Replacement of single-paned patio windows in a family room addition
My house includes a cheaply built flat-roofed family-room addition, which was constructed about 45 years ago. Its dimensions are 30 ft. x 16.5 ft. The exterior side faces SW and contains two 12 ft. single pane patio windows. This side also contains an overhang, which limits direct sunlight entering the windows during the summer months. The shorter NW side contains a 10 ft. patio window. Radiant heating is installed in the concrete slab floor. I would like to rebuild this structure with a pitched roof and new 6 ft. 8 inch tall windows covering about 27 ft. on the SW side, 9 ft. on the NW side, and about 6 ft. on the protruding SE side. We are located in Lafayette, California.
To optimize thermal efficiency, I would like to know what benefits would accrue from the use of double paned or triple paned windows, and other specifications relating to window design and performance, i.e., what reduction in heating and air conditioning costs might I expect? Also, what degree of overhang would be optimum in curtailing overheating during the summer months, while permitting passive solar heating during the winter months? I am also interested in the selection of materials for the window frames, and potential manufacturers.
I would be most appreciative in getting some guidance, especially from those with expertise using software to perform the needed calculations, and who are familiar with climatic conditions east of San Francisco Bay.
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The windows facing southwest are likely to result in lots of solar heat gain on summer afternoons (since roof overhangs don't work very well as the sun sinks in the west). I'm sure that you have experienced overheating in this room on summer afternoons, so you probably know what I'm talking about.
The theory behind sizing roof overhangs is to aim for your south-facing windows to be fully shaded at noon on the summer solstice, but fully exposed to the sun at noon on the winter solstice. If you understand how the sun moves in the sky, you'll realize that this design guide is imperfect. You'll still get sun when you don't want it, and you'll find that the windows are shaded when you wouldn't mind a little heat.
While this approach (sort of) works for south-facing windows, it definitely won't work for windows facing southwest, because you are up against low-angle sun.
Briefly: Choose low-solar gain glazing for the windows facing southwest, and consider reducing the glazing area. You may want to hire an architect to help you.
For more information on glazing, see this article: All About Glazing Options.
-- Martin Holladay
Thank you so much, Martin, for your helpful suggestions, especially the reference to your article, which I read, and others you have written. Reference to RESFEN5 (and thus RESFEN6) was particularly valuable, as it will now allow me to make some semi-quantitative, if not quantitative estimates of the consequences of window replacement in my area.
As part of the permit process in California you will need to do an energy calculation commonly referred to title 24. The computer program models the building. You can search online of a company that can do that or even better find someone locally. The online provider usually charges about $250.00 for the required permit documents. Once the building is modeled the windows can be changed and dialed in to what you want.