Hey everyone! I have some R21 fiberglass insulation that's meant for 2x6 framing, but I only have a 2x4 framing left to insulate. I understand that fiberglass shouldn't be compressed since it loses R-value when it's squished down. If I were to compress the R21 insulation into a 2x4 frame, would the R-value drop below what's typically expected for a 2x4, like R15? Or is it possible it would just fall to an effective R18 or something similar?
2 Answers
Honestly, I wouldn’t recommend compressing the R21 insulation. The R-value is based on thickness, and if you shove it into a 2x4 frame, you're definitely going to lose some performance. If you're trimming it down to fit, just shave off the excess rather than compressing. You might want to check the packaging for comparison between R21 and R15 to see how much R-value you can expect after trimming.
If you can, just return the R21 and grab some R15 instead. It's designed for that frame size, and you won't have to worry about messing with compression or anything like that.
I actually got the R21 for free off Facebook. That’s why I didn’t think about returning it!
Thanks for clarifying! So if I trim it down to 3.5 inches, the R21 should stay at that rating?