Short answer
The most robust way to insulate a rim joist in an older home is to air-seal and insulate it with closed-cell spray foam (pro-applied), or as a DIY approach, install rigid foam board cut to fit each bay and seal the edges with one-part spray foam (“cut-and-cobble”). Avoid using fiberglass batts alone; they don’t stop air movement and can trap moisture against the wood.
Why the rim joist needs special treatment
Rim/band joists are thin, cold exterior framing members riddled with cracks, penetrations, and gaps. In older homes, they’re major air-leak paths and condensation points. The right approach must:
- Stop air leakage (primary goal)
- Provide sufficient R-value to keep the interior surface warm
- Control vapor so the wood stays dry
Closed-cell spray foam and rigid foam both do these well. Fiberglass alone does not.
Best methods
1) Closed-cell spray foam (professional)
- One-and-done solution: air seal + insulation + vapor control
- Excellent for irregular bays and around wires/pipes
- Typical target: 2–3 inches
2) DIY cut-and-cobble rigid foam
- Cut rigid foam (polyiso, XPS, or EPS) to the shape of each bay, adhere it, and seal the perimeter with gun-applied foam. Optionally add a second layer with staggered seams.
- Cost-effective and friendly to weekend DIYers
Recommended R-values and thickness
Target R-value: R-15 to R-20 at the rim joist
Cold climates (IECC zones 5–7):
- 2–3 in closed-cell spray foam (R ~ 12–20)
- 2–3 in XPS (R ~ 10–15) or 2.5–4 in EPS (R ~ 10–16)
Mixed/mild climates (zones 3–4):
- 1.5–2 in closed-cell spray foam, or 2 in rigid foam
Check your local code/energy program for specifics.
DIY step-by-step: Cut-and-cobble
Tools and materials
- Rigid foam board: Polyiso (R~6/in), XPS (R~5/in), or EPS (R~4/in)
- One-part spray foam with a foam gun (more control than straw cans)
- Foam-board adhesive (compatible with foam)
- High-quality sealant/caulk (polyurethane or acoustical) for sill and large gaps
- Utility knife with snap-off blades or insulation saw; straightedge; tape measure; marker
- PPE: safety glasses, gloves, long sleeves, dust mask or P100 respirator
- Work light/headlamp, vacuum, putty knife/scraper
Steps
1) Inspect and prep
- Look for moisture, stains, mold, insects, and air gaps. Fix bulk water issues first. Address any obvious leaks at the sill plate.
- Remove any existing fiberglass batts and debris. Vacuum the area.
- If you see old wiring (e.g., knob-and-tube), deteriorated pipe insulation (possible asbestos), or heavy mold, pause and consult a pro.
2) Air-seal penetrations
- Seal cracks at the sill plate/foundation with polyurethane sealant.
- Foam around pipe/wire penetrations. Larger gaps may need backer rod or scrap foam first.
3) Measure and cut foam panels
- Each bay is slightly different. Measure width and height, then cut foam about 1/8 in undersized for a snug but workable fit.
- Dry-fit each piece.
4) Adhere and seal
- Run beads of foam-safe adhesive on the rim joist. Press the foam panel in place.
- Seal the perimeter with a continuous bead of gun foam, ensuring no gaps remain.
- Tip: Two thinner layers with staggered seams outperform one thick layer and are easier to fit.
5) Optional: Add interior batt
- After continuous foam is installed and edges sealed, you can add an unfaced mineral wool or fiberglass batt in the cavity to boost R-value. Do not install batts without the foam layer.
6) Address the sill and band transitions
- Seal where the sill plate meets the foundation and where joists penetrate the rim. Small pieces of foam and sealant can eliminate tricky leaks.
Time and cost
- A typical basement (60–100 linear feet of rim) is a 1–2 day DIY project.
- Materials: $1.50–$3.00 per square foot of rim area for rigid foam + sealants.
Professional spray foam option
- Expect 2–3 inches of closed-cell foam applied to each rim bay. This delivers air sealing, R-12 to R-20, and good vapor control.
- Pros handle irregular framing and obstructions quickly.
- Cost: typically $3–$6 per square foot of foam area, often with minimum job charges.
- Ask about ignition/thermal barrier requirements; rim joists are often exempt from additional coatings, but local code rules.
Safety and code considerations
- Wear PPE; foam dust and old debris are irritating. Ventilate the area.
- Combustion appliances: Don’t block combustion air or vent clearances. Install CO detectors if you don’t have them.
- Termite inspection gap: In some regions, leave a visible 2–3 in gap at the top of the foundation for inspections—check local requirements.
- Brick veneer walls: Because brick can hold rainwater, closed-cell foam or sufficiently thick rigid foam is preferred to keep the interior wood warm and dry.
- Fire and ignition barriers: Local codes vary on exposed foam in basements/crawlspaces. Some jurisdictions require a thermal or ignition barrier; ask your inspector.
Tips for best results
- Use a foam gun rather than straw cans—better control, less waste.
- Keep foam cans warm (65–75°F) for proper expansion.
- Label utilities and take photos of penetrations before covering.
- Don’t compress batts if you add them; compressed insulation loses R-value.
- Aim for continuous, unbroken foam coverage with sealed edges; the air seal is as valuable as the R-value.
Common mistakes to avoid
- Stuffing fiberglass batts against a cold rim without an air/vapor control layer.
- Leaving gaps or missing bays—air will find the weak spots.
- Sealing active bulk water issues behind foam. Fix water first.
- Blocking dryer vents, flues, or weep holes.
When to call a pro
- You see mold, significant moisture, or suspected asbestos/lead hazards.
- The rim bays are obstructed or access is tight.
- You prefer closed-cell spray foam or want a fast, warrantied job.
Done right, insulating the rim joist tightens the building envelope, cuts drafts, and reduces energy bills—often with a noticeable comfort improvement on the main floor.