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Re: [Phys-l] Insulation R-value



On Feb 17, 2006, at 9:43 AM, Dwight K. Souder wrote:

Greetings everyone. I have a student who recently heard of homes being
built with Styrofoam walls and another home with miniature wheat/straw
bails. He's wanting to do a project to measure their R-value (Resistance to
heat flow) of different materials. He's done some research on it, but
hasn't come up with how the R-value is determined. I think I once saw that
the R = ((delta)Fahrenheit x ft^2 x hours)/Btu...is this correct? The
formula doesn't look right. Has anyone done a lab on this or knows how it
is determined?

The book I am looking into first defines the heat conductivity , k. It is used to calculate H ( energy lost per square meter and per second). By definition,

H=k*dt/d

where d is the wall thickness and dt is the temperature difference on its two sides. Thus the SI unit of k is J*m/C. The R of a material is the same thing as k but in British units (BTU*ft/F). The conversion factor is 6.93. I have never measured k but the definition suggests what could be done.

a) A box containing a controllable electric source of heat is placed in air whose temperature is constant.
b) Use the heater to establish the desired dt, for example, 30 C. Make sure it remains constant (by adjusting the wattage of the inner heater).
c) Measure that wattage, P. Suppose P=10 W. Then you know that 10 J of heat is lost per second. Divide this by the area of walls to get H.
d) Calculate k (or R) from the known values of H, dt and d.

Ludwik Kowalski
Let the perfect not be the enemy of the good.