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Re: Heat loss question



Will Stewart <will1000@ix.netcom.com> wrote:

>...The basic formula for heat transfer is;
>
>Q = -kA(T1-T2)/X
>
>Where
>    Q is the heat transfer rate
>    k is the thermal conductivity
>    T1 is the inside temp
>    T2 is the outside temp
>    X is the thickness of the insulating material

It looks like A must be the surface area of the wall, in ft^2, and perhaps X
would be measured in inches (?) Using a negative sign in the equation seems
an odd convention, for houses. Sorta makes sense, tho. Heat out of the house
is minus, and heat in, from an oil burner, the sun, etc., is plus.

>To apply this formula, one just needs some thermal conductivity values for
>various materials, as the following in Btu/hr*ft*degreeF

Seems like these are in Btu/hr-ft^2-F-INCH of thickness, not as above.
Then again, perhaps they are per foot... That would make sense for most
of the numbers in your left column below...
 
I've added some numbers to the right, for thermal conductivities, per inch,
from the ASHRAE Handbook of Fundamentals and other sources.

>Cork                .o4         0.29 
>Glass wool(fine)    .31 avg     0.32 (close, per INCH...)
>Masonry Brick       .38         5.0
>Glass              ~.45         31   (air films make insulation)
>Rock Wool           .017        0.25

Nick