Hybrid Car – More Fun with Less Gas

Are replacement windows ever worth it? or What's wrong with my math? - Page 4

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Posted by Jeff on May 13, 2008, 3:31 am
daestrom wrote:

   I'm sure this varies, but I wonder how people set their thermostats.
It would seem reasonable that if you feel a cold spot that you would
compensate by turning up the heat. Likewise, a blast of solar sunshine
heating may prompt you to adjust the AC down.

So, factoring in these little intangible comfort factors may have
outsized influences.

   Drapes or some kind of window treatment could have a dramatic effect..

   In my old house adding an extra layer of (vinyl) glazing and covering
up the window ACs made a big difference in the heat I was using. I felt
more comfortable at a lower setting.


Posted by daestrom on May 12, 2008, 10:08 pm

Your numbers look about right.  I think replacement windows can save you
more if there is a lot of air leakage around/through them.  Somewhere I read
that a 'typical' house loses half of its energy via air-exchange.  Your
calculations are only looking and conduction through the window.  If the old
windows are loose, poorly sealed and drafty, you can see a much higher
savings.  So the 'hype' may be exploiting that, but without measurements,
it's just guesswork.

But if you have good windows that seal well and are already double-glazed,
the savings to go to triple glaze is pretty small and usually not worth it.

After the obvious insulation in attic and walls, I'd opt for a 'blower-door'
test and start looking for air infiltration/exfiltration.  This *may* lead
you to the windows, but it may lead elsewhere.

P.S.  Of course cooling costs are the price of electricity, not natural gas.
But between the COP of air-conditioner and the difference in cost of
electric energy versus natural gas, it comes out close.

Posted by Kitep on May 12, 2008, 11:29 pm

Thanks.  That's pretty much the conclusion I've come to.  My windows don't
seem to be drafty, but a blower-door test may show otherwise.

I wonder why I was thinking my AC ran on NG.  I've had to replace the fuse
for it, so I should've known better.

Posted by mike on May 12, 2008, 10:38 pm
 Kitep wrote:

You've left out the time value of money.
You have to borrow money (or forgo investing it) to pay for the windows.
Payback keeps getting worser and worser.

Return address is VALID!
Bunch-O-Stuff Forsale Here:

Posted by Paul M. Eldridge on May 12, 2008, 11:43 pm

Hi Mike,

I'm not so sure.  I expect the cost of energy to exceed borrowing
costs and normal rates of return for the foreseeable future.  As noted
before, wholesale natural gas prices have more than doubled in the
past year alone (currently $1.32 per MM BTU, up from $.38 last
August).  Given how things stand with regards to natural gas, there's
an excellent chance the payback will be *a lot* sooner than any of us


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