Filling stud spaces with fiberglass
batts is the cheapest, easiest way for you to insulate new walls. It’s also the
best way to upgrade wall insulation during remodeling. Installing the batts
doesn’t require any special skills, but it’s slow, tedious, itchy work. It’s
often done poorly, and even small gaps can reduce efficiency as much as 25
percent. In this article, we’ll show you how to cut and fit fiberglass batts
and how to work around electrical outlets and cables to get the best job with
the least hassle.
Fill All Voids
The key to a quality insulating job is
tight-fitting batts that completely fill the stud cavity with no voids or gaps.
You can do top-quality work with only a few basic tools. You’ll need a utility
knife with a good supply of sharp blades, a tape measure and a straightedge,
and a 3- or 4-in. putty knife for stuffing insulation around doors and windows.
Fiberglass can irritate your throat and skin, so wear protective gear. Buy a
two-strap mask rated for fiberglass insulation (3M No. 8210 is one example) and
wear a hat, gloves, a long-sleeve shirt and goggles to keep fibers out of your
eyes.
Click image to enlarge.
Push batts all the way to the back of each stud
space and then pull out the front edges until they’re flush with the face of
the studs.
Split batts to fit around electrical cables.
Tear the batt in half, starting from the bottom. Slide one half behind the
cable and lay the other half over the top.
Fit Batts Tightly Around Electrical Cables and
Boxes
Running a full batt in
front of electrical cables leaves an uninsulated space behind. Avoid this by
splitting the batt as shown. Then when you come to an electrical box, trim the
insulation to fit snugly around it. Run your knife blade against the outside of
the box to guide the cut. But don’t cut too deep or you risk nicking the wires.
If you have plumbing pipes on an outside wall, insulate behind them, but leave
the side facing the interior uncovered to allow heat from the house to keep the
pipes warm.
Split and cut batts to fit behind and around
electrical boxes. Slide half the batt behind the box. Then cut the front half
to fit tightly around the box.
Fit First, Then Cut to Length
We’re using unfaced batts that are sized to friction-fit
into standard stud spaces (either 16-in. or 24-in. on-center studs). They’re
also available precut to lengths that fit standard 8-ft. and 9-ft. walls.
Buying precut batts eliminates some work, but you’ll still have to cut some
batts to length. You could measure the space and cut the batt to fit, but a
quicker method that’s just as accurate is shown below. Leave an extra 1/2 in.
of length for a snug fit.
We’re using unfaced
batts because they’re easier to cut and install. In most climates, you’ll have
to staple 4-mil plastic sheeting over the batts to form a vapor barrier. Check
with your local building inspector for the recommended practice in your
area.
Click image to enlarge.
Cut batts to length by setting the top of the
batt into the space and cutting against the bottom plate with a sharp utility
knife. Leave an extra 1/2 in. of length for a tighter
fit.
Trim Batts in Place
Accurate cutting is essential (actually, slightly
oversized batts are best). A batt cut too small leaves gaps and one cut too
large bunches up and leaves voids.
The photos show two
methods of cutting batts to width. If you’re having trouble getting an accurate
cut with the “eyeballing” technique, measure the width of the stud space and
use the straightedge method instead. Add about 1/2 in. to the width to ensure a
tight fit. It’s better to compress the batts a little than to leave gaps. Don’t
worry if the batts bulge out a bit. The drywall will compress them
tightly.
Leave the batt folded in half and hold one edge
against the edge of the stud. Slice down the length while holding the top of
the batt. Cut against the stud face.
Alternate method: Press a straightedge down on
the batt at the desired width and use it as a guide for the utility
knife.
Fill Gaps Around Windows and Doors
The shim space around
windows and doors is a prime spot for air leakage. Stop these leaks by reaching
to the back of this space with the straw-type nozzle included with a can of
expanding foam insulation and applying a bead around the perimeter. Let it cure
at least an hour before stuffing the remaining space with a thin strip of
fiberglass. Don’t pack the fiberglass too tight or it will bow the jambs and
cause trouble with the operation of the window.
Stuff skinny strips of batting into spaces
around windows and doors with a 3-in. wide putty knife. The insulation should
fit snugly, but don’t pack it.
