How to Air Seal a Basement Rim Joist
Why this matters
The rim joist (also called the band joist) is the board that caps the ends of the floor joists and sits on the sill plate, which sits on the foundation. Where it meets the sill, where the sill meets the concrete, and where the subfloor lands on top, there are three long seams running the whole perimeter of the house, plus every pipe, cable and vent the trades have punched through it. In winter the basement is the bottom of the house and the stack effect pulls it negative, so outdoor air comes in along that line all day. That is the cold floor along the outside walls of the first floor, the frost on the inside of the rim in January, and the moldy fiberglass batt somebody stuffed in each bay years ago.
This card is the basement counterpart of How to Air Seal Penetrations Into the Attic. The attic seals the top of the house, where air leaves; the rim seals the bottom, where it comes in. Doing one without the other still helps, but the stack effect is driven by both ends.
What you are building, one bay in section
SUBFLOOR above
========================================
| rim | seam 3: subfloor to rim (bead)
| joist| [foam block, cut 1/4 in under]
| | edges sealed to rim, joists,
| | subfloor and sill with foam
| ---- | seam 2: rim to sill (bead)
| sill plate | seam 1: sill to concrete
+--------------+ (sill gasket or sealant)
| foundation |
| wall | BASEMENT side ->
The block is the insulation, but the seal is the bead around it. A block that fits snugly with no bead still leaks around all four edges, because neither the rim nor the subfloor is flat.
Before you start: the things that stop the job
- Wet or soft wood. Probe the rim and sill with an awl. A rim that is dark, damp, or lets the awl sink in has a water problem from outside (grade sloping toward the house, a missing deck ledger flashing, a leaking hose bib, a gutter dumping at the corner). Foam over it traps the water against the wood. Stop, photograph it, and hand the source to the trade that fixes it; seal that stretch after the wood is dry and sound.
- Pests. Look for termite mud tubes on the foundation and sill, frass, droppings and gnaw marks. In termite country, foam over the sill hides the evidence a pest inspector needs; follow the inspection gap or practice your local building department and pest-control trade expect, and call a licensed pest professional when you see activity.
- Combustion and fuel. Note every flue, vent connector, dryer vent and gas line that passes through or near the rim. Foam never goes against a flue or vent connector, and nothing gets sealed in a way that hides a gas joint. If you smell gas at any point, stop, leave the house without operating switches, and call the gas utility from outdoors.
- Electrical. Note cables through the rim and any junction box. Do not bury a box. Knob-and-tube wiring (single conductors on porcelain knobs) is not to be enveloped in insulation under NEC 394.12 as adopted; flag it for an electrician and leave that bay alone.
Steps
1. Clear the perimeter and remove old batts. Pull out any fiberglass stuffed in the bays and bag it. A batt is not an air barrier; moist basement air filters through it, meets the cold rim, condenses and feeds mold on the batt's back face. Old batts and the dust behind them are an irritant and can carry mold; wear gloves, eye protection and a respirator matched to the dust under your shop's written program per 29 CFR 1910.134 (applied to construction work such as this retrofit by 1926.103). Skip it and the foam goes in front of a wet, moldy batt, sealing the problem in.
2. Inspect, then log every bay. With the rim bare, walk it with a light and write down bay by bay what is there: clear, cable, pipe, vent, wet, damaged. Why: the material for each bay depends on what passes through it, and the count of clear bays is what you cut blocks for. Skip it and you cut a block for the dryer vent bay and find out at the wall.
3. Seal the sill to the foundation. Run a bead of polyurethane sealant where the sill plate meets the concrete, both sides where you can reach. Older houses rarely have a sill gasket, and the concrete is rough, so this joint can leak more than the rim itself. Skip it and you have sealed a box whose bottom is still open.
4. Seal the rim-to-sill and subfloor-to-rim seams. Caulk the visible seams in each bay. On the walls parallel to the joists, the rim is a continuous run behind the last joist; seal both of its seams the full length. Why before the blocks: the blocks hide these seams, and a seam you cannot see after the block goes in is one you cannot fix.
5. Seal the penetrations by gap size. A common shop default: caulk to about 1/4 inch, one-part low-expansion foam from about 1/4 inch to 3 inches, and a rigid patch sealed at its edges for anything bigger. Plastic pipe moves with temperature, so it gets a flexible caulk rather than a rigid foam that cracks loose. At a metal dryer vent or a vent connector, keep foam off it entirely and close the gap with a sheet metal collar and a high-temperature sealant suited to the vent; a flue keeps the clearance to combustibles that its listing or the code gives, and that clearance stays open air, not foam. One-part foam releases isocyanate vapor while it cures and its propellant is flammable: ventilate, wear gloves and eye protection, and keep it away from the water heater and furnace while they can fire. Skip it and the cables and pipes become the leaks the blocks were meant to stop.
6. Cut and set the blocks. Measure the bay opening (for 2x10 joists at 16 inches on center, typically about 14-1/2 by 9-1/4 inches), cut rigid foam about 1/4 inch under in each direction, press it to the rim, and seal all four edges with low-expansion foam. Two inches of foam board is a common thickness; check your adopted energy code for the R-value it asks of a rim. Why undersized: the gap is where the sealant goes, and a tight block crushes the bead to nothing.
7. Cover the foam if its listing does not allow it exposed. Under IRC R316 as adopted, foam plastic inside the house generally has to be separated from the interior by a thermal barrier, commonly 1/2 inch gypsum board. There is an allowance for foam at the rim, in substance letting spray-applied foam within stated thickness, density and flame-spread limits stand without that barrier (R316.5.11 in the 2021 edition; check yours). That allowance is written for spray foam. Rigid board left exposed needs either the barrier or an evaluation report that permits it exposed in that use, so read the board's report and ask your inspector. Skip it and you have put an exposed fuel load along the whole basement ceiling.
Worked example: a 30 by 44 foot basement
The house has 2x10 floor joists at 16 inches on center, spanning the 30 foot direction and bearing on the two 44 foot walls. The two 30 foot walls run parallel to the joists.
The bay count. Along each 44 foot wall: 44 times 12 inches is 528 inches, divided by 16 is 33 bays. Two walls give 66 bays. The 30 foot walls have no bays; their rim is a continuous strip 9-1/4 inches tall and 30 feet long, two of them for 60 feet.
The log. Of the 66 bays, 3 carry something: one has the metal dryer vent, one has the gas service entry, one has a hose bib line. One more, at the northwest corner under a downspout, shows a darkened sill that the awl sinks into a quarter inch. That bay is photographed, flagged to the owner for a gutter extension and a carpenter, and left unsealed. That leaves 62 bays for plain blocks, plus the 3 penetration bays sealed in step 5 around their penetrant and blocked with notched foam, 65 blocks in all.
The board, counted two ways. By area: 66 bays at 14.5 by 9.25 inches is about 61.5 square feet, and the two 30 foot strips at 9.25 inches tall are about 46.3 square feet, 107.8 in all. Divided by a 32 square foot sheet that is 3.37 sheets; with a 10 percent waste factor, 3.7, so four sheets.
By cutting layout: a 4 by 8 sheet cut into blocks 14.25 by 9.0 inches yields 3 across the 48 inch width (42.75 inches) and 10 along the 96 inch length (90 inches), 30 blocks per sheet. Two sheets give 60 blocks, 5 short. The strips come from full-length rips: 5 rips of 9.0 inches from a sheet's 48 inch width (45 inches), each 8 feet long, so 40 feet of strip per sheet. Cut a third sheet all into blocks and the strips still need 2 more sheets, 5 in all. Plan it instead: on the third sheet, take the last 5 blocks from one 9 inch rip (5 times 14.25 is 71.25 inches of its 96), then rip 4 more strips from the remaining 39 inches of width, 32 feet of strip. The other 28 feet come from a fourth sheet. Total, 4 sheets.
The area method said 4 and the job needs 4, but only because the cut layout was planned; cut sheet by sheet without a plan and it takes 5. Draw the layout before you buy.
The finish. The basement is unfinished and the owner plans to frame it next year. The board chosen has no report permitting it exposed, so the shop quotes 1/2 inch gypsum over the foam along the rim, or leaves that line item for the finish carpenter in writing, so nobody assumes the foam can stay bare.
What would change it. If the joists bore on the 30 foot walls instead, the bays move: 30 times 12 is 360 inches, divided by 16 is 22.5, so 22 or 23 bays per wall depending on where layout starts, and the long walls become the continuous strips. If the owner wants to skip the covering, spray foam applied within the rim allowance's limits by a trained installer is the usual alternative to board plus drywall.
How to verify it worked
On a cold, windy day, or with the house depressurized for a blower door test, run the back of your hand or a smoke pencil along each block edge and the sill-to-concrete bead. Any edge that moves smoke gets another bead. See related: How to Find Air Leaks While the House Is Depressurized. Check the flagged bay again after the water source is fixed and the wood has dried, then seal it.
The invoice records the bay count, every penetration and what sealed it, the clearance kept at each vent, the flagged bay with its photo and the reason it was left open, and whether a thermal barrier was installed or deferred and to whom.
References
- See related: How to Air Seal Penetrations Into the Attic; The House as One Pressure System; Stack Effect and the Neutral Pressure Plane
- IRC R316 (foam plastic), as adopted by your jurisdiction; 2021 edition consulted for the rim-joist allowance
- NEC 394.12, knob-and-tube wiring and insulation, as adopted
- OSHA 29 CFR 1910.134, respiratory protection, applied to construction work by 29 CFR 1926.103
- Foam board and sealant manufacturers' data sheets and evaluation reports