Friday, November 03, 2006

Passive Solar in the Northern Tier

Recently, an email discussion group that I moderate and prod here in Wisconsin had a raucous discussion on Passive Solar.

Passive solar homes seem to be getting some interest again. But the examples are always in the sunny-winter parts of the country.. not cloudy-winter parts of the country. That gets my ire up. Also, the general public think - "ahh, passive solar how cool, I will use it to heat my home" (with no other heat source). Up goes my ire again.

So after our raucous discussion here is what we came up with. First, passive solar will only met about 20% to 30% of a home's space heating needs. Second, a passive solar house needs to be "super insulated." (Yes, it is time to use that 7-'s phrase again.) But this time that super insulated home needs to have (third) a mechanical ducted fresh air ventilation system (with a air to air heat exchanger).

Oh yeah, and for passive to work, your need either really good, high R-value windows or really good window shades that are used religiously. And the windows need to have a high solar heat gain coefficient

And to quote one discussion group member:
"Thermotech, a Canadian window, is the only one I know that uses Libby Owens Ford Energy Advantage II hard coat low-e, U-0.3, SHGC 0.49 (whole window). So, with 100sf of south glazing here, a house would have a net gain of 4 MMBtu/yr."
"5-9% of floor area in south facing glass is an inexpensive way to get 20-25% of heat energy needs. On a 2,500sf house, that's 175sf, an ordinary looking amount of window."
The Germans have basically figured this out. They call it the "Passive House" they dropped the "Solar" (we should too). Google them; they are online.

Their passive house is built with whatever thermal shell that is needed to reduce the building's heating load to 10-watts/square meter. A house built to this standard in Northern Minnesota has R-70 walls and a R-100 ceiling with German made triple pane windows.

The German's passive house ends up meeting it space heating needs with 1/3 passive solar, 1/3 internal heat gain (people, appliances, cooking etc.), and 1/3 some heating source (they seem to dig pellet stoves). But the heating load is a small fraction of the average home - thanks to all the insulation.
So what we need now are decent high R-value and reasonably priced windows that allow the sun's warmth to enter a building. Time for some market transformation.

I am interested in reading more of your blog. We have just moved into a passive solar bermed home in Maine (long way from being done). In the few weeks since then, we are very happy - but have a lot to learn about living in this type of home.
If you are interested check us out:
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