Is R-13 x 2 greater than R-21 (for a 2×6 wall)?
Keith92705
| Posted in Energy Efficiency and Durability on
Two layers of R13 batt insulation is cheaper than one layer of R21 batt (for a 2×6 wall). Can I place 2 layers of R13 in the wall and achieve a greater R value than one layer of R21?
One other note: Unfortunately the R13 is faced. Would two faced layers cause a problem?
Location-Southern California-92705
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Keith,
Two layers of R-13 batts are likely to have an R-value of about R-20 when compressed into a 5.5-inch-deep stud cavity. In other words, the R-value will be very close to, or slightly less than, the R-value of R-21 batts.
Frankly, when it comes to R-1 differences, it's safe to say that the quality of the installation job will make more of a difference than the batt choice in this case. Choose either type of batt, and strive to do a very careful job of installation, without voids.
Compressing a fiberglass batt reduces the R-value of the batt but increases the R-value per inch of insulation. (See attached image, below.)
Having two layers of kraft facing wouldn't cause any problems. But if the kraft facing bothers you, it can easily be peeled off and discarded.
.