Short #31 - U-Factor and R-Value

What is the difference between r-value and u-factor? Why should we care about the differences? In this short podcast, we'll explain what those differences between the two are and why you should care.

R-value and u-factor are actually pretty close to the same thing; they are inverse coefficients of the same phenomenon. R-value is the resistance to heat energy moving through conductance. R-value is not concerned with radiant gains, such as the sun's UV rays passing through a window; the heat gains occur strictly through conduction, molecule-to-molecule, like heat passing from the wall insulation to the actual wall upon contact. In terms of insulation, a higher r-value is desirable, Inversely, we like to see a lower u-factor.

The u-value is the coefficient of heat transfer. So, the r-value's resistance to heat acts directly against the heat transfer of the u-factor. You can convert the u-factor to r-value by dividing the u-factor into 1 (1/u-factor). Similarly, you can get your u-factor from your r-value by dividing the r-value into 1 (1/r-value).

We use these values in load calculations and plug them into Manual J programs. We figure out our BTUs per hour in an equation where we multiply the square feet by the u-factor and the delta t. So, our insulation plays into equipment sizing. Some products also have a rated u-factor. You also need to average out the u-factors if you use multiple materials. (Note: sometimes, manufacturer u-factor ratings are not entirely accurate.)

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