Elizabeth writes from Sacramento, California: "I hired a contractor to insulate and sheetrock the garage in my 1960s home. The contract specified R-13 in the walls (2x4 stud) and R-19 in the ceiling (2x8 joists). He made a mistake and put R-19 in the walls. He completed the sheetrock before I discovered the mistake. I know that if you compress insulation, you lose R-value. From the compression chart on the Owens Corning website, it appears that R-19 compressed into a 2x4 cavity will achieve R-13. Is this correct? I noticed that the insulation seemed bumpy and not smooth and straight. By packing insulation so tightly, will the unevenness cause even further loss of R-value? Or, will the insulation even out over time? Do I need to worry about moisture problems, since there is little airflow? What about buckling or warping of the drywall or outside siding, due to too much pressure from the insulation?"

Answer: You are correct! The resulting R-value from compressing R-19 fiberglass insulation into a 2 x 4 stud wall will be R-13. Without seeing the installation it is hard to say whether the R 19 insulation will exert enough pressure on the sheetrock or siding to cause damage but I don't expect any and you should have no further loss of R-value. In addition, compression does not contribute to moisture problems. Generally, airflow is not necessary in walls. We do recommend the use of Kraft- faced insulation. You may need to confirm with the installer that material with a Kraft vapor retarder was installed with the Kraft facing installed toward the warm-in-winter side of the wall.