Thursday, November 12, 2009

Solar house design

I've been thinking about building a house. Ideally it will be cheap, modest, super energy efficient, comfortable, beautiful, and low maintenance.

My friend Andy loaned me a book called The Passive Solar House, by James Kachadorian. There are a bunch of worksheets in the back, and I turned them in a Google Spreadsheet. (I accidentally drank caffeinated tea in the evening, so I'm up late.)

Then I plugged in data for my family and my house, and found a lot of bugs. Fixed the bugs, then fixed some more, and found some interesting data.


I can stay comfy in winter for ~1 cord of wood per year. That's dry, seasoned hardwood in a woodstove. A masonry stove will be more efficient with that wood.

This area doesn't get that cold (good) but doesn't supply much solar energy (bad). Having lots of big windows helps, but not enormously.

Most of the heat loss is through windows (especially at night) or from replacing air.


The house design I'm using is built in to a hillside. The north side is fully bermed. The east and west sides are bermed part-way, but leave space for some egress windows and a doorway.

The south side is mostly windows. Just how much is a variable I'm playing with. Some web sites recommend floor-to-ceiling windows, which is expensive, but creates an in-home greenhouse, which can be very beautiful.

The book points out that windows may allow a lot of solar gain in the summer, but then lose a lot at night. In many climates, it's a wash: adding windows only increases temperature fluctuation. Then you need to add thermal mass to compensate. Glass insulation ranges from R-1 to R-3, which isn't much.

The design is 48' x 32', for 1500 sq. ft. That's more than we really need, but if I'm going to build a house in the ground, I figure I should design a little extra room. Size can matter a lot, so I will ponder these numbers carefully.

In my calculations I'm ignoring 2 areas of thermal mass. The design includes large planters in the greenhouse area. The moist soil can hold a lot of heat. The walls may be built out of concrete block, filled with dirt or concrete, which will add a lot more thermal mass. I haven't accounted for either of these factors, but I should.

I'm calculating wall and ceiling insulation at code requirements, approx R-20 and R-30 respectively. Glass is double-paned, which is R-2 (ouch!). Both can be improved, for greater cost.

The book recommends replacing 2/3rds of the interior air every hour, to keep it healthy and fresh. I don't know hot that compares to conventional homes, but it seems like a lot.

The calculations assume 72 degF as the ideal internal temperature. I'd be happy with 68, but I can't figure out how to work that in yet, but I will.

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