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How to Insulate Rim Joists

Rim joists are the beams that run around the perimeter of your home above the foundation wall. You can see these joists lining the top of unfinished basement walls. If you live in an older home, it’s likely that your rim joists aren’t insulated. Learn why and how to add the proper type of insulation to this part of your home.

Why Insulate Rim Joists?

The only materials separating uninsulated rim joists from the outside air are sheathing and siding, which offer little protection from the elements. This is also the place where many building components come together, including the foundation wall, mudsill and subfloor. As a result, air leaks are common.

Rim joists are located above ground, so it makes sense to insulate them to the same level as first-floor walls. Expect insulated rim joists to improve home comfort and decrease heating and cooling costs.

Which Type of Insulation is Best?

It was once common practice to insulate rim joists with fiberglass batts, but things have changed. Air-permeable fiberglass does nothing to stop air and moisture from passing through the rim joists. This can cause condensation to form, leading to mold and wood rot.

To avoid these issues, you have two air-impermeable options to choose from: rigid foam insulation and spray foam insulation.

Insulate Rim Joists with Rigid Foam

This is the more affordable option of the two, but it requires a bit more labor. You must cut the sheets to fit awkward spaces, an especially tricky task anywhere wiring or pipe penetrations exist.

Whether you choose polyisocyanurate (polyiso), expanded polystyrene (EPS) or extruded polystyrene (XPS), two inches of rigid foam should be adequate for our warm Houston climate. To seal the rigid foam against air leaks, plan to apply caulk or canned spray foam around each piece.

Insulate Rim Joists with Spray Foam

While the cost is higher, it’s must faster and easier to insulate and air seal rim joists using spray foam. Thanks to our warm climate here in Houston, you can use either open- or closed-cell spray foam with great success. Two or three inches of thickness should provide sufficient insulation.

Leave Rim Joist Insulation Work to the Professionals

Installing rim joist insulation can be a tedious, messy job. For results you can count on, choose IBP Houston to insulate your rim joists with rigid foam or spray foam. To schedule services, please contact us today.

Insulation Products We Install
Radiant Barrier

We can help keep your home cooler and more comfortable with an attic radiant barrier.


Our team installs both batt and blown-in fiberglass insulation.


Environmentally friendly and effective, cellulose works well in homes and buildings.

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