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Double-glazing only tiny gain in San Francisco?

Posted by sfjeff (My Page) on
Mon, Oct 19, 09 at 19:45

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

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?

Follow-Up Postings:

RE: Double-glazing only tiny gain in San Francisco?

So your math is telling you that only $10 per year of fuel is going out each window in your home.

How many windows in your home?

Average heating bill?

RE: Double-glazing only tiny gain in San Francisco?

My math tells me that something like $10 a pane ($20 a window, as they are double hung) goes through the glass. What leaks around the sashes (as well as other infiltration and conductive losses) is another question! Maybe 30 "panes" worth of glass area throughout the house, so $300 a year.

Total gas bill is around $100-200 a month (including "summer"), which includes heat, hot water, and gas range. Based on that, I'll back-of-the-envelope $500-750 a year for heating. It all seems to fit together. I'm just surprised that it's so low.

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