How many south facing windows for a 100% passive solar home?


How many south facing windows for a 100% passive solar home?

Asked by Christopher Lund

Dear pros, I am currently in the designing stages of my home. My design is going to be as close as I can to a 100% passive solar home. The home will be around 1400 sqft at an approximate 20' depth. All flooring, internal walls, counter tops ect. will be thermal mass. Intended 2-3' solid (or potential) Insulation space at the "attic." I have 90% south exposure. The 10% that I don't are from mountains on either the extreme East and West Sides. Altitude is 9500 ft at 38º parallel. What I am attempting to figure out is, do I glaze the entire south side. Or something less so. Formulas will be wondrous! Even if I need to write a program to perform the calculations. Thank you!

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Polly Osborne, FAIA, LEED AP's picture

Dear Christopher,

Where are you located? To figure these calculations, more is needed than knowing that you are 38º parallel and at 9500ft.

  • Completely glazing the Southern exposure could drive you out of your house for the summer even in in some fairly cold locations!
  • Keep in mind what you can do to maintain a comfortable home with subtleties of light and warmth as well as achieving passive solar success.

A handy free of charge calculator that will do virtually all your calculations for you is Murray Milne's HEED available from UCLA. If you really get to know this free calculator, you can figure your passive solar heating, thermal mass, ventilation, night flushing and daylighting needs.

Probably less glass than you'd think

In many locations, the calculations will reveal that far less South facing glass should be used than what people originally think might be the case.

Also, when you determine whether heating or cooling are the ruling factor, you'll want to decide on a window system that enhances the performance of your calculations.

For instance, depending on your climate, you'll need to decide whether the low-E coating on multiple glazing is on the inside of the outside panel of glass (hot climates) or the outside of the inner pane (cold climates). Kind of a tongue twister there!

It's nice that you have such an excellent exposure, but you'll also want to be sure you plant or shade all that lovely glass. One of the reasons the solar homes of the sixties and seventies fell out of favor was the excessive and unshaded South facing glass that was used in the badly designed houses.


I'd also study whether you will want to bounce light deep into your space with light shelves.

  • As well as heating, the more natural light you can get to permeate your house, the less energy you'll use.
  • The trick here is to try and do that without too many skylights or West and North facing glass, which generally work against you in heating and cooling.

Consider working with a mechanical engineer

Another option to get you going is to find a mechanical engineer who is dedicated to sustainable principles and innovative in the understanding of solar design.

Often such a green professional can figure in a short time for a low cost a great deal of useful design data.

I hope this helps.

For more information:

Read "We want to design a passive-solar home for a hot climate. Can we still have views to the east?", a Q&A answered by Greg Upwall.

UCLA has more design tools here.