Rim Joist Insulation — Foam Board and Spray Foam at the Basement Rim
The rim joist is the leakiest foot of wall in the house and the fiberglass stuffed in it does nothing. Foam it right and the kitchen floor stops being cold in January.
JMB Builders insulates rim joists in basements across the Twin Cities metro. The old fiberglass batts come out of each bay, the rim and the sill plate are cleaned and sealed, and either rigid foam board is cut to fit each bay with the edges sealed in spray foam, or the whole rim is sprayed with closed-cell foam. The drafts stop and the floors above warm up. Priced per project, quoted from your basement photos within one business day.
- Cut-and-seal foam board or closed-cell spray foam
- Stops the draft, warms the floor above
- Quote from photos in 1 business day
- Licensed MN contractor BC809005
Look up at the edge of your basement ceiling. Where the floor joists sit on the foundation, that band of wood running around the house is the rim joist. It is a single board between the house and outside with a sill plate under it that was never sealed to the concrete. In most houses there is a scrap of fiberglass shoved into each bay, black on the outside face from years of air being pulled through it. That is your cold floor, your drafty first-floor baseboard and, in a lot of houses, the spider highway.
Fiberglass does not stop air, so the fix is foam. The batts come out, the bay is cleaned, and the rim and sill get sealed. Then either rigid foam board is cut to each bay and its edges are sealed with canned spray foam so it is airtight, or the whole rim is sprayed with closed-cell foam, which seals and insulates in one pass and is the better job around wiring, pipes and odd-shaped bays. The Minnesota code allows foam at the rim joist within limits without a separate thermal barrier; we stay inside those limits and cover it when it is required.
We do rigid foam and canned foam with our own crew. Sprayed closed-cell foam is done with our spray-foam partner when a rig is needed, usually when the whole rim is being sprayed in one visit or the rim is paired with basement walls.
What you supply
- Photos of the rim joist from inside the basement: a few bays, the corners, and any spot with pipes or wires
- The rough perimeter of the house or the basement square footage
- A cleared path around the basement walls so we can reach every bay; we move light items
- Whether the basement is finished and the rim is covered, which changes the job
What JMB supplies
- Removal of the old fiberglass from every bay
- Sealing of the sill plate to the foundation and the rim to the subfloor above
- Rigid foam board cut to each bay and sealed at the edges, or closed-cell spray foam on the whole rim
- Foam around every pipe, wire, dryer vent and hose bib penetration through the rim
- Thermal barrier over the foam where the code requires it
- Cleanup of the old batts and foam trimmings
- Photos of the finished rim in your job folder
How it is priced: per project
Quoted in writing from your photos within one business day. What moves the number:
- Perimeter of the house; the number of bays is what sets the time
- Foam board cut-and-seal vs. sprayed closed-cell foam
- How crowded the rim is with pipes, wires, ducts and the electrical panel
- Bays that are hard to reach: behind the furnace, water heater, or a finished wall
- Sill plate that needs sealing to a rough or uneven foundation
- Basement walls insulated at the same time
A photo of a typical bay and one of the busiest corner is enough. Tell us the house's rough size and whether the basement is finished.
How long it takes
Most rims with foam board: one day. A large or crowded rim, or spray foam with the partner rig: a day to a day and a half.
How your project works
- 1
Send photos, get a written quote
Send three photos of the space and a sentence about what you want. You get a clear, itemized quote within one business day. We only come out to measure first when the job needs a tape measure — no pressure, no games.
- 2
Pick materials, or bring your own
We help you choose profiles and materials that fit the house and the budget, or match your existing millwork exactly — or you buy the door or trim yourself and we install it. Everything is confirmed before the crew is scheduled.
- 3
Clean, precise installation
The crew shows up on time, protects floors and furniture, and installs with tight joints and consistent reveals. Photos of the finished work go in your job folder.
- 4
Walkthrough and invoice
We walk the finished work with you. If anything is not right, we fix it before we call the job done. Then one invoice, paid by check or direct deposit — no card fees, no payment links.
Situations and options
Cut-and-seal rigid foam
A piece of rigid foam board is cut to each bay, pressed in, and the edges are sealed with canned spray foam so no air passes. Two layers where the code range calls for it. Best on open, regular bays; slower where there are a lot of pipes.
Closed-cell spray foam
The whole rim is sprayed with closed-cell foam in one pass, sealing every crack and covering pipes and wires. Done with our spray-foam partner when a rig is needed. The right choice on a crowded rim or a house where the sill plate is rough.
Rim plus basement walls
If the basement is being finished, the rim and the walls get done together: foam board or spray foam on the walls, sealed at the rim, so the finished basement is dry and warm from the start.
Finished basement rim
When the rim is behind a finished ceiling, we open the soffit or the ceiling drywall along the wall, do the rim, and patch. It costs more than an open basement, and it is often still worth it on a house with cold floors and drafts.
Rim Joist Insulation — Foam Board and Spray Foam at the Basement Rim: questions, answered
Why are my floors cold above the basement?
The rim joist is leaking outside air into the floor system. The band of wood at the top of the foundation is thin, the sill under it was never sealed to the concrete, and the fiberglass stuffed in it lets air straight through. Cold air moves under the floor and the first floor feels cold at the edges. Sealing and foaming the rim is the fix.
Is spray foam or foam board better for a rim joist?
Both work when they are sealed airtight. Cut-and-seal foam board is done with hand tools, costs less, and is the right choice on open bays. Closed-cell spray foam seals everything in one pass and is better on a rim full of pipes, wires and ducts, or on a rough sill plate. We quote both when the rim could go either way; spray foam is done with our spray-foam partner when a rig is needed.
Should I remove the fiberglass in the rim joist?
Yes. Fiberglass in the rim does not stop air, it filters it, which is why it is black on the outside face. It also holds moisture against the wood. It comes out, the bay gets cleaned and sealed, and foam goes in. Leaving the fiberglass and foaming over it traps whatever moisture and dust is in it.
Does rim joist insulation need to be covered?
Sometimes. The Minnesota code allows foam at the rim joist without a thermal barrier within set limits on thickness and product; beyond those, or where the basement is finished, the foam gets covered with drywall or an ignition barrier. We stay within the limits or cover it, and we tell you which on the quote.
Will insulating the rim joist help with spiders and bugs?
It helps. The gap between the sill plate and the foundation, and the cracks around every pipe through the rim, are how most of them get in. Sealing the sill and foaming every penetration closes those paths. It will not make the basement bug-free, but the rim is the biggest door and it gets shut.
Can you insulate the rim joist if my basement is finished?
Yes, with more work. The rim is behind the finished ceiling or a soffit along the outside wall. We open enough drywall to reach every bay, do the rim, and patch it ready for paint, with painting quoted as its own line. On a house with cold floors and drafts it is still worth doing, and the patch is a lot cheaper than living with it.
Get a written quote for this job
Three photos and a sentence is enough. You hear back within one business day. Or text the photos to 763-204-2099.
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Read morePage reviewed September 24, 2026 · JMB Builders LLC, MN license BC809005 · All insulation pages
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