Closed cell spray foam is dense, rigid and water resistant. It insulates at R-6.9 to R-7.1 per inch and becomes a vapor retarder at about 1 1/2 inches. Open cell is soft and light, insulates at R-3.9 to R-4.2 per inch, air seals once it is thick enough, and lets water vapor pass. Closed cell costs more per inch.
Use closed cell where water, vapor or depth is the problem: rim joists, basement and crawl space walls, anything below grade, metal, shallow rafters. Open cell fits above-grade walls and roof decks with room for the thickness, with the vapor rules below followed, for less money.
Open cell vs closed cell spray foam compared
R-value per inch
Open cell runs R-3.9 to R-4.2 per inch; closed cell R-6.9 to R-7.1. R-49 takes about 11 3/4 to 12 1/2 inches of open cell or about 7 inches of closed cell. Closed cell gets its higher number from a blowing agent gas trapped in its cells that conducts heat less than air, according to the Spray Polyurethane Foam Alliance. The Department of Energy notes closed cell R-value can drift down as some of that gas escapes, mostly in the first two years; open cell’s doesn’t change.
Density and rigidity
The Alliance puts open cell at about 0.5 to 0.8 pounds per cubic foot and closed cell at 1.8 to 2.3, which is why the trade calls them half-pound and two-pound foam. Open cell stays soft and dents under a thumb. Closed cell cures hard. In a Building America guide published by the Department of Energy, typical closed cell tested at 22 psi in compression and 28 psi in tension, against under 2 and under 4 psi for open cell.
Air sealing
Both are air barriers once they are thick enough; the Alliance puts that at about 1 1/2 inches for closed cell and 3 1/2 to 5 1/2 inches for open cell. The code calls foam that passes the air-permeance test air-impermeable insulation (IRC Section R202), a term that matters for unvented roofs. Gaps and shrinkage cracks leak either way.
Vapor permeance
The International Residential Code classes vapor retarders by perm rating in Table R702.7(1): Class I at 0.1 perm or less, such as polyethylene; Class II above 0.1 and up to 1.0, such as kraft facing or vapor retarder paint; Class III above 1.0 and up to 10, such as latex paint.
- Closed cell: one data sheet lists 1.1 perms at 1 inch and 0.61 perm at 1 1/2 inches, and the Alliance says 1 1/2 inches is typically under 1 perm. That makes it a Class II vapor retarder at normal thickness.
- Open cell: one data sheet lists 21 perms at 1 inch, and the Building America guide gives 5 to 10 perms at 5 inches. Even thick, it is at best in the Class III range, not the Class I or II retarder the code asks for in cold-climate walls and roofs.
Water
The Building America guide calls closed cell hydrophobic: it won’t absorb water. Open cell absorbs and holds liquid water, up to about a third of its volume depending on the product. FEMA’s January 2025 flood-resistant materials bulletin rates closed-cell foam insulation acceptable below flood elevation and open-cell plastic foams unacceptable, and the Department of Energy says open cell should not be used below ground level.
Sound
Open cell is the one tested for sound: the Building America guide lists a 2x4 wall with open cell at STC 37 and gives no sound data for closed cell. Both cut noise that leaks through air gaps; neither soundproofs a wall or floor.
Strength
Closed cell bonds to sheathing and framing and stiffens them; the Building America guide cites studies finding wall racking strength substantially increased. It is a bonus, not a structural repair. Open cell adds no meaningful strength.
Cost per inch
Closed cell costs more mainly because it uses more material: it expands about a third as much as open cell and ends up about four times as dense. Per unit of R-value the gap narrows, since each inch of closed cell insulates about 1.6 to 1.8 times as much. Thick closed cell also takes several passes; one data sheet caps a closed cell pass at 3 inches, while one open cell manufacturer allows 6.
Which spray foam to use where
Roof decks, unvented attics and cathedral ceilings
Both can go under the roof sheathing of an unvented attic or cathedral ceiling (IRC Section R806.5). The Building America guide rates closed cell preferred on sloped roofs in all climates and open cell acceptable.
- Closed cell fits the energy code’s R-49 (R-60 in Connecticut) into fewer inches, which matters in a 2x8 rafter only 7 1/4 inches deep, and carries its own vapor control. The trade-off: a roof leak may not show, because water won’t pass through the foam, and a worn roof’s leak is harder to find.
- Open cell costs less and lets a leak show; wet open cell dries in time once the leak is fixed. It needs about 11 3/4 to 12 1/2 inches for R-49. In climate zone 5 and colder, the code requires air-impermeable insulation under an unvented roof to be a Class II vapor retarder or to carry a Class II coating in direct contact with it, so open cell there gets vapor retarder paint.
- Hybrid: closed cell against the deck, with fiber insulation directly under it. Table R806.5 sets the minimum foam R-value for condensation control: R-15 in zone 4A, R-20 in zone 5 and R-25 in zone 6, or just over 2, just under 3 and about 3 1/2 inches of closed cell.
Which zone you are in: all of Connecticut is zone 5. In New Jersey, Bergen, Hunterdon, Morris, Passaic, Somerset, Sussex and Warren counties are zone 5 and the rest of the state zone 4. In New York, Westchester, Nassau and Suffolk are zone 4; Rockland, Putnam, Dutchess, Orange, Columbia and Greene are zone 5; Ulster and Sullivan are zone 6. The Department of Energy’s insulation guide goes further than the code and calls for closed cell, or open cell with vapor retarding paint, in zones 4 and 5 alike.
Walls
Above grade, both work. Open cell fills a 2x4 or 2x6 bay for less. Closed cell puts more R-value in the same bay, controls vapor and stiffens the wall; the prescriptive energy code tables in New Jersey, Connecticut and New York accept R-30 of cavity insulation alone in a wood-framed wall, which closed cell reaches in a 2x6 and open cell doesn’t.
Rim joists
Closed cell, which the Building America guide rates preferred there. It air seals the joints between rim board, sill and subfloor in one pass, and within the code’s limits for exposed rim joist foam (no more than 3 1/4 inches thick, 0.5 to 2.0 pounds per cubic foot, flame spread index 25 or less, smoke-developed index 450 or less; IRC Section R316.5.11, R303.5.11 in 2024) it can stay uncovered.
Basement walls
Closed cell. The Department of Energy’s insulation guide says closed cell gives the best moisture control of all interior foundation insulations; open cell costs less but is not a good vapor retarder and must go on much thicker, eating into the room. The Building America spray foam guide rates open cell not acceptable below grade. Foam on basement walls needs a thermal barrier such as drywall. See basement insulation.
Crawl spaces
Closed cell on the walls of a sealed crawl space; the Department of Energy lists 3 inches of it, against 10 inches of open cell. Under the floor of a crawl space that runs damp, the same guidance says at least 3 inches of high-density closed cell is an adequate vapor retarder under any floor covering, vinyl included. See crawl space insulation.
Below grade and anywhere water can reach
Closed cell only. The Alliance says it is the only spray foam that can be used where contact with water is likely, such as below-grade concrete walls, in contact with the ground, or on the exterior side of the building, and FEMA’s bulletin draws the same line for flood-prone spaces.
Metal buildings, pole barns and metal roofs
Closed cell. Humid air that reaches cold steel condenses, so the foam against metal has to be the air seal and the vapor control, and open cell is vapor-open and holds water. One manufacturer notes metal may need a primer or surface treatment before foam will bond.
Vapor retarder code rules for spray foam
These come from the International Residential Code: the 2021 edition in Connecticut and the 2024 edition as adopted in New Jersey and New York, Section R702.7 for walls and R806.5 for unvented roofs.
- Frame walls in zone 5 and colder need a Class I or II vapor retarder on the interior side, such as kraft facing or vapor retarder paint. In zone 5, Class III, ordinary latex paint, is allowed instead where the wall has vented cladding, such as vinyl siding or brick veneer with an air space, over plywood, OSB, fiberboard or gypsum sheathing, or enough continuous insulation outside (Tables R702.7(2) and R702.7(3)).
- Closed cell can count as that continuous insulation. Under Section R702.7.1, spray foam of 1.5 perms or less at its installed thickness, applied to the inside of the sheathing, meets the continuous insulation condition if its R-value reaches the table’s number, R-5 in a 2x4 wall or R-7.5 in a 2x6 wall in zone 5. A little over an inch of closed cell reaches R-7.5. Open cell, at about 21 perms per inch, doesn’t qualify.
- Frame walls in zone 4 may use a Class II or III vapor retarder. A Class I retarder such as polyethylene is not permitted on the interior, except the variable-permeance type the code allows in every zone.
- Basement walls and below-grade walls are exempt from the interior vapor retarder requirement.
- Unvented roofs in zone 5 and colder: air-impermeable insulation must be a Class II vapor retarder or be coated with one. Closed cell at normal thickness already is; open cell needs vapor retarder paint. In any zone, no Class I vapor retarder may go on the attic floor or ceiling side of an unvented attic.
The Building America spray foam guide adds a warning for open cell walls in cold climates: it reports a moisture failure in northern Vermont with no interior air or vapor retarder and an exterior that couldn’t dry. Its answer is interior air sealing plus vapor retarder primer or several coats of latex paint.
Common myths about open and closed cell foam
- “Closed cell is always better.” Per inch and against water, yes. Above grade, in a wall or a roof with depth and vapor control handled, open cell does the job for less.
- “All spray foam is a vapor barrier.” Only closed cell at thickness. Open cell tests at 5 to 10 perms at 5 inches.
- “Open cell rots roof decks.” Moisture does, from indoor air or roof leaks. The code’s vapor retarder coating in zone 5 handles the first, a sound roof the second. Closed cell has its own risk: it can hide a leak.
- “Foam can’t grow mold.” The Alliance says foam is not a food source for mold, but dust on its surface plus moisture can grow it in any poorly designed assembly.
- “One thick pass is better.” The Alliance says closed cell sprayed past the manufacturer’s pass limit traps heat that can shrink the foam, cut its R-value and, in extreme cases, ignite it.
- “Closed cell doesn’t need a fire barrier.” Both foams are combustible. Both need a thermal barrier in living space, with narrow exceptions for rim joists, attics and crawl spaces. See spray foam insulation.
Common questions about open cell and closed cell spray foam
What is the difference between open cell and closed cell spray foam?
Closed cell is dense, about 2 pounds per cubic foot, rigid, R-6.9 to R-7.1 per inch, water resistant, and a vapor retarder at about 1 1/2 inches. Open cell is light, about half a pound, soft, R-3.9 to R-4.2 per inch, an air barrier at enough thickness but vapor-open, and it absorbs water. Closed cell costs more per inch.
Which is better, open cell or closed cell spray foam?
Neither everywhere. Closed cell for rim joists, basement and crawl space walls, anything below grade or near water, metal buildings, and shallow rafters. Open cell for above-grade walls and roof decks with room for the depth, where it costs less. In climate zone 5 and colder, open cell under an unvented roof needs a vapor retarder coating.
Do I need a vapor barrier with open cell spray foam?
Often. Under an unvented roof in zone 5 or colder, which covers Connecticut, seven New Jersey counties and the Hudson Valley counties outside Westchester, the code requires a Class II vapor retarder coating on it. In a zone 5 frame wall, the interior needs a Class I or II retarder unless the wall has vented cladding over sheathing or enough exterior insulation. In zone 4, latex-painted drywall meets the code.
Is closed cell spray foam a vapor barrier?
It is a vapor retarder. At about 1 1/2 inches it is typically under 1 perm, which is Class II. It is not Class I, 0.1 perm or less, at the thickness used in houses, and it doesn’t need to be: the code doesn’t allow ordinary Class I retarders on the interior of frame walls in zone 4 or on the attic floor of an unvented attic.
Can open cell spray foam get wet?
It absorbs and holds liquid water, up to about a third of its volume depending on the product, and dries in time once the source is fixed. That is why it doesn’t belong below grade, on basement or crawl space walls, or where flooding is possible. Closed cell doesn’t absorb water, and FEMA rates it acceptable in flood-prone spaces.
Can closed cell spray foam be left exposed?
On a basement or crawl space rim joist, yes, up to 3 1/4 inches thick within the code’s density and flame-spread limits. In an attic or crawl space entered only for repairs or maintenance, it needs an ignition barrier or a fire-tested coating. Anywhere else inside the house, it needs a thermal barrier such as 1/2-inch drywall. Open cell follows the same rules.
Does closed cell spray foam add structural strength?
Some. The Spray Polyurethane Foam Alliance says 2 to 3 inches can significantly increase the racking strength of certain framed walls and the uplift strength of roof decks. It doesn’t replace structural repairs. Open cell adds no meaningful strength.
Does open cell spray foam reduce noise?
It helps with airborne sound. A Building America guide lists a 2x4 wall with open cell at STC 37 and has no sound data for closed cell, and both foams cut noise that leaks through air gaps. Neither stops footsteps through a floor; that takes mass and decoupled drywall.
Should I use open cell or closed cell under my roof deck?
Closed cell if the rafters are shallow, if you want vapor control built in, or for a hybrid with fiber insulation under the foam. Open cell if there is depth for the full R-value, you want a lower price and a roof leak that shows, and a vapor retarder coating goes on where the code requires one.
Why does closed cell spray foam cost more?
It uses more material. Closed cell is about four times denser than open cell because it expands far less, and thick applications take several passes. Per R-value the difference is smaller, since each inch insulates about 1.6 to 1.8 times as much. Most of our projects run between $3,500 and $12,500.
Open cell or closed cell: we decide at the site
We look at the spot before we pick the foam: above or below grade, rafter or stud depth, climate zone, siding and sheathing, any water, and how the space is used. Then we tell you which foam, how thick, what covers it and what it costs. The energy audit is free.
Call 914-554-2289 or use the contact form, 8am–8pm, seven days. The Home Insulators is family-owned, based in New Rochelle, NY since 2015, with a 100-plus-person in-house crew that never subcontracts. We work across all of New Jersey and Connecticut, New York’s Hudson Valley and Long Island; see our service area.