing range.

    That said, the exterior finish and window choices will have effects that may or may not suit your plans. I took care of a local home with passive design features, mostly a south facing wall of windows, for a few weeks in July, which proved very instructive. Despite very high mass interior walls and thick saltillo tile floors, the heat would build up into the high 80's around 4 pm. The sun doesn't let up until closer to 7:30 pm that time of year, so trying to stay in the great room was not fun til well into the evening. The sun gain is nice in the winter, though the room cools a bit much for my taste in the evening thanks to the window losses. The in floor radiant helped a lot, but I wasn't paying for the propane.

    When considering my own build I opted for minimizing southern window area and chose the lowest SHGC for all the windows. I found the lowest U factor windows I could afford and followed up with a total of 6 1/2" of outsulation on the walls and about 8" on the roof. The net result is a house that is slow to gain or lose heat if one is mindful. The total mass of the framing, flooring and drywall is essentially all inside the insulation envelope. The house is slow to stablize come fall mostly due to having to argue with my spouse about opening windows when it is 50F outside. Once settled in though, it is pretty even and annualized BTU usage is about 1.4BTU/sf/degree day (if I have done my calculations correctly).

    While I did not order a blower test, I feel confident that my endless fussing with air sealing details has paid off. A power outage a few winters back provided a taste of how long we could go before it chilled down to 55F. That was about 12-14 hours, a little more than a degree an hour. Window losses are still the killer.

    We have fake stucco on the walls which is not a significant heat store. The metal roofing is so far from interior surfaces that nothing shows on the inside. It does get very very hot in the summer as well as the sunny days in the winter. Snow melt on the roof is driven by the sun not interior heat loss through the roof deck. If you did chose the popular rusted metal corrugated siding, my perception would be that reflected heat might make patios or porches a bit dicey certain times of the day. A side note, the rust does stain everything below, slowly, but inevitably.

    A stone cladding might provide a buffering effect from the days heat, though with some risk of inducting a bit of heat during the night. I avoided stone cladding due to weight issues with thick exterior insulation. The fake stucco seems fine so far and doesn't keep much heat in it. Maybe a winter loss, but I designed more for summer. The heat lag effect in stone over certain thickness makes the storage value moot if placed over a sufficiently insulated wall.

    Hope this helps a little bit. There may not be much to reference about outside materials simply because architects choose based on other properties than the thermal capacity.

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