04 / HIDDEN LEAKS
The best rim joist air sealing products, and how much foam the job actually takes
It is the longest continuous air leak in most houses and almost nobody looks at it, because it is above head height in a basement behind the stuff you put down there.
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Seal the rim joist with canned foam sealant at the seams: where the sill plate meets the foundation, where the rim board meets the subfloor, and at each joist. Use a gaps-and-cracks formulation published to 1 in for almost all of it, and a big-gap can only for voids you can see are wider.
What a rim joist is, and why it leaks so much
Stand in an unfinished basement and look up at the wall where the concrete stops. On top of the foundation there is a horizontal board laid flat, the sill plate. On top of that, running around the outside, is a board on edge closing off the ends of the floor joists. That board is the rim joist, sometimes called the rim board or the band joist.
What makes it matter is not the board, it is the seams. There is a joint where the sill plate sits on the concrete, a joint where the rim board sits on the sill plate, a joint where the subfloor lands on the rim board, and a joint at each end of every joist. Those seams run around the entire perimeter of the house, on every floor, and they were made by carpenters fitting timber to concrete rather than by anyone trying to make an airtight assembly.
So it is not one leak of any size. It is a very long, thin leak that happens to be continuous, and continuous is what makes the total large. This is why it turns up near the top of most serious air-sealing lists while remaining almost invisible to the people living above it.
Sealing is not insulating, and the order matters
The thing most often found in a basement is fiberglass batts pushed into the joist bays, and the thing most often reported afterwards is that the basement is still cold. The reason is that those are two different jobs solving two different problems.
Insulation slows heat conducting through an assembly. Air sealing stops air physically moving through gaps in it. Fiberglass is good at the first and, because it is air-permeable, close to useless at the second. A batt in front of an unsealed seam has cold outside air moving through it, which also means it is not doing much of the job it is good at either.
Seal first, then insulate. That order is the single most useful thing on this page, and it is also the cheaper way round, because a few cans of foam is a much smaller purchase than a basement's worth of board.
Match the part to your gap
Measure at the widest point. Then read across. If your measurement sits on a boundary, take the larger band — a seal that is slightly too big compresses, and one that is too small does nothing at all.
| Your gap | Part type | Our pick | Price |
|---|---|---|---|
| Ordinary framing seams, up to 1 in | Gaps and cracks foam, 12 oz reclosable |  Great Stuff Smart Dispenser Gaps and Cracks Insulating Foam Sealant, 12 ozThe default can, and the reclosable dispenser survives a job done over two weekends. | $9.59 · CHECK ON AMAZON |
| A whole basement perimeter | Gaps and cracks foam, 20 oz |  Great Stuff 157911 Insulating Foam Sealant, 20 oz, IvoryTwo-thirds more foam per can. Work out your linear feet before deciding how many. | $16.99 · CHECK ON AMAZON |
| Voids from 1 in to 3 in | Big gap filler foam |  Great Stuff Smart Dispenser Big Gap Filler Insulating Foam SealantFor beam pockets and out-of-square bays only. Wrong, and harmful, in a narrow seam. | $9.98 · CHECK ON AMAZON |
| Same job, whichever is on the shelf | Gaps and cracks foam, 12 fl oz |  Loctite Tite Foam Gaps and Cracks Insulating Foam Sealant, 12 fl ozThe other brand at the same point in the lineup, usually cheaper per can. | $6.28 · CHECK ON AMAZON |
| Sill plate line in a basement with mice | Pest-deterrent foam |  Great Stuff Smart Dispenser Pestblock Insulating Foam Sealant, GrayThe air seal for a pest-prone spot. Mesh goes in first; foam is not rodent-proof. | $10.97 · CHECK ON AMAZON |
Working out how much foam to buy
This is where the job goes wrong logistically, because a rim joist is measured in length rather than in area, and the length adds up fast. Do the arithmetic before ordering rather than halfway through a Saturday.
- 1Measure the outside perimeter of the house. That is the run of the sill plate seam, and the run of the subfloor seam, so double it straight away.
- 2Count the joist bays along that perimeter. Each one has two vertical seams where the joist end meets the rim board.
- 3Add the penetrations: every pipe, cable, dryer duct and hose bib that comes through the rim board is its own small job.
- 4Add the things you will find once you are up there, because you will. Old holes, a gap where a beam pockets into the foundation, and at least one place a previous owner improvised.
That total is why the 20 oz can is on this page. A perimeter job is not a two-can job on most houses, and swapping cans mid-run means more clogged straws and more wasted foam sitting in a discarded nozzle.
Doing it without making a mess you regret
Canned foam is unforgiving in one specific way: cured, it is extremely difficult to remove from anything it should not be on, including skin, hair, clothing and the finished side of a floor joist. Wet, it wipes off. That asymmetry should shape how you work.
- Wear disposable gloves, eye protection and clothes you have written off. Foam goes on overhead here, which means it drips.
- Ventilate, and read the can. These are solvent-based products used in a confined space with poor air movement, which is the worst combination this site recommends anything in.
- A bead, not a fill. Foam expands, and the job is to seal a seam rather than to fill a bay. Overfilling is the main cause of both mess and foam pushing where it should not.
- Work in sections and come back. Foam skins over quickly but keeps expanding, so judging your bead by what it looks like at the moment you lay it is how you end up trimming all afternoon.
- Have the trimming tool ready before you start. Cured foam trims flush with a serrated blade, and doing it the same day is far easier than doing it next month.
The rim joist is also how mice get in
The sill plate line is one of the routes rodents use, for the same reason it leaks air: it is a long seam between two materials that do not meet perfectly, at exactly the height a mouse can reach from the ground outside. If you have mice in the basement, the air sealing job and the exclusion job are happening in the same place at the same time.
The important thing is the order. Foam does not stop a rodent; a mouse will chew cured foam if there is a reason on the other side. What works is packing the gap with a metal fill fabric or hardware cloth first, then foaming over it. The mesh stops the animal, the foam stops the air, and neither does the other's job. The same principle applies at the door, and the parts are covered in best door sweeps to keep mice out.
What we would skip, and why
Nobody in this category ever declines to recommend something. That is what makes universal praise worth ignoring. Here is what we would not buy.
Stuffing fiberglass batts into the joist bays and calling it sealed
This is the commonest thing found in an unfinished basement and it is the reason so many people have insulated rim joists that still leak. Our reasoning, stated as reasoning: fiberglass is air-permeable by construction, so air moves through it rather than being stopped by it. A batt pushed into a joist bay slows heat moving through the assembly and does very little about air moving through the seams around it. Air sealing and insulating are two different jobs, and the seal has to happen first or the insulation is working in a breeze.
Buying one 12 oz can for a whole house
A rim joist is not a repair, it is a perimeter, and the arithmetic surprises people. Every joist bay has a seam at the bottom where the rim board meets the sill plate, a seam at the top where it meets the subfloor, and two vertical seams at the joists. On a modest house that is well over a hundred linear feet of bead before you have touched a penetration. Work out the length first, buy the cans together, and expect to use more than the listing photographs suggest.
Using big gap filler everywhere because it fills more
The high-expansion formulation is published for gaps up to 3 in and it is genuinely the right product there. In a narrow seam it is the wrong one, and not merely wasteful: foam that keeps expanding in a space that cannot take it pushes against the framing around it rather than compressing. Match the can to the gap. Most of a rim joist is an ordinary seam that wants the standard formulation, with the big-gap can kept for the two or three places you can see it is needed.
Trusting foam to keep mice out
The sill plate line is a rodent route as well as an air leak, and foam alone does not close it, whatever additive is in the can. A mouse will chew cured foam. Where there is an active route, the fix is a metal fill fabric or hardware cloth packed into the gap first, with foam over the top to seal the air path. The pest-deterrent can here is the air seal in a pest-prone spot, not the exclusion itself, and the entry says so.
Foam, caulk, or backer rod
Canned foam is the right tool for the rim joist specifically because the seams are irregular, often wide, frequently overhead and rarely straight. It is not automatically the right tool everywhere else in the house, and using it everywhere is how people end up with foam squeezing out of trim in the living room.
For a clean, narrow, visible joint, caulk is neater and paintable. For a gap too wide to caulk sensibly, backer rod goes in first and caulk over it. That three-way decision is worked through in spray foam vs caulk vs backer rod, which is worth reading before you carry a can of foam upstairs.
And if you have not yet confirmed the rim joist is where your problem is, start with how to find drafts in your house. A cold floor in particular has more than one possible cause, and cold air from under the house covers the crawl space version of the same complaint.
Complete the kit
The parts that decide whether the seal you just bought actually holds. Skip these and you will be doing the job again in March.
The corners a sweep cannot reach
Xcluder Rodent Control Fill Fabric, Large DIY Kit
Stainless steel wool blended with poly fiber, pushed into the gaps at the door corners that every sweep leaves open.
Deep gaps before caulking
Frost King C21CP-C Caulk Saver Backer Rod, 3/8 in x 350 ft
Backer rod: the foam you push into a deep gap so the caulk over it is a seal rather than a bottomless hole.
Removing old weatherstrip adhesive
Goo Gone Pro-Power Adhesive Remover Spray, 16 fl oz, with Scraper
The step everybody skips: the new tape will not stick to the residue the old tape left behind.
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How we picked
- We rank on published gap capacity first, because that is the one number that separates these cans functionally. Gaps and cracks formulations publish up to 1 in; the big-gap formulation publishes up to 3 in, and the picks are placed against the seams a rim joist actually has.
- Can size against the real length of the job second. A perimeter job is measured in linear feet, so we treat a 20 oz can as a different proposition from a 12 oz one rather than a better one.
- Dispenser type third. A reclosable dispenser is worth real money on a job worked along a wall over several sessions, and we say which picks have one.
- No maker here publishes an air-leakage figure for a bead of foam, and we publish none either. We did not test these. Prices come from the live price layer and are dated; the full method is published.
We have not tested these products and we do not claim to have. The full method, including what we refuse to claim, is on how we pick.
Questions people actually ask
- What is the best way to air seal a rim joist?
- Canned foam sealant along the seams: where the sill plate meets the foundation, where the rim board meets the subfloor, and at each joist end. Use a gaps-and-cracks formulation published for gaps up to 1 in for nearly all of it, keeping a high-expansion big-gap can for the few voids that are visibly wider than that.
- How much spray foam do I need for a rim joist?
- More than people expect, because the job is measured in linear feet rather than area. The perimeter of the house counts twice, once at the sill seam and once at the subfloor seam, then add two vertical seams per joist bay and every pipe and cable coming through. Measure it first and buy the cans together rather than running out mid-run.
- Should I put insulation in the rim joist too?
- Usually yes, but after the air seal, not instead of it. Sealing stops air moving through the seams; insulation slows heat moving through the assembly. Fiberglass batts pushed into unsealed bays are the classic version of doing only half of it, and they are why plenty of insulated rim joists still feel cold.
- Can I use fiberglass insulation in a rim joist?
- It will slow heat transfer but it will not stop air movement, because fiberglass is air-permeable. If the seams behind it are unsealed, outside air moves through the batt, which undermines the insulating job as well. Seal the seams with foam first; what goes in the bay afterwards is a separate decision.
- Will spray foam in the rim joist keep mice out?
- Not on its own. A mouse will chew cured foam. Where the sill plate line is an active rodent route, pack the gap with a metal fill fabric or hardware cloth first and foam over the top. The mesh handles the animal and the foam handles the air, and neither substitutes for the other.
- Is the rim joist worth sealing if my basement is unfinished?
- It is one of the better returns available in an unfinished basement, because everything is accessible and you are sealing a leak that runs the whole way around the house. Doing it before finishing the space is also far easier than doing it afterwards, when the seams are behind a ceiling.
Read next
Spray foam, caulk or backer rod?
Three fillers, sorted by gap width and whether the joint moves. Including the backer-rod sizing rule.
How to find drafts in your house
Five tests, no equipment, an hour — ordered so you find the big leaks before the small ones.
Cold air coming up from under the house
Cold floors are usually the rim joist. The biggest accessible leak in a house, and rarely sealed.
Why is my house so drafty?
One mechanism, a handful of holes, and why the fix for a downstairs draft is often in the ceiling.
The best foundation vent covers for a cold floor
Cold floor over a crawl space? Covers help — but only after you have checked which kind of crawl space you have.