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And, here is a quick explanation of R value and U value...

R-value measures the resistance to heat flow of a material.
U-value measures heat conduction thru a material.

The formula for computing U-value is: Btu / (hr x degrees F x sqft)
The formula for computing R-value is: (hr x degrees F x sqft) / Btu

To illustrate a simple example, imagine that we have one sq foot of fiberglass insulation (thickness not important for this exercise).

We have a temp of 70° on one side of the material and 0° on the other, we can now find the U-value if we know how much energy it took to keep the 70° side at 70° for one hour.

If it took 3.68 BTU's to keep the 70° constant for that hour, then we have 3.68 / (1 hr x 70° x 1 sqft) = a U-value of .05257.

The same calculation for R-value would be (1 hr x 70° x 1 sqft) / 3.68 or an R-value of 19.02.

19.02 = 1 / .05257 or .05257 = 1 / 19.02 - Thus the inverse or reciprocal relationship between R-vale and U-value.

So why use U-value for windows and R-value for about everything else?

Walls are built to stop the outside from coming in while windows are added to allow the outside to come in. Walls stop heat and light - windows pass heat and light.

Really simplified - when a person builds a wall they want to know how much of the outside is going to stay outside - and when a person installs a window, they want to know how much of the outside the window is going to allow inside. Walls resist heat flow and R-value measures resistance to heat flow; U-value measures heat flow and windows pass heat.