Air leakage
The joints between foundation, sill, and rim, plus pipe and wire holes, add up to one of the largest leaks in many crawl spaces — and the biggest one left once vents are closed.
Learning guide
Rim Joist: What It Is, How to Insulate It, and Rot Repair
Where the rim joist sits, how it differs from a band joist, sill plate, header, and ledger, why it matters so much in a crawl space, and how insulation, rot, and repair options compare.
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Where the rim joist sits
A section cut through one joist bay at the edge of the house. The rim is the only thing between outside air and the bay.
Terms
Rim joist vs. band joist, sill plate, header, and ledger
Inspection reports and quotes use all of these. Here is which board each one means.
| Term | Where it is | What it does |
|---|---|---|
| Rim joist | On edge around the outside of the floor, nailed to the joist ends, resting on the sill plate | Closes the joist bays, keeps joists upright, carries the wall above to the sill |
| Band joist / band board / rim board | Same board | Regional and trade names for the rim joist; "rimboard" usually means an engineered LVL or OSB rim used with I-joists |
| Floor joist | Spans across the crawl space, inside the rim | Carries the floor load to beams and the foundation |
| Sill plate | Flat on top of the foundation wall, under the rim | Anchors the floor framing to the foundation with bolts |
| Header | Over a door or window opening in a wall | Carries load across the opening; some builders also call the rim a "band header" |
| Ledger | Bolted to the outside of the rim or a wall | Supports another structure — most often a deck — hung off the house |
Why it matters
Why the rim joist matters in a crawl space
It is a small part of the floor with an outsized share of the air leaks and moisture problems.
Condensation
The rim is cold in winter and the bay behind it is full of humid air in summer. Where warm, moist air meets a cold surface, it condenses — often behind batt insulation where nobody sees it.
Water and insects
Siding, deck ledgers, hose bibs, and high soil all put water near the rim. Wet wood at the perimeter is also where termites and carpenter ants start.
Insulation
Rim joist insulation options compared
The job is air sealing first and R-value second. The method that fails is the one that insulates without sealing.
| Method | Air sealing | Moisture / condensation | Cost | DIY? |
|---|---|---|---|---|
| Rigid foam, cut and cobbled | Good — if every edge is sealed with canned foam or caulk | Low risk: foam keeps the rim face warm, so indoor air cannot condense on it | Lowest material cost; slow to cut and fit | Yes — the most common DIY method |
| Closed-cell spray foam | Excellent — fills gaps and the sill joint in one pass | Lowest risk: about 2 in also acts as a vapor retarder | Highest; usually a hired crew or a large two-part kit | Kits exist; fumes and technique favor a pro |
| Fiberglass batt alone | None — air passes straight through | Highest risk: humid air reaches the cold rim and condenses behind the batt | Cheapest | Yes, but it is the method most likely to cause problems |
| Air seal first, then batt | Good — the sealing does the air work | Moderate: the batt adds R-value, the sealing controls moisture | Low | Yes — acceptable if every gap is sealed first |
How cut-and-cobble works
Cut rigid foam board (commonly 1½–2 in) to fit each bay against the rim, then seal all four edges and the sill joint with low-expansion canned foam. The foam, not the board, is what makes it airtight. Seal pipe and wire penetrations while you are there.
Fire and code notes
Foam plastics generally need a thermal or ignition barrier. The model code has a specific exception for spray foam at sill plates and headers up to 3¼ in thick when the product meets its density and flame-spread limits; exposed rigid board usually needs a product rated for that use or a covering. Check the product's listing and your local inspector.
As planning estimates, air sealing and insulating the rim of a typical crawl space commonly runs about $400–$1,600 when added to an insulation or encapsulation job, depending on perimeter length, access, and method. Price your own with the crawl space insulation cost calculator. In a sealed crawl space the rim is part of the air and thermal boundary, which is why encapsulation quotes should list rim sealing as its own line; in a vented crawl space it still pays to seal it along with the floor insulation.
Damage
Signs of rim joist rot or insect damage
Check from the crawl space with a flashlight and a screwdriver. Hard wood resists the tip; rotted wood lets it sink in.
Soft, dark, or crumbling wood, often below a door, deck ledger, hose bib, or downspout.
Dark staining or mold-like growth on the rim face behind old batt insulation.
Mud tubes on the foundation just below the sill, or wood that breaks in layers.
Frass (fine sawdust) or ants around damp perimeter wood.
Siding or trim outside that is swollen, stained, or pulling away at the bottom.
A floor that feels soft or dips along one exterior wall.
Repair
Rim joist repair and replacement
Repair follows the damage: stop the water, then restore the wood. Ranges are planning estimates, not quotes.
| Approach | Usually fits when | Planning estimate |
|---|---|---|
| Dry out, treat, and monitor | Staining only; wood still hard | Cost of the moisture fix |
| Sister the rim | Localized rot with sound wood on either side | Often quoted around $40–$50 per foot |
| Replace a rim section | Rot through the board, often with siding opened from outside | Varies widely with length, access, and siding |
| Replace rim with sill | The sill below is also damaged | Sill plate work commonly runs $100–$200 per linear foot |
The rim carries the wall above, so replacement needs temporary support and should be done by an experienced framing contractor. Damage that runs down into the sill plate or the floor joist ends usually becomes one perimeter repair. If termites are possible, have a licensed pest professional inspect first, and fix the source — grading, gutters, a deck ledger, or high crawl space humidity — in the same plan.
Related pages
Related framing and moisture guides
The boards around the rim, and the work that keeps them dry.
Sill plate
The treated board under the rim — what it is, damage signs, and repair.
Learn moreFloor joist repair
When rot or insects have reached the joist ends behind the rim.
Learn moreCrawl space insulation
Floor vs. wall insulation, and where the rim fits in each plan.
Learn moreCrawl space encapsulation
Rim air sealing is one of the steps in a sealed crawl space.
Learn moreCrawl space vapor barrier
Covering the soil so less moisture reaches the perimeter framing.
Learn moreInsulation cost calculator
Price rim, floor, and wall insulation for your own crawl space.
Open the toolFAQs
Rim joist FAQs
Short answers to the questions homeowners ask most.
What is a rim joist?
A rim joist is the board that runs around the outside edge of a floor, fastened to the ends of the floor joists and resting on the sill plate. It closes off the joist bays, keeps the joists from rolling, and carries the exterior wall above down to the sill and foundation.
Is a rim joist the same as a band joist?
Yes. Rim joist, band joist, band board, and rim board all describe the same perimeter board. The name changes by region and trade; "rimboard" often refers specifically to engineered LVL or OSB rim material used with I-joist floors.
What is the difference between a rim joist and a regular floor joist?
Regular floor joists span the building and carry the floor load. The rim joist runs across their ends at the perimeter, tying them together and closing the bays. On the two walls that run parallel to the joists, the outermost joist plays a similar role and is often called an end or rim joist too.
What is the difference between a rim joist and a ledger?
The rim joist is part of the house's own floor framing. A ledger is a separate board bolted to the outside of the rim (or a wall) to hang another structure, usually a deck. A deck ledger that is poorly flashed is a common cause of rim joist rot.
What is the best way to insulate a rim joist?
Air seal it and insulate it with something that keeps its inside face warm: closed-cell spray foam, or rigid foam board cut to fit each bay with every edge sealed with canned foam. Both stop humid air reaching the cold rim. Foam left exposed in a crawl space may need an ignition barrier, so check the product and local code.
Can you use fiberglass insulation on a rim joist?
Fiberglass alone is a poor fit, because air moves through it and moisture can condense on the cold rim behind the batt, leading to mold or rot. If you use batts, seal every gap and the sill joint first with foam or caulk, then press the batt in without compressing it.
Sources
Research notes and review date
Building-science, code, and cost references used for this guide.
- U.S. Department of Energy — air sealing your home for rim and sill air leakage.
- GreenBuildingAdvisor — insulating rim joists for cut-and-cobble, spray foam, and batt condensation risk.
- UpCodes — IRC R316.5.11 sill plates and headers for the spray foam thermal-barrier exception.
- American Wood Council — I-joist rimboards for engineered rim board.
- Acculevel sill plate and sistering costs for rim/band board sistering and sill repair per foot.
Rim air sealing ranges come from this site's insulation and encapsulation cost calculators. Code requirements vary by jurisdiction; confirm with your local building department.
Last updated: .