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re: home insulation - plan of attack?
20 dec 1995
mike andresen wrote:
>i live in a single level 1700 ft2 home in phoenix.
with 1700 ft^2 of ceiling and 1700 ft^2 of walls, as you said,
and perhaps 200 ft^2 of windows?
>i have r=11 in the
>ceiling, nothing in the block walls (r=3) and single pane windows (r=0.9).
let's see, if it's 68 f inside and 32 outside, how much heat is flowing through
each part of the house each day? roughly:
before after
ceiling: 24(68-32)1700/r11 = 134 k btu ---------------> 48 k
walls: 24(68-32)1500/r3 = 432 k btu (wow) 68 k
windows: 24(68-32)200/r.9 = 192 k btu 86 k
air infiltration: 24(68-32)1700x8'x2/55 = 214 k btu/day 112 k
total 972 k btu 314 k
how about putting 4" of dri-vit on the outside of the block walls, or the
water-based equivalent system? that would leave the thermal mass of the block
inside the house, and increase its time constant to about 3 days, so you
could solar heat this phoenix house with windows or some sort of low-thermal-
mass sunspace... you might put some holes in the tops and bottoms of the
walls inside the house to increase their heat transfer area. this would cost
about $2/ft^2 for the materials, and make the walls about r19 instead of r3,
reducing the heat loss to 68 k btu/day, for a net savings of about 100
btu/32f day/$ materials invested. you might make it look like adobe, carving
and painting some little brown foam poles to stick out of the eaves.
another layer of glazing might cost another $/ft^2, making the windows r2,
and reducing their loss by about half, to save about 500 btu/$.
>i am wondering if it is worth while to add another insulation layer in the
>attic to make r=22 up there when the walls and windows are less than r=3?
i think these things are only related by marginal economics. adding some
25 cent/ft^2 r-19 fiberglass in the attic would reduce its loss to 48k btu
saving about 200 btu/$. adding another r11 would cost about 15 cents/ft^2,
saving 250 btu/$. some 15 cent/ft^2 foil, 4' wide, would help more in the
summer, but not so much in the winter.
and plugging up holes in the house to reduce air filtration from 2 air changes
per hour to 1 is labor intensive but rewarding. $20 worth of caulk might save
112k btu, ie 5000 btu/$. this becomes the largest heat leak, after the house
is better insulated.
if you do all these things, you can probably heat your house 100% with a low-
thermal-mass sunspace with about 300 ft^2 of glazing. say, a $400 commercial
plastic film greenhouse, 8' tall, extending 36' along the south wall, with
fresh red tomatoes and basil growing inside:-)
nick
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