Fiberglass insulation, when properly installed, doesn’t move or settle so it maintains its R-value and does so for a long time. But when fiberglass insulation is compressed during installation, it loses its R-value.

R-value is important because it’s what insulation is identified and labeled by. If you have a 2×4 wall, then you buy insulation that’s listed for a 2×4 wall. If you buy insulation that’s listed for a 2×6 wall, then you’d have to cram it into your 2×4 wall. This may not seem like a big problem, but it is.

Fiberglass insulation needs some airflow in order to work correctly. Tiny air pockets inside the insulation basically trap air between the fibers and are what make the fiberglass insulation R-value effective. Compressing fiberglass insulation is counterproductive. It means you’re squeezing all the air out of it, and it needs that air in order to work how it’s made to work.

Something else you have to watch out for are gaps and holes. If there are any gaps or holes due to compression, then it’ll lose R-value. Let’s say you have a 5 percent insulation gap. That doesn’t seem so bad, right? Well actually it is because it means you can have up to a 25 percent drop in your R-value.

Here are the main issues with buying fiberglass insulation and then compressing it: you’re going to be paying more money and putting in more work but not receiving a greater benefit, and if you compress it too tightly then you can damage the area you’re using it for. None of which you want to deal with.

In order to make sure you’re getting the full R-value during pre-
installation and post-installation, you need to buy the correct size, make sure there are no gaps or holes and under no circumstances compress the fiberglass insulation. Do these things and you’ll receive the benefits guaranteed by your fiberglass insulation and its R-value.