What Is The "r" Value?
Years ago we assumed that the best measure of insulation was thickness. Times and products have changed. This rule-of-thumb is no longer reliable.Insulation is now manufactured and sold by "resistance value*' (called the "R" value) — a precise measurement of the insulation's resistance to heat transfer. The higher the resistance value, the less heat will escape through the insulating material. The following chart shows the resistance value of some better-known building materials. Notice that one inch of polyure-thane, rated at 5.88 R value, heads the list.
One brand of insulation may be thicker or thinner than another, but if they both show the same R value, they'll perform equally well. The R value is stamped in large letters on the cover of packaged insulation. If it isn't there, check with your dealer who should know the value.
Calculations using the R value
Let's say you decide to check the R value of the insulation in the walls of your home. Suppose it is
has retained its full insulating properties over the years). Let's say you want to double this R value to 20. Looking at the table, you have 3 choices:
(1) a 3" layer of batt-type insulation
(2) 2 1/2" of cellulose fibre loose fill (with an R value of 4.00 per inch)
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May 19, 2012