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Basement Rim Joist Insulation in Ontario: The Leakiest Three Feet of the House, Spray Foam vs Cut-and-Cobble Foam Board, Cost per Linear Foot and the Minimum Charge Problem
Insulation & Air Sealing
The Rim Joist: One Band of Wood Sitting on Cold Concrete, Open to the Weather on Three Sides, and Usually Stuffed With a Loose Batt
Go down to the basement, look up at the top of the foundation wall, and you are looking at the leakiest part of a wood-frame house. The rim joist is a 1.5 in. board on edge, R-2 or so, sitting on a sill plate that sits on concrete, with the floor joists butting into it every 16 in. and the outside air on the other side of the sheathing. Each joist bay is a small box with three cold sides, and the batt somebody pushed in there in 1988 is doing nothing except hiding the frost. Two inches of closed cell spray foam fixes it, $5.50 a linear foot on an average house, and the catch is that nobody sends a truck out for the rim alone without a minimum charge. Here is what it costs, what the alternative is, and how to get it done without paying for the truck twice.
Why the rim leaks more than the wall
The foundation wall is one piece of concrete with foam against it. The rim is dozens of pieces of lumber, nailed together on a sill plate that was never quite flat, on concrete that was never quite level. The sill gasket, if there is one, is a strip of foam that was flattened the day the house was framed. Every joist end, every sheathing joint, every anchor bolt and every hole a plumber or electrician drilled through the rim is an air path, and the stack effect in winter pulls cold air in at the bottom of the house through exactly these holes. Put your hand along the top of the foundation wall on a January night and you will find them.
A batt in that bay is a wool sweater in the wind. Air goes through it, condenses on the cold rim behind it, and the rim stays wet half the winter, which is why the sheathing behind a batt-insulated rim is often black. The fix has to be an air seal, not a filter, and that means foam. The air sealing article on this site goes through the whole house; the rim is the basement end of it.
The numbers
$5.502 in. closed cell, per linear foot of rim
$2.08of that is foam; the rest is prep and crawling
$900 to $1,800rim alone on a house, minimum charge
R-12from 2 in. of closed cell, and an air seal
The arithmetic behind the $5.50: a 10 in. rim at 2 in. deep is 1.67 board feet per linear foot, and at $1.25 a board foot for closed cell that is $2.08 of foam. The other $3.42 is the mask over the duct and the wires, pulling the old batts out and bagging them, foaming the top of the wall and the sill, and the hours spent on a creeper under a 7 ft ceiling. The range is $4 to $7: the low end is a clear, dry, open rim on an unfinished basement; the high end has a furnace, a water heater, a dryer duct and forty years of shelving in the way. A 36×28 house is 128 lf, $704.
And then the minimum. A spray foam crew is two men, a truck and a heated rig, and it is not worth their day to drive out for 128 lf of rim and 214 board feet of foam. Rim alone, expect $900 to $1,800 regardless of what the per-foot arithmetic says. That is not gouging; the rig costs more to warm up than the foam is worth. The spray foam cost calculator shows the minimum beside the board feet so you can see where the line is.
Spray foam vs cut-and-cobble
| 2 in. closed cell spray foam | Cut-and-cobble foam board | |
|---|---|---|
| What it is | Foam sprayed into each bay over the rim, the sill and the top of the concrete, 2 in. thick | 2 in. XPS or EPS cut to fit each bay, pushed in, every edge sealed with canned foam or acoustical sealant |
| Air seal | Complete. It sticks to everything and fills the gaps at the sill and the sheathing joints. | Only as good as the sealant bead on four edges of every piece. Sixty bays, 240 edges, one missed and that bay leaks. |
| R-value | R-12 at 2 in., R-18 at 3 in. | R-10 at 2 in. of XPS, less at the edges |
| Cost | $4 to $7 per lf, subject to the minimum | Material is the small part. A carpenter at a day and a half for 128 lf lands about where the spray foam does, without the minimum, and without the air seal. |
| Obstructions | Masked and sprayed around; a few minutes a bay | Every pipe and wire is a notch; a bay with a duct in it gets skipped or botched |
| Where it makes sense | Any basement, and any job where the crew is already coming for the walls | A handy owner on a weekend, an ICF house with a wood rim and no other foam work, or the dozen bays a furnace install exposed |
Fibreglass batts are not on the table because they are not an answer. If the bays already have batts, they come out, and a wet or black one tells you the rim has been condensing for years.
Getting it done without paying for the truck twice
- Bundle the rim with the basement walls. Assembly B, 3 in. on the walls and 2 in. at the rim, is one visit, one setup, one masking. The rim is the cheapest line on that invoice. The basement cost page runs it.
- Or bundle it with the attic. Cathedral sections, a knee wall, the attic hatch surround and the rim can be one foam visit. A foam contractor doing the attic air sealing is already there.
- Clear the rim before they come. The $7 end of the range is the shelving, the paint cans and the Christmas boxes against the wall. An hour of your own time is $200 of theirs.
- Pull the old batts yourself if they are dry and clean. Bag them, and photograph any bay that is black or damp before the foam covers it.
- Backing in open bays. A cantilevered floor, a bay over a porch, the ends of an I-joist floor: the foam needs cardboard or OSB to shoot against, $2.00 a square foot, and it is extra.
The thermal barrier, and the two things that must not be foamed
Spray foam and foam board burn, and they burn fast with black smoke. The OBC in 9.10.17 wants a thermal barrier over foam in a living space, which is 1/2 in. drywall or an intumescent coating at $1.25 to $2.00 a square foot. In an unfinished basement the rim is the one place most people leave foam exposed, and the honest answer is that it needs the coating or a drywall strip before the basement is finished and lived in; a mechanical room with a furnace a few feet from bare foam is the case an inspector will not let go. Write the barrier into the quote and the argument never happens.
The two bays that get skipped: the one with the chimney or the B-vent through it, which keeps its 3 in. (75 mm) clearance with mineral wool packed around a sheet-metal collar, never foam; and any bay where knob-and-tube wiring runs through the rim, which gets no insulation of any kind touching it under the Ontario Electrical Safety Code until the wiring is replaced. A foam installer who sprays over either one has told you what the rest of his work looks like.
On the houses I build, the rim gets the same closed cell as the walls, and on an ICF wall it is the only foam job in the basement, because the wall itself is the insulation; the ICF basement comparison shows what that saves.
Questions people ask
How much does it cost to spray foam a rim joist?
$4 to $7 a linear foot, $5.50 on an average house, for 2 in. of closed cell including the sill and the top of the wall. A 128 lf house is $704, but rim alone trips a minimum charge of $900 to $1,800 because the crew has to mobilize a truck and a rig. Bundle it with the walls or the attic.
Can I insulate the rim joist with foam board myself?
Yes. 2 in. XPS cut to each bay with every edge sealed with canned foam. It is slow, it is only as tight as the worst edge, and a bay with a duct in it will defeat you, but on a dry, open rim it is a real improvement over a batt and costs a weekend.
Is fibreglass OK in a rim joist?
No. Air goes through it and condenses on the cold rim behind it. Black sheathing behind a rim batt is the usual find. Pull it, foam the bay.
Does spray foam at the rim need drywall over it?
In a living space, yes: a thermal barrier of 1/2 in. drywall or an intumescent coating under OBC 9.10.17. In an unfinished basement it is the one spot people leave bare, and it should be coated before the basement is finished, especially near the furnace.
What R-value does a rim joist need?
The rim is part of the above-grade wall in the code, so R-22 nominal or the effective equivalent is the target; 2 in. of closed cell at R-12 with the air seal does more for the house than an R-22 batt with none. 3 in. at R-18 if the crew is there anyway.
See where the minimum charge lands
The spray foam cost calculator prices the rim by the linear foot and shows the minimum beside it, then lets you add the walls, the cantilever or the knee wall to see when the per-foot price takes over. If you have a quote for “rim joist insulation” that does not say the thickness, the $99 review will.
Sources and further reading
- Ontario’s Building Code: the official page with the current regulation; vapour barriers are 9.25, attic ventilation 9.19, thermal barriers over foam 9.10.17, radon 9.13.
- CUFCA Quality Assurance Program: Canadian Urethane Foam Contractors Association: certified installers with photo ID, licensed contractors, daily work sheets and the CAN/ULC S705 standards a spray foam job is held to.
- Electrical Safety Authority: Ontario: knob-and-tube wiring, the inspection rules, and why thermal insulation cannot be placed around it.
