When insulating your home, the rim joists can sometimes be overlooked, but they could be an important part of your home’s energy-saving strategy. Uninsulated rim joists may be responsible for drawing cold air into your home, making it harder for your heating system to maintain a comfortable temperature.

Insulating your rim joists with rigid foam can help to air seal a property, improve comfort levels in the home, and is a pretty straightforward job. With a few basic tools and adequate access, you can start feeling the benefit in no time.

Here’s our comprehensive guide to insulating your rim joists with rigid foam board.

What Rigid Foam Board to Use

Whether you use Styrofoam, expanded polystyrene, or polyiso, it doesn’t really matter too much. Each has its own advantages, however, as polyiso and Styrofoam are more robust and less attractive to pests, they are perhaps the better choice in this location.

Use a minimum thickness of one inch. Two inches is better; however, it might be harder to cut around and shape for ductwork and other penetrations.

In some climate zones, it might be advisable to use more than one layer of foam board to achieve a required R-value. Always check local codes to ensure compliance, and to make sure you will get the maximum return for your efforts.

What is a Rim Joist?

The rim joist is what you will find on top of your foundation or basement wall. It will run right around the perimeter of your house between, and beside, your main floor joists. The floor may already be insulated up to it with fiberglass or other insulation. Very often the fiberglass is discolored where cold air has entered the space and brought dust and other contaminants with it.

Oftentimes, insects, rodents, or other pests may find their way into the space beneath your home via a rotted, or gappy section of rim joist. The first thing to do would be to clear out the space at the top of the foundation wall and check the rim joist is in good condition.

Make good any loose, damp, broken, or rotten sections of the rim joist before proceeding.

How Will I Know Whether my Rim Joist isn’t Air-sealed?

Up to 30% of cold air ingress into your living space can be attributed to the lack of insulation or air sealing to your rim joists.

There are three things to look for, dirt, damp, and spiders. Dirt will get drawn in on the air, moisture too, and spiders prefer spaces where there is air movement as it delivers their prey to them.

Check out the external corners too. Oftentimes you will find the biggest drafts, and spiderwebs here.

On the other hand, should your crawl space or basement be vented, it is important that any grilles or other ductwork are left open to enable proper ventilation, and prevent condensation, unless you intend to condition the space.

What Do I Need to Insulate My Rim Joists?


You don’t need to purchase any special tools, however, this is a list of what we recommend you use.

  • Tape measure-clear and easy to read. If yours is covered in rust spots, now’s the time to upgrade.
  • Craft knife or box-cutter.
  • A straight edge, and/or a carpenter’s square.
  • Hand saw and drywall saw.
  • Gloves and eye protection.
  • A step ladder that allows adequate access without over-reaching.


The exposed space between the main joists that is inhabited by the rim joist is not huge, but it does follow the entire perimeter of the house, so measure up carefully in order to calculate how much rigid foam board you will need.

To get that all-important air seal, and fix the board in place, you will need some low-rise spray foam.

Silicon caulk: Use this for sealing up the smallest joins and junctions between the rim joist, wall plate, and other timber sections.

How to Insulate the Rim Joists

First things first: Make sure the rim joists are clear of debris, clean and dry.

  1. Make sure you have clear access to the wall you are working on and that any items that cannot be moved are protected from debris or spills during the work.
  2. Using spray foam, fill any large openings such as redundant penetrations, where ductwork or other services have been installed, or where their access points have been altered, made bigger, or changed direction.
  3. Use caulk to fill any narrow gaps.
  4. While the spray foam and caulk are drying and hardening off you can start measuring up and cutting the rigid foam board to fit.
  5. Measure the width and height of the rim joist space and subtract a quarter inch.
  6. You will need to leave a 1/8th of an inch all around the rigid foam so there will be enough space for the spray foam to fix and seal it in place.
  7. Cut the individual rectangles of rigid foam and put them in place.
  8. Don’t spray foam them in until you have at least ten cut, and ready to be fixed, if possible.

Pro Tips

Make sure to insert the end of the spray foam nozzle at least half the depth of the foam board to get the depth of spray foam and seal required for the job to be effective.

Repeat a layer of foam board, if necessary, to achieve the desired R-value, but make sure the two layers are fitted as close together as possible. With a good air seal already in place you can simply glue a second layer onto the first and run a bead of caulk around the edge to finish.

To improve the R-value and sound deadening performance you could also glue rubber spacers between the panels. It will reduce heat and sound travel through conduction.

Local codes may require fire protection to be fitted over the insulation. This can be drywall, and, or fire-retardant mineral wool insulation. Always check to ensure compliance.

If a basement is conditioned and is to be used as part of the living space of the home, then enclosing the insulated rim joist with drywall by fixing boards to the entire ceiling would be adequate protection in most cases.

Using Foam Board With Batts

Flash and batt is the name sometimes given to the process whereby mineral wool or other insulation batts are used alongside rigid foam board to insulate a rim joist to a higher specification without breaking the bank.

It is sometimes used to describe a combination of foam board and spray foam, or fiberglass and spray foam too.

Rigid foam can be expensive, but it is also a very efficient insulator, and perfect for air-sealing rim joists.

However, in order to reach R-value requirements, sometimes it makes better economic sense to reach compliance by using a mix of materials.

Mineral wool works well with foam board. Fitting it to the interior, inside the vapor and air barrier created by the spray foam, is the preferred method.


Justin's been in construction for over 20 years in both new build and renovation. With experience in both commercial and residential construction, he specializes in healthier and more energy-efficient homes.

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