A rim joist is the framing member that runs around the outside edge of a floor, capping the ends of the floor joists and sitting on the sill plate or the wall below. It closes off the joist bays, keeps the joists upright, carries the wall above down to the foundation and, in the basement, is one of the leakiest spots in the whole house.
This guide covers what the rim joist does structurally, how it differs from a band joist and rim board, what goes wrong with it, and how to air seal and insulate it properly.
Where the Rim Joist Sits
In a typical wood-framed house over a basement or crawlspace, the order from the bottom up is: foundation wall, sill plate (bolted to the foundation), then the floor joists bearing on the sill. The rim joist is nailed across the ends of those joists, standing on edge on the sill plate, the same depth as the joists. Subfloor sheathing goes over the top, and the wall above sits on the subfloor directly over the rim.
The sill plate itself is held down to the foundation with anchor bolts or straps; our guide to anchor bolts explains how those connections work in concrete.
What a Rim Joist Does
Closes the floor and steadies the joists
Joists are tall and thin. Nailed to a continuous rim at their ends, they are held vertical and evenly spaced, and the floor edge is closed off so the subfloor and wall above have something to bear on.
Carries the wall above
On a bearing wall line, the rim sits right in the load path: roof and upper-floor loads come down the studs, through the subfloor, into the rim and sill, and into the foundation. HUD’s Residential Structural Design Guide, Chapter 5: Design of Wood Framing (checked October 2026) notes that over a wall opening, the floor band joist and the sole plate of the wall above also span the opening and act as part of the system resisting the load, along with the header. That is why cutting a rim joist above a window or door is not a harmless change.
Transfers wind uplift
The same HUD chapter cites the transfer of moderate wind uplift through the band or rim joist of a floor to the construction below as an application of cross-grain tension in wood, and says that where more strength is needed, designers should increase member size or change the detail. In high-wind areas, straps or connectors that tie the wall framing past the rim to the framing below are common. Their size and spacing come from the engineer or the local code.
Rim Joist vs Band Joist vs Rim Board
These terms overlap, and builders use them differently from region to region. HUD’s framing guide uses “band or rim joist” as one term. Here is how they are generally used:
| Term | What it usually means | Typical material |
|---|---|---|
| Rim joist | The perimeter member at the edge of a floor; often used for the member capping the joist ends | Dimension lumber the same depth as the joists |
| Band joist | Used interchangeably with rim joist; some builders reserve it for the side that caps the joist ends | Dimension lumber |
| End joist | The outermost joist on the sides that run parallel to the joists | A regular joist |
| Rim board | A manufactured rim product used with engineered floor systems | Engineered wood (LVL, OSB or similar) matched to the joist depth |
| Track / edge member (steel) | The perimeter member in a cold-formed steel or structural steel floor | Steel track, channel or edge angle |
Engineered floors built from wood I-joists normally use a rim board specified by the joist manufacturer for that system rather than ordinary lumber. Joist depths are listed in our guide to wood I-beam (I-joist) sizes. In light-gauge steel framing, the rim is a track section with the same web depth as the joists; how steel stud and track sizes are designated is explained in metal stud sizes.
Common Rim Joist Problems
- Rot. The rim sits just above grade, behind siding and next to the sill. Poor flashing at doors and decks, high soil or gutters that dump water at the foundation can keep it wet. Probe it with a screwdriver from the basement; soft, punky wood needs attention.
- Insects. Being close to soil and often damp, the rim and sill are common places to find termite or carpenter ant damage.
- Large holes. Dryer vents, plumbing and service entries often pass through the rim. Engineered rim boards have manufacturer limits on hole size and location; on a bearing line, any large opening needs to be checked.
- Deck ledgers. Decks are commonly bolted to the house through the rim. A failed ledger connection is a serious safety issue, so ledger fastening must follow the local code or an engineered detail.
If the rim or sill under a bearing wall has to be replaced, the wall above must be shored first, the same way it would be for a beam swap. Our article on basement support beams covers shoring and replacement for the beam in the middle of the house; a licensed structural engineer should specify any structural repair at the perimeter.
How to Insulate and Air Seal a Rim Joist
The rim joist is a weak spot in the building envelope because it is a single thickness of wood with a lot of joints around it: sill to foundation, rim to sill, and subfloor to rim. The U.S. Department of Energy notes that air can infiltrate through the framing, including the band and rim joists, and that defects in air sealing and insulation at floors over unconditioned basements or crawlspaces cause heat loss and cold floors (DOE Building Science Education, Insulating and Sealing Rim Joists, checked October 2026).
Step 1: Air seal first
ENERGY STAR’s basement air sealing guide (checked October 2026) recommends sealing the top and bottom of the inside of each rim joist cavity, even where you cannot see cracks, plus the gap between the sill plate and the foundation. It recommends caulk for gaps of 1/4 inch or less and expanding spray foam for gaps from about 1/4 inch to 3 inches, and high-temperature caulk where a metal flue sleeve passes through. Bump-outs such as bay windows deserve extra attention.
Step 2: Choose the insulation
| Method | Air seals? | Notes from DOE and ENERGY STAR guidance |
|---|---|---|
| Rigid foam board | Yes, if the edges are sealed | Cut to fit each cavity; seal the edges with caulk or spray foam |
| Spray foam, closed-cell | Yes, at 1.5 in or thicker | Air seals and insulates in one step |
| Spray foam, open-cell | Yes, at 5.5 in or thicker | Thinner applications need a separate air barrier |
| Fiberglass batts | No | Seal the cavity first; cut to fit and place against the rim without compression, gaps or voids |
The spray foam thicknesses above are DOE’s minimum finished thicknesses for the foam to act as an air barrier. Foam plastics in basements usually have to be covered by a fire-rated barrier unless a code exception applies, and the rules differ by code edition, so check with your local building department before leaving foam exposed. Never cover or block combustion air openings, and keep insulation clear of flues.
Step 3: Watch the moisture
Insulating the rim makes it colder in winter, because less house heat reaches it. A rim that is already damp from outside leaks should be fixed at the source before it is covered. If you insulate and find wet or soft wood, stop and repair it.
Rim Joists in Steel-Framed Buildings
Steel floors have the same edge problem. In a structural steel building, the floor edge is usually a perimeter beam with a bent plate or angle that stops the concrete on the deck; channels are also used as edge members. Section dimensions for those are on our C-channel sizes chart. The structural engineer sizes these members; they carry the facade as well as the floor.
Rim Joist FAQ
Is the rim joist structural?
Yes. It steadies the joist ends, carries the wall above to the sill and foundation, and helps resist wind uplift and loads over openings. Do not cut or remove it without an engineer’s detail.
Is a rim joist the same as a band joist?
In most usage, yes. HUD’s design guide uses the terms together. Some builders use “band joist” only for the member that caps the joist ends and “end joist” for the parallel sides.
Is it worth insulating the rim joist?
In a basement or crawlspace that is part of the heated space, usually yes. DOE and ENERGY STAR both identify it as a common source of air leaks and heat loss, and sealing it is a relatively small job.
Can I use fiberglass on a rim joist?
Only after air sealing the cavity. Fiberglass batts slow heat flow but do not stop air, so on their own they let moist indoor air reach the cold rim.
How do you replace a rotted rim joist?
The wall and floor above have to be supported with temporary shoring, the damaged section cut out and replaced, and the cause of the water fixed. Because the rim is in the load path, have a licensed structural engineer or experienced contractor specify the repair.
Reference information only, not engineering advice. Disclaimer.