The rim joist is the worst single air-leakage point in 90 percent of pre-1995 Cincinnati homes. It sits at the basement ceiling at the cold-air infiltration band; uninsulated, it acts as a chimney for winter stack-effect heat loss. 4-6 hours of closed-cell foam here typically delivers 10-15 percent total utility savings, the highest ROI of any single insulation project.
More detail
The rim joist (also called the band joist) is the structural framing element that runs around the perimeter of the home at the top of the foundation wall, where the floor joists meet the foundation. In a typical 2-story Cincinnati home, there are two rim joists: one at the basement ceiling and one at the second-floor level. The basement rim joist is the priority because it sits at the cold-air infiltration band and the stack-effect chimney start point. The physics. In a pre-foam Cincinnati home in January, warm air rises through the conditioned envelope (stack effect). The warm air has to come from somewhere; that somewhere is cold air infiltrating at the lowest building penetration, the rim joist. Air leaks through the gaps between the rim joist and the foundation, around plumbing and electrical penetrations through the rim, and through the rim joist material itself (typically dimensional lumber that has no insulation value). The cold air enters the basement, drops to floor level (because cold air is denser), and chills the basement floor and adjacent first-floor flooring. The heat the furnace generates has to replace both the cold air mass and the thermal mass that the cold air absorbed. Sealing the rim joist with 2-3 inches of closed-cell foam (which fills cavities, seals penetrations, and adds R-13 to R-19 of thermal resistance in one pass) interrupts this cycle. Most Cincinnati homes show 10-15 percent total utility reduction after rim joist work alone, with payback in 18-24 months. The scope. A typical 2,500 sqft Cincinnati home has 100-150 linear feet of rim joist around the basement perimeter. Closed-cell foam at 2-3 inches across that band totals 200-450 board feet at $1.80-$2.20/bf average, or $360-$990 in product cost. Add 4-6 hours of crew time at $80-$120/hour and the typical project ranges $800-$2,200 total. The work happens entirely in the basement; no living-space disruption. The scheduling is the easiest of any foam project (most Cincinnati installers can do a rim-joist-only project within 1 week of quote). Cincinnati installer recommendation: if a homeowner can only afford one foam project, the rim joist is the universal first answer.