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Q.What happens to the R-value when you stuff R-19 insulation into a 2x4 stud cavity?

A.Mike Lacher, Technical Services Manager in the Insulation Group at Certainteed Corp., responds: What happens is that as you compress fiberglass batt insulation, the R-value per inch goes up, but the overall R-value goes down. For every x% that you compress the material, you lose approximately 1/2 x% of R-value. Refer to the chart below to see how this affects common sizes of insulation. This chart refers specifically to Certainteed products, but can be used for other similar fiberglass batt insulation products.

Compressed R-Values for Certainteed Building Insulation
Nominal Lumber Size>Depth of CavityR-15 3 1/2"R-19 6 1/4"R-21 5 1/2"R-22 6 1/2"R-25 8"R-30 8 1/4"R-30 10"R-38 10"R-38 12"
2x43 1/2"15141515    
2x65 1/2" 182120202221  
2x87 1/4" 19 222427263028
2x109 1/4"    2530293633
2x1211 1/4      303837