Q.What happens to the R-value when you stuff R-19 insulation into a 2×4 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 Cavity | R-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″ |
| 2×4 | 3 1/2″ | 15 | 14 | 15 | 15 | |||||
| 2×6 | 5 1/2″ | 18 | 21 | 20 | 20 | 22 | 21 | |||
| 2×8 | 7 1/4″ | 19 | 22 | 24 | 27 | 26 | 30 | 28 | ||
| 2×10 | 9 1/4″ | 25 | 30 | 29 | 36 | 33 | ||||
| 2×12 | 11 1/4 | 30 | 38 | 37 |