A builder’s net zero envelope course by Xerox Builders · October 2026
Every floor line is a weak spot: plates, rim joist and subfloor meet in a small area with little wood between inside and out. This Detail shows how to carry the air barrier past the floor, outside or inside, how to insulate the rim without trapping condensation, and how to choose between spray foam and cut-and-seal board.
Why floor lines are weak spots
At every floor, the wall is interrupted by a band of framing: the top plates of the wall below (or the sill plate on the foundation), the rim joist and the subfloor. Many joints meet in a small area, there is often only one thickness of wood between inside and outside, and the rim bay is easy to under-insulate. Building Science Corporation points out that air leakage in houses comes mostly from connections like these, not from the open field of the wall, and singles out the rim joist as especially troublesome.
The goal at every floor line: the air barrier unbroken, insulation as good as the wall, and the rim kept warm and dry.
Two kinds of floor line
- Rim on the foundation: sits on the sill plate (Detail 05), often beside a basement or crawlspace.
- Rim between storeys: sits on the top plates of the wall below.
Both cut straight through an interior air barrier. An exterior air barrier passes both without a break, which is one reason BC Housing’s airtightness guide describes the exterior approach as simpler: it doesn’t have to connect to all the interior framing.
Exterior approach: run it straight past
- The sheathing membrane or taped sheathing carries on over the rim joist, unbroken.
- Tape any horizontal sheathing joint at the floor line.
- Floor framing shrinks as it dries, so the joint at the rim can move. Use a flexible membrane or tape that can take some movement.
- Exterior insulation keeps the rim warmer, which lowers the condensation risk.
- Still insulate the rim bay on the inside, to the same level as the wall.
Interior approach: seal every joint
With sealed poly or airtight drywall, the air barrier stops at each floor and must be joined across it. The R22+ effective walls guide for BC shows the wall polyethylene sealed to the floor above and to the top plate below, with the rim insulated to the same level as the wall. At the basement rim it shows blocks of air-impermeable insulation cut and fitted between the joists, sealed around their edges. Seal the joints between the plates, rim joist and subfloor as well. Some builders wrap a strip of poly around the rim before the floor goes on, so it can be joined to the wall poly later.
Why rim joists get wet
In winter the rim is one of the coldest pieces of wood in the wall. Fibrous insulation such as batts lets warm, moist room air pass through to that cold surface, where the moisture condenses. The cure is an air-impermeable layer tight against the rim, either spray foam or rigid board sealed at every edge, with batts behind it if more insulation is needed. For the physics, see our Building Science Deep Dive.
Spray foam or cut-and-seal?
| Spray foam | Cut-and-seal rigid board | |
|---|---|---|
| Air seal | Fills every gap in one step | Good if every edge is sealed |
| Who installs | Certified installer with equipment | Framing or insulation crew |
| Labour | Fast on site | Slow, many small pieces |
| Watch for | Thin spots, wet wood; open-cell foam is not a vapour barrier | Missed edges, gaps at the joints |
Cut-and-seal in five steps: seal the sill (or plate), rim and subfloor joints first; cut each board slightly undersize; seal all four edges with sealant or low-expansion foam; add insulation to match the wall; cover or protect as required.
Spray foam: the BC and Vancouver codes reference CAN/ULC standards for spray polyurethane foam, and installers must be trained and certified. Spray only onto clean, dry wood in the conditions the maker allows, and run the foam continuously from the concrete or top plate, over the rim, to the underside of the subfloor.
Whichever method you choose, check the rims with smoke or a thermal camera during the mid-construction blower door test (Detail 18), while they are still easy to reach and fix.
Embodied carbon also differs between foam types and boards; we cover that in Detail 20. Confirm details with your designer or envelope consultant.
Site checklist
- Exterior membrane or taped sheathing continuous past every rim
- Horizontal sheathing joints at the floor line taped
- Interior air barrier sealed to the floor above and the top plate below
- Plate, rim and subfloor joints sealed
- Air-impermeable layer tight against every rim; no batts alone
- Rim insulated to the same level as the wall
- Spray foam continuous, on dry wood, by a certified installer
- Foam covered or protected as your building official requires
- Rims smoke-checked at the blower door test and photographed
Test yourself
Try to answer each question, then click it to check.
Why shouldn’t batts alone go against a rim joist?
Indoor air passes through the batt to the cold rim, and the moisture in that air condenses on the wood.
What is the advantage of an exterior air barrier at floor lines?
It runs straight past the rim joist without a break, so there are far fewer joints to seal.
Name two ways to air seal and insulate a rim joist bay.
Spray foam, or rigid board cut to fit and sealed at every edge, with more insulation behind it if needed.
Next: Detail 07, Window Rough Openings. Want the physics of condensation? See our Building Science Deep Dive.
Planning a high-performance home in Vancouver, Burnaby or Surrey? Talk to our team about getting the envelope right from day one.
Sources: Building Science Corporation, Info-408: Critical Seal (Spray Foam at Rim Joist); Illustrated Guide: R22+ Effective Walls in Residential Construction in BC; Illustrated Guide: Achieving Airtight Buildings (BC Housing / RDH); Fine Homebuilding: Air Sealing a Basement; BC Building Envelope Council: Spray foam presentation