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Re: energy-efficient windows

This is my second posting of this message because in six hours it has
failed to appear in my mail or in the phys-l archive.

I don't think too much of argon fill because it leaks away in 5 to 7
years. Therefore I do not view argon fill as a long term investment.
However, if you buy low-e double or triple-glazed windows, you might
automatically get argon fill.

When selecting low-e glass, there are roughly three types of coating to
consider... (1) high solar gain, (2) moderate solar gain, (3) low solar

(1) High solar gain allows a higher percentage of solar infrared to
enter. This would be good in a climate where you figure the heating
season is your most expensive season, and you want to get as much solar
energy into your house as possible on a sunny winter day.

(3) Low solar gain allows a lower percentage of solar infrared to enter.
This would be good in a climate where you figure summer cooling is your
most expensive season, and you want to block as much solar energy as
possible on a sunny summer day.

(2) Moderate solar gain is intermediate and is best in a climate where
your heating and cooling costs are both significant.

In Ohio (where I live), and in Utah (from where the original question
was asked) the heating cost dominates, so the recommendation is (1) high
solar gain low-e coatings if you used double-glazed windows. In a
typical Ohio home, this is estimated to save 7% per year in heating and
cooling costs. In Utah this is estimated to save 8% per year. These
percentages assume you are comparing high solar gain low-e double-glazed
windows to double-glazed windows with ordinary glass.

In both Ohio and Utah the best combination is predicted to be
triple-glazed windows with moderate-gain low-e glass. The savings with
these windows compared to double-glazed plain glass would be about 10%
in Ohio and 19% in Utah. I'm surprised by that difference. I suppose
it is caused by low temperatures coupled with more sunshine in Utah.

These numbers can give you a bit of an idea what the pay-back periods
might be. In my case I have double-glazed plain glass. My
heating/cooling cost for last year was about $1400 for my 3000
square-foot house. If I could save 10% per year by replacing my
windows, I would save about $140 per year. I live in a Victorian house
with lots of tall/narrow windows. One estimate for replacing all these
was about $6000. That would give me a payback of 43 years. I decided
not to do it.

There are some helpful websites for getting these kinds of numbers. One
I found helpful was...

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817