Insulating and Air Sealing Basement Rim Joists: Cutting, Fitting and Sealing Foam Board in One Weekend

This is the part most guides skim, and it is the part most likely to matter if you ever sell the house, file an insurance claim or have an inspector in the basement.

Foam plastic insulation burns. Building codes across Canada deal with that through a long-standing article in Part 9, numbered 9.10.17.10 and titled Protection of Foamed Plastics. In the Ontario wording, foamed plastics that form part of a wall or ceiling assembly must be protected from the adjacent space in the building, with exceptions for concealed spaces inside attics, crawl spaces and wall assemblies, as shown in this reproduction of the Ontario article. An open basement where you do laundry and store the hockey bags is not a concealed space. That page reproduces an earlier edition and notes that Ontario's 2024 code is now in force, so use it to understand the rule and confirm current wording locally.

The federal homeowner guidance says the same thing in plainer language. When installed on interior surfaces, all plastic-based rigid board insulation must be covered with a fire-resistant material, typically 13 mm (1/2 in.) drywall, that is mechanically fastened to the building structure, according to Natural Resources Canada's materials chapter. The last phrase matters. The drywall is screwed to wood, not glued to the foam.

Why the rule exists

The reasoning is about time. Foam insulates so well that it does not soak up heat from a fire the way wood or drywall does, so a room lined with it heats up faster and reaches flashover sooner, which shortens the time people have to get out, as a Manitoba planning district bulletin explains. A layer of drywall buys that time back.

What counts as a covering

Local building departments publish their own lists, and they are fairly consistent. One Alberta municipality's guideline names 12.7 mm gypsum board, plywood, hardboard, fibreboard or OSB of an appropriate thickness, masonry, concrete, or a product that has passed the Canadian test standard for protective coverings, in the Town of High River's safety codes guidance. The same document notes that foam in a crawl space needs protection once the space is used for storage or as a service room, which shows how much the use of the room shapes the answer.

For a rim joist, the usual approach is a strip of 12.7 mm drywall cut to fit each bay and screwed to small wood cleats fastened to the sides of the joists. If you plan to finish the basement ceiling with drywall, that ceiling can do the covering for you, and the federal guide makes the same point about spray foam in this space.

Where the leave-it-exposed advice comes from

You may have read that rim joist foam can stay bare. That advice traces back to the United States, where the model residential code allows thin spray foam at rim joists to remain exposed. It is not a Canadian provision. The clearest illustration is a foil-faced polyiso board that is marketed for exposed use south of the border, whose own installation guide states that in Canada the board must be covered with a thermal barrier when exposed to the interior, as DuPont's Thermax guidelines spell out.

In researching this piece we did not find a rigid board with a published, blanket exemption for exposed use at rim joists in Canada. That does not mean no product has ever been accepted anywhere.

Most bays are empty and easy. A few will have something running through them, and those few deserve your slowest, most careful work.

Gas appliance vents

If a metal vent from a furnace or water heater passes through or beside a bay, do not pack foam against it. Canadian instructions for one common double-wall gas vent call for a 25 mm (1 in.) minimum air space to combustibles and say to keep building insulation out of that space, in Selkirk's Type B vent installation instructions. Foam board and canned foam are both combustible, so they stay outside that gap. The exact clearance for your vent is printed on its label, and other vent types, including single-wall pipe and wood stove chimneys, need considerably more room.

The gap around a vent still leaks air, so it still needs closing. The right materials are a collar of sheet metal and a non-combustible sealant, and high-temperature silicones are made for flue pipes, as Natural Resources Canada's air sealing chapter advises. Plastic vent pipes from high-efficiency furnaces run much cooler, and their own installation manuals say what may touch them.

Canned foam has a temperature ceiling of its own. One widely sold product is labelled as combustible once cured, with a limit of 116°C, and its maker says it should not be used around heaters, furnaces or anywhere it may touch heat-conducting surfaces, according to the Great Stuff Pro product sheet.

Gas piping, wiring and exhaust hoods

Where a gas line passes through the rim, seal the small gap around the pipe and leave everything else alone. Do not bury shutoff valves, unions or flexible connectors in foam, and do not lean on or shift the pipe while you work. The federal advice here is to be careful around electrical wires and gas pipes and not to disturb connections.

Electrical cables can be foamed around where they pass through the wood. Junction boxes need to stay accessible, so cut the board around them and keep their covers clear. Dryer and bathroom fan hoods should be sealed around the outside of the duct, with the damper left free to swing.

Here is the whole job in order. The times are our planning estimates for a first-timer working alone in a house with roughly 50 to 60 open joist bays plus the two end walls. Your basement may be quicker or slower, and obstructions such as ductwork add time.

A shopping list that matches the table

Most of what you need fits in one trip:

• Rigid foam board, 50 mm thick, in sheets you can carry down the stairs
• Low-expansion canned foam, with one more can than you think you need
• Flexible sealant for the sill joint, plus high-temperature silicone if you have a metal vent
• A long-blade utility knife or fine-toothed handsaw, a straightedge and a tape measure
• Nitrile gloves, safety glasses, long sleeves, a respirator and a drop cloth
• 12.7 mm drywall, short lengths of wood for cleats, and screws
• A moisture meter, borrowed or bought

How much to buy

With joists spaced 400 mm apart, you will have about two and a half bays for every metre of wall on the two sides where the joists run into the rim. A typical bay between 38 by 235 mm joists is roughly 370 mm wide and 235 mm tall. By our arithmetic, one 610 by 2,440 mm sheet gives 12 to 14 bay pieces once you allow for offcuts.

Canned foam is harder to predict. One manufacturer gives a theoretical figure of about 238 metres of 3/8-inch bead from a 340 g can and cautions that real yields vary with conditions, in GE's technical data sheet for its gap-filling foam. Filling a 10 mm gap the full depth of a 50 mm board takes several times more foam than that thin bead, so we would plan on 12 to 15 bays per can and keep the receipt for any you do not open. Treat both numbers as estimates.

Saturday and Sunday

Saturday morning is for inspecting, clearing and cleaning. After lunch, seal the sill joint all the way around, then measure and cut. Numbering each piece and its bay with a marker saves a surprising amount of walking back and forth.

Sunday morning is fitting and foaming. Work one wall at a time. Set a board in place, hold it against the rim board, and run the foam around all four sides so that it bridges from the board to the wood. Let the foam do the holding. The two walls where joists run parallel to the foundation are handled as one long strip each, cut into lengths you can manoeuvre, and they are often the tightest reach in the basement.

Pick a cold, windy day for your final check. Run the back of your hand slowly along each wall at the top of the foundation. A bay that still feels cool and breezy has a gap in its bead, and a short burst of foam fixes it. Take photos of every wall before any covering goes on. They cost nothing, and they answer questions from a future buyer, inspector or contractor.

Will the house be too tight now?

It is a fair question, and the answer for most houses is reassuring. Most older houses are so loosely built that enough air still comes in for ventilation even after extensive air sealing, according to Natural Resources Canada's overview of how a house works. The same guidance adds two sensible cautions for homes that have had a lot of tightening done: indoor humidity can rise, and fuel-burning appliances that draw their air from the house have less to draw on. If windows start fogging more than they used to, or if you have an older furnace or water heater with a draft hood, mention the work to your heating contractor at the next service visit.

Tying it into the rest of the basement

If insulating the basement walls is on your list, the rim joist work you have just done becomes the top edge of that project. New-construction energy rules treat this band as part of the wall's air barrier, and a British Columbia government guide describes the goal as making all joints at the rim joist assembly airtight. When the wall insulation goes in, its air and vapour control layers should meet your foam so there is no gap between the two.

With the bottom of the house sealed, the top is the natural next project.

Our overview of attic insulation upgrade options is a good place to start planning it.

You have now dealt with a part of the house that most people never look at. The floor along the outside walls should feel less drafty, the basement should feel calmer on a windy night, and you know exactly what is behind the foam because you checked it yourself.