Spray foam insulation, where it earns its place

What spray foam is, and the two kinds

Spray foam starts as two liquids mixed at the tip of a spray gun. It goes on wet, expands within seconds and hardens in place. Because it sticks to whatever it hits and fills every crack as it expands, it insulates and air-seals in one pass, which is the whole appeal.

There are two kinds, and they're not interchangeable:

• Open-cell foam is soft and spongy, expands a lot and lets water vapor pass through. It's the lower-cost foam per square foot and it fills irregular cavities well. It also dampens sound.
• Closed-cell foam is dense and rigid. It packs more R-value into each inch, slows water vapor and adds some stiffness to what it's sprayed on. It costs more per square foot, and it's the one to use where space is tight or moisture is a concern.

The choice comes down to where the foam is going and what it has to do there, not which one sounds better.

Where foam makes sense in a Bay Area house

Rim joists over a crawl space. The band of framing that sits on top of the foundation is full of corners, wires and pipes, and batts never fit it well. Closed-cell foam seals and insulates it in one go, and it's a natural add-on to crawl space and underfloor insulation.

Vaulted and cathedral ceilings. In hillside homes and remodels with vaulted ceilings, the rafters are the only place for insulation and the space between them is shallow. When a rafter roof or cathedral ceiling is opened up, the 2025 Energy Code sets a mandatory minimum of R-19 there. Closed-cell foam can reach that in depths where batts struggle.

Kneewalls and bonus rooms. Rooms over a garage or tucked under a sloped roof have short walls and angled ceilings that leak air and are hard to insulate any other way.

Unvented attic conversions. Foam goes on the underside of the roof deck instead of the attic floor, so the attic and its ducts end up inside the insulated house. It's a design project, not a product swap; see our unvented attic conversion guide.

Open walls during a remodel. When the drywall comes off for a kitchen remodel or a garage being turned into living space, the stud bays are open for a few days before new drywall goes up. That's the one time foam can go into a wall without drilling. Open-cell fills the bays and quiets the room; closed-cell earns its higher price on exterior walls where the framing is shallow. For walls that are staying closed, see existing-wall insulation instead.

Small awkward gaps. For gaps between a quarter inch and about 3 inches during air sealing, ENERGY STAR's guide calls for expanding foam from a can. That's the small-scale version of the same idea.

Spray foam vs blown-in for an open attic

On an ordinary vented attic floor, the comparison is mostly about money and what you're trying to fix. Blown-in reaches the code's R-49 target on an open floor for a fraction of foam's price, and with the gaps sealed underneath first it does the same job. Foam's strengths, sealing and insulating in one pass and packing R-value into a shallow space, matter where there's no room for 16-odd inches of loose fill or where the insulation has to move to the roof deck. So the answer for most Bay Area attics is blown-in on the floor plus air sealing, and foam for the rim joists, vaulted ceilings and kneewalls where loose fill can't stay put.

Where it doesn't

• An open, vented attic floor. As above: blown-in insulation does that job for much less.
• Rodent holes. Rats chew through foam. Entry points need metal: see rodent proofing.
• Flat roofs with no attic. Most Eichlers and other mid-century flat-roof homes have no attic at all; the roof deck is the ceiling. The Eichler Network notes the original roofs had under an inch of insulation, and the fix is foam or rigid board added on top of the deck during a reroof. That is roofing: California's license classification for roofing explicitly includes urethane foam. We don't do roofing work, so that one belongs to a roofer.

How we do it

1. Inspect and measure. What's being sprayed, how much surface there really is (a roof deck has more area than the floor under it), moisture, wiring and what's in the way.
2. Pick the foam and the thickness for the R-value the space needs, and write both on the quote, along with what happens to existing insulation and vents.
3. Prep. Mask and cover what shouldn't get foam on it, and set up ventilation for the work area.
4. Spray in passes, trim where it needs to be flush, and clean up.
5. Re-entry. The house stays clear during spraying and for the re-entry time the manufacturer sets for that product.

Safety: re-entry time, coverings and old wiring

Spray foam gives off fumes while it's being applied and while it cures, and each manufacturer's instructions say how long people should stay out afterward. That time varies by product, which is why we give you the re-entry time for the specific foam on your quote and ask that people and pets be out of the house until it has passed.

Foam plastic generally has to be covered or coated where it's exposed, under the building code, and what that means depends on where it is: a living space, an attic you walk into, or a crawl space. That's part of planning the job, not an afterthought.

And as with every insulation job, foam doesn't go over live knob-and-tube wiring. If your house has it, the next call is to an electrician; see the knob-and-tube guide.