Some spaces cannot take fiberglass or cellulose. A converted attic has a few inches of rafter depth, no ventilation path and slopes that run straight into the living space. A flat-roof house in Jersey City Heights, Bayonne or Union City has a roof cavity only a few inches tall. A rim joist is a ring of gaps where the framing meets the foundation, and it leaks all winter. These are the places where heat and conditioned air escape and where summer heat gets in.
Spray foam works in all three because it is insulation and air barrier in a single pass. On a roof deck we spray the underside of the sheathing, which moves the insulation to the roofline and keeps the attic or cavity much closer to the temperature of the house. At the rim joist, the foam seals the gaps and insulates in the same step.
What's included
- Inspection of the roof deck for leaks, rot and damp wood before anything is sprayed. Foam never goes over an active leak.
- Open-cell or closed-cell foam, chosen area by area. Open-cell is sprayed at about 3 to 5.5 inches, closed-cell at about 2 to 3 inches.
- Masking of the work area, ventilation during spraying and clear instructions on when it is safe to come back in.
- Foam sprayed in passes to the thickness the area needs, then trimmed flush where framing requires it.
- Rim joist sealing and insulating, either on its own or as part of a basement or crawl space job.
- An ignition or thermal barrier over exposed foam where code requires one.
- Cleanup of overspray and debris, and a walkthrough of the finished work.
What it costs in North Jersey
Typical range
$3.50 to $7.50 per sq ft, by foam type
Open-cell spray foam (about 3 to 5.5 inches, used on roof decks and rim joists) typically runs $3.50 to $5.00 per square foot. Closed-cell foam (about 2 to 3 inches, denser, better where moisture is a concern) typically runs $5.00 to $7.50 per square foot.
For a roof deck of about 1,200 square feet, the typical range is $6,000 to $12,000 depending on foam type and thickness. Foaming the rim joist as part of a crawl space or basement job typically adds $800 to $2,000.
What moves the price
- Foam type: closed-cell costs more per square foot than open-cell.
- Thickness: deeper rafters and higher targets use more material.
- Access and height: steep pitches, tight eaves and multi-story rim joists take more labor.
- Repairs: damp or damaged decking has to be fixed before foam goes on.
How the work goes
- 1
Assessment. We inspect the roof deck or rim joist, note leaks or moisture and agree on the foam type for each area. You get a written scope with ranges.
- 2
Prep and masking. We cover floors and anything that must stay clean, then set up ventilation. We tell you how long to stay out of the area based on the product being installed.
- 3
Spraying. Foam goes on in passes to the target thickness. Most roof deck jobs take one to two days.
- 4
Trim and barrier. We trim high spots and add an ignition or thermal barrier where it is required.
- 5
Walkthrough. We walk the finished work with you and clean up. Spray foam on a roof deck sometimes needs a permit, so we confirm with the local construction office before we start.
When to choose spray foam vs blown-in insulation
Choose spray foam when the insulation has to go on the roof deck or rim joist, when the space is too shallow for blown-in depth, or when you want air sealing and insulation done in one step. Closed-cell is the pick for areas where moisture is a worry.
Choose blown-in insulation for an open, vented attic floor or for existing wall cavities. It costs less per square foot and reaches R-49 to R-60 in an unfinished attic without trouble.