insulating window shades in a passive solar house

leeelsonApril 19, 2006

We're building a passive solar house with 18- 7' high south facing windows. Energy-10 calculations suggest that leaving the windows uncovered at night has a big effect on heat loss.

We plan to use cellular shades to insulate all the windows, but have the dilemma of what to do when we are not home. The windows on the south side must be raised and lowered each day. We'd like to use motorized shades with a timer, but find they are very expensive (~$700/window + wiring and controls). Does anyone have any experience with these? Are there better solutions?


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Thanks for the input.

Ours is a passive solar house, so just burning a little fuel is what we want to avoid. Not that we're zealots, but... ;-)

When I did an Energy-10 calculation with shades up, I found that south windows were a net heat *loss*, i.e. a *very* big effect.

I was impressed with your URL. Quite a nifty system. I'd be willing to try it, but the wife is against "experimenting" with her dream house. The main problem I see is that one would still have to anticipate the situation when the system must be in place.

Looks like we'll be installing an automated motorized system if we can find it.

    Bookmark   April 25, 2006 at 11:53AM
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Home Depot has/had a relatively economical motorized cellular shade system available. It's battery powered, so no wiring, but it can be wired if changing batteries 18' up is a problem. The same thing can be found on the web as well.

BUT cellular shades don't seal against air leakage well enough! Does Energy-10 allow you to input an infiltration rate?

The main problem with air leakage through and around cellulars isn't the heat loss, but the moisture condensation. If you get a close-fitting cellular (even with a vapor barrier like mylar) it prevents heat loss just fine. But if no heat is reaching the clear glass window, that means that window will reach dewpoint every night in the winter. That coupled with an imperfect air seal makes for drippy windows. (been there done that and got mold as well.) This is all for 35%-50% indoor RH.

So it's a tough problem with the only existing solution being those magnetic-strip sealing window quilts. Unfortunately, they haven't been automated.

My wife also tries to discourage experimentation, but she loves our 1 yr old passive dreamhouse. It's a lot like Phil's house at

    Bookmark   May 21, 2006 at 5:47AM
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Hi Kevin,
I think the condensation problem depends a lot on the window and on your inside humidity level.
We have cellulars that have "energy tracks" along the side and appear to seal quite well. We almost never get any condensation on the windows (perhaps once or twice in a full winter).
We do live in a dry area (SW MT).


    Bookmark   May 23, 2006 at 9:30AM
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You're right, it's not much of a problem here in the West unless you have the humidifier going, which we do. We even run it all summer as well since the swamp cooler isn't on all the time.

Also, the "energy tracks" you mention above are a better design than the "c" channel style I tested. But even if you had something airtight, water vapor can diffuse right thru most common honeycomb cellular shades.

    Bookmark   May 25, 2006 at 9:43AM
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