Installing fiberglass insulation is a great project for a do-it-yourselfer. Fiberglass has a reputation for being itchy and dusty, but there are new formulations that make it easier to work with and more comfortable to use.
Our motto for installing fiberglass is, “Fluff – Don’t Stuff.”
If you’ve ever opened a tightly bundled package of fiberglass insulation you know first hand how much it can be compressed. But it doesn’t insulate as well when it’s compressed. You need it to stay fluffy – with all those little pockets of air inside it – to get the maximum insulating value.
So always pick the right thickness of fiberglass to perfectly fill the area you’re trying to insulate. And there’s probably a perfectly sized “batt” available for whatever space you need to fill.
Most exterior wall cavities are either 3-1/2” thick (if they’re built with 2x4 studs) or 5-1/2” thick (if they’re built with 2x6’s).
The cavities are usually 14-1/2 inches wide and 92” high. Fiberglass batts come pre-cut to 15” wide and 93” high, so they’ll fit snugly into a standard wall cavity.
Wall insulation is often available with a paper vapor barrier on the inside surface. We’ll explain about vapor barriers later.
We prefer to use “unfaced” batts and apply a continuous plastic vapor barrier once all the insulating is finished.
Because fiberglass is so fluffy, you need to compress it in order to cut it. We usually keep 2-foot scrap of lumber with us when we’re insulating, but we’ve also been know to use a 2-foot level.
When we need to cut a batt to length (say, for under a window) we lay the batt on the floor, lay the level across the batt at the desired length, press down on the level to compress the fiberglass as much as we can, and then run a sharp utility knife along the side of the level.
First bit of advice: if the room has a concrete floor, find a of scrap wood to put UNDER the fiberglass when you cut it. Running your knife across a concrete floor will dull the blade very quickly.
Second bit of advice: don’t go crazy trying to cut fiberglass to an exact dimension. Within half an inch will probably be just fine.
Check out the video for other tips for cutting fiberglass.