Re: How is the insulation strength or R-value of a material calculated?

I am using Physics by John Cutnell and Kenneth Johnson (John Wiley & Sons, 5th edition, 2001) as my reference, but there are actually lots of places to find this information.

The heat flow through a slab of a material is calculated using the equation
Q = k A DT t / L
where Q is the heat flow, k is the thermal conductivity of the material, A is the surface area through which the heat flows, DT is the temperature difference across the slab, t is the time interval, and L is the thickness of the slab. In the SI unit system, Q is in Joules, k has units of Joules per second per meter per Celcius degree, A is in square meters, DT is Celcius degrees, t is seconds, and L is meters. The units for the British units of heat are BTU, k is in BTU per hour per foot per Fahrenheit degree, A is in square feet, DT is in Fahrenheit degrees, and L is in feet.

If we rearrange the equation by dividing through by time and taking k to the denominator we get
Q/t = A DT / (L/k) = A DT / R
which is convenient because Q/t is the power (energy per unit time). The term L/k is what engineers call the R value, and it is typically given in British units. Obviously we want larger values of R for good insulation, so we want large L and small k. If a slab is made of composite materials in layers, the individual R values are added to derive a composite R value.

Rearranging the equation again to get L/k alone on the left side we get
R = L/k = A DT t / Q
so R values can either be calculated by knowing the thickness and the thermal conductivity of a material, or they can be measured (and/or verified) by measuring the heat energy flow in a given amount of time with a measured temperature difference for a slab of known dimensions .

John Link, MadSci Physicist