Double-glazing only tiny gain in San Francisco?
Am I missing something on the "value" of even double glazing in San Francisco? We run about 3000 degF-days (heating) here, and don't really have A/C. Ignoring infiltration losses, what could better glazing material gain me?
Single pane glass -- 1.2 BTU/sq.ft./hr/degF
1.2 * 9 sq.ft. * 3000 degF-days * 24 hr/day = 777,600 BTU 8 therms * $1.25/therm (gas) ~ $10/year per 3'x3' Victorian pane at 100% efficiency.
Now, I'll admit, the heater isn't going to be 100% effective, but we also set-back to 60 or lower at night and the heat loss even through low-E glass isn't zero.
I'm guessing the savings are well under $1/sq.ft. for our climate, making the additional expense of double-glazed windows (at least for thermal purposes) very hard to justify.
Am I missing something, or do those numbers sound right?