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re: payback period on retrofit vinyl windows
26 apr 2003
j f  wrote:

>i'm looking into buying retrofit vinyl windows.
>
>i presently have single pane aluminum frame windows.
>
>my annual heating bill is only $250 since i live in
>a mild climate.  cooling would be about $50 annually.
>
>so how much would i save going from r-1 (u=1) to r-3...

>so for a single story, 2000 square feet house, what percentage 
>would i save?

you might have about 8% of the floorspace as windows, say 160 ft^2.
changing from r1 to r3 would reduce the house thermal conductance by
160-160/3 = 107 btu/h-f. it might also lower the air leakage (good)
and solar gain (bad.) 

>i've got about 6 inches of cellulose (r=3.5/inch) for
>a total attic insulation of r-21.

so the ceiling contributes about 2000/21 = 95 btu/h-f...

>walls are 2x4 studs with fiberglass (r=3.2/inch) giving
>about r-12.8 plus r-1 for drywall giving about r-14.

maybe 1400ft^2/r14 = 100 for walls, or less, with the studs.

>maybe nick pine can figure out the rest.

maybe 1 house volume air change per hour, ie about 2000x8/60 = 268 btu/h-f
for 268 cfm of natural air leakage. (ashrae recommends 15 cfm/occupant.)
g = 160+95+100+268 = 623 btu/h-f with the old windows and 516 (17% less) with
the new ones. if you make the house more airtight and your electrical usage
stays the same, you might see more than a 17% savings with the new windows.

>oh well, i just found some information that windows account
>for about 25% of heat loss approx, so if i go from r-1 to r-3,
>it might save 16% of the $300 annual bill = $48 /year.

not much.

>since it costs about $5,000 to do the whole house, 
>$48/year is about a 1% yield.

you might improve the payback by leaving the old windows in place and adding
oversize storm windows with sliding screens on the outside, esp below and to
the sides of windows on the south wall. windows look dark from a distance, so
you might paint the wall between the old and new windows dark for aesthetics
and to collect solar heat and warm the inner window in wintertime, making a
"parasitic heater," as in "khanh's radically new approach... " described in
bill shurcliff's 1979 book new inventions in low-cost solar heating. an extra
square foot of south facing window might save you $1/year in heating cost,
or more, with some passive plastic film dampers. 

nick




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