Our Green Home: Effective Passive Solar Heating

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Like I said before, I am designing a new house for my partner and me. Along with deciding on a floor plan, we had to place the south facing windows for passive solar heating.

There is living space along the south side of the 40’x24’ building. Of course, we went wild and lined the whole wall with floor to almost-ceiling windows! As I imagined this space in my mind, I KNEW this was too much glazing. I envisioned us roasting like turkeys in November.

Optimum passive solar heating

For the best passive solar heating, a home should have its long axis running east-west. The south wall should be oriented to true south, which is not magnetic south. I’m not going to go into how to find true south, but this helpful website will show you many, simple options.

The best orientation is no more than 5% from true south. This allows the most winter sun to reach and heat the house. You will get effective solar gain with orientation as much as 15-30% off true south. However, the house could overheat, because the glazing will be facing the low-lying sun. In a situation like this, it’s better to have southeast glazing than southwest, since the morning sun is much cooler than afternoon sun.

I retrofitted my current house with a passive solar greenhouse. It faces due southwest, because that was the only place I could put it. I have curtains closed in summer, because the hot afternoon sun hits it directly. It does a great job of catching late day sun in winter, and it does heat the house, but in summer, I have to be especially careful that there is not so much sun that the house gets hot, but that there is enough for the plants I grow.

Glazing and thermal mass

Windows in a conventional house make up about 3% of the square footage. In a passive solar home, there are two options. A sun-tempered house has glazing equal to 7% of the square footage, and there is no thermal mass. This is a small investment to lower heating bills a bit.

The better option is to increase the glass to 12% and add thermal mass. This is called direct gain. Thermal mass is any material that will absorb heat and radiate it back to the room when the temperature drops after dark. This can be concrete, adobe, water, brick or stone. Glazing combined with thermal mass creates a very effective heating system, lowers utility bills and conserves natural resources.

Working out the square foot to direct gain ratio for passive solar heat

Our house is 1024 square feet. To sun-temper it (7% of the square footage) would mean placing four 3’x6’ windows along the 40’ long south side. When I showed this to Daniel, it was clear he wanted more sun than that. ‘Is that all the windows we can have?!’ he asked in disbelief. We did the math for direct gain (12% of the square footage) and came up with seven 3’x6’ windows. When I drew them into the AutoCAD design, he seemed relieved.

The thermal mass is a concrete slab floor, and two adobe walls in the kitchen and dining area. Sun does not have to hit the mass directly. It soaks up ambient heat. The floor is the main mass, but adding more can’t hurt. It can get expensive, though, so you have to plan carefully. I have seen fireplaces and water features as thermal mass, so you can get creative!

Rough plan of my passive solar heated home

This is a rough draft showing the windows. The mass has not been added yet.

Click image to view larger version in new window

Along with passive solar heating, we will probably have solar radiant floor heat. I’ll be talking to a solar installer about this soon. PV is also a consideration, but we need to crunch some numbers first. The next thing we are going to think about, though, is water catchment and the most effective roof designs. Stay tuned!

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Nan Fischer

Nan Fischer has been living and building green for over 35 years. Nan’s emphasis on the BuildDirect blog is about how to make your dollar stretch further, while also moving toward a more sustainable lifestyle, as well as upcoming and existing technology to help us live in an ecologically-friendly way. Nan also authors posts on the website of her seed business, sweetly seeds.