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Are replacement windows ever worth it? or What's wrong with my math? - Page 2

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Posted by Eeyore on May 12, 2008, 1:46 am

Kitep wrote:

Good Luck with it anyway whichever route you take.

One other nice thing about double glazing is they you're relatively unlikely to
get condensation (and possibly resultant mould) from condensation on the window

Also look to fix any excessive draughts you may have. Note that it's important
that your home needs some air circulation (so don't make it 'hermetically
sealed') or humidity will increase to unpleasant levels which may then (read
WILL) also lead to internal condensation.


Posted by J. Clarke on May 12, 2008, 12:09 pm
Kitep wrote:

Won't help--they'll just replace the broken glass, not the entire
window, unless it's made in such a way that the glass is not

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(was jclarke at eye bee em dot net)

Posted by Bruce Harvey on May 13, 2008, 12:16 am

Also in my experience the insurance companies will only replace 'new for
old' or 'like for like'. So if your single glazed window gets broken the
insurer will only pay for a single glaze pane of the same size plus fitting.
At least thats how they work 'down under' where I live. You may be able to
have the upgrade done at the same time but you will still have to pay the
difference. Just a thought.

Posted by Ecnerwal on May 12, 2008, 12:46 pm

In point of fact, it's good to run a (necessarily ballpark) calculation
on the whole house, broken down into components. That lets you see where
the heat and $ are going, which makes it easier to figure out what it
will make sense to spend money on. Since you are already in the as-built
phase, you can also see if the ballpark estimate lines up with actual
use, and adjust the estimate to be more accurate. A spreadsheet works
well for this sort of thing. Use fields and calculations so that you can
enter all the things you might change, such as furnace efficiency.

RESFEN from LBNL is another approach to this analysis, though I find
myself unhappy with some of its built-in assumptions. But it does try to
calculate the effect of sun on the windows, which the simplistic
approach based only on R-values and degree days does not. That can make
quite a difference in some situations.


As for your original question - it almost never makes pure heating
payback sense to replace windows in good condition, which is why buying
good ones when you need to buy them anyway is important. Caulking the
old ones does make sense, and replacing the old ones because you are
dissatisfied with them aesthetically (or they are not in good condition)
does make sense, if you can afford that for that reason.

While your calculations don't include rising fuel prices, they also
don't include (as hard-eyed accountants do) the value of your $44
invested and providing you income, which can make your payback look even

One figure that is hard to track down (though you might get an insight
if your utility or  an efficiency organization offers a blower door
test) that will make a difference is air infiltration, or ventilation
rate. When cold air leaks in and hot air leaks out, that costs you
additional heating $. You need some air exchange, but old houses often
have far more than is advisable, and an "overly tight" house can save a
lot of heat $ by doing its air exchange through an air-air heat

In "British" units the British mostly don't use anymore:

Figure volume of house (conditioned space) in cubic feet.

Divide by 13.9 to get pounds of air.

Multiply by the estimated or measured air changes per hour. Start with
1.0 for a typical older house if estimating.

Multiply by 0.24 (the specific heat of air).

This gives the BTU/HR*DegF

If going by degree days (HDD and CDD) multiply by 24 (day) and the
appropriate degree day figure to get yearly figures.

Add that to the radiant heat loss (simpler calculation you already
grasp, if you can add up square feet of similar R-values) and see if it
tallies with your utility bill, though domestic hot water,
refrigeration, and lights will all have an impact, depending on how your
energy sources are distributed in supplying your needs.

Replacing one window does not make a typical house all that much
tighter. But reducing air infiltration by caulking, sealing, and making
minor repairs is the quickest payback for most houses. Additional
insulation in an unfinished attic space is typically the next - it's a
large area (unlike a window), insulation is cheap (unlike a window), and
adding it to an unfinished attic is low labor cost. Depending on your
water heater and hot water pipes present state of insulation, adding
insulation to those is also a typical good payback project. If your
electricity is expensive, refrigerator replacement can make sense, if
the new fridge is chosen for high efficiency.

Cats, coffee, chocolate...vices to live by

Posted by Kitep on May 12, 2008, 11:42 pm

Thanks. I bookmarked the page.  I'll figure out air-exchange later this

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