In just a couple of hours, you
can seal and insulate your rim
joists, which are a major source of
heat loss in many homes. Properly insulating
and air-sealing rim joists takes
patience, so most builders simply stuff in
some fiberglass and walk away.
Figure A Insulated rim joist
Airtight insulation reduces heat
loss through the rim joist.
Fiberglass insulation and expanding
foam seal the open top of
hollow concrete blocks.
If you have an unfinished basement,
you can properly insulate the rim joists in
two or three hours. (This will also block
tiny passages where spiders and other
insects enter your basement!) The materials
will cost about $1 per foot of rim joist.
Call your local building inspections
department before you begin this project. The inspector may require you to
cover the new insulation with drywall
(as a fire block) or leave some areas
uncovered to allow for termite inspections.
You can insulate second-floor rim
joists following the same steps shown here
if you happen to tear out a ceiling during
remodeling.
Rigid foam is the best insulation for rim
joists. We chose 2-in.-thick (R-10) “extruded
polystyrene” ($20 per 4 x 8-ft. sheet).
Don’t use “expanded polystyrene,” which
is a less effective air and moisture barrier.
Cut the foam into 8-ft.-long strips 1/8
in. less than the height of the rim joist. A table saw is the fastest way to “rip” these
strips, but you can also use a circular saw.
Then cut the strips to length to fit between
the joists, again cutting them 1/8 in. short
(Photo 1). A heavy-duty box cutter ($6) is
the best knife for making short cuts and
trimming foam; the long blade slices
cleanly through the foam (a utility knife
blade is too short). Use long sections of
foam to cover the rim joists that are parallel
to the floor joists (Photo 2). Don’t
worry about cutting the foam for a tight
fit around pipes, cables or other obstructions;
you can seal large gaps with expanding
foam sealant later.
It’s important to create an airtight
seal around each section of foam
using caulk or expanding foam
(Photo 2). Otherwise, moist inside air
could condense on the cold rim joist.
The resulting dampness can lead to
mold and rot. If you have a solid concrete
foundation, also run a bead of
caulk where the sill plate meets the
concrete. If you have a concrete block
foundation, also seal the openings on
top with expanding foam. Stuff a wad
of fiberglass insulation into each
opening to support the foam as it
hardens (see Figure A).
Click Image to enlarge.
1. Cut rigid foam insulation into
strips with a table saw or a
circular saw. Cut the strips to fit
between floor joists using a box
cutter.
2. Run a bead of acrylic caulk around
each section of foam to form an airtight
barrier. Fill gaps larger than 1/4 in.
with expanding foam sealant.
