Open-cell vs closed-cell spray foam in an attic

By Insulation Report Editorial. Last reviewed August 2026. How we research this.

Every attic material is compared against every other one on the insulation types hub, which carries the master table. Below are only the two spray foams, and the decision that outranks the choice between them: whether the insulation belongs on the attic floor or under the roof deck.

The one-line answer, and who each foam is for

Closed-cell where depth is the constraint or the assembly needs vapour control at the roof deck. Open-cell where there is room for the thickness and the assembly can tolerate a vapour-open layer. Neither one on an ordinary vented attic floor.

Open-cell spray polyurethane foam expands into a soft, low-density layer with cells that connect to each other. That structure is why it is vapour-open, why water runs through it, why it absorbs airborne sound better than its sibling, and why it costs less per inch: less chemical goes into the same volume. It is the usual choice at the roofline in mild and hot climates, where the sheathing has a drying path and the vapour openness is not working against you.

Closed-cell spray polyurethane foam cures dense and rigid with sealed cells. It carries the highest R per inch of any material this site tracks, acts as its own vapour retarder at ordinary installed thickness, resists liquid water, and stiffens the assembly it is sprayed into. It is the choice where the inches are not available, where the code path for an unvented attic asks for air-impermeable insulation directly against the deck, and in the tight triangle at the eaves where nothing else fits.

Side by side

The thermal half of the comparison is arithmetic, so it is worth doing exactly rather than describing. The R-value targets used here are the ENERGY STAR recommendations for an uninsulated attic: R-49 in climate zones 2 and 3, R-60 in zones 4 through 8, which cover most of the country.

The depth rows are arithmetic rather than a specification: foam is designed to a thickness by the contractor and sprayed in passes of limited thickness, not ordered in inches from a table. The bay depth row uses 9.25 inches, the dressed depth of a nominal 2x10 under American Softwood Lumber Standard PS 20; framing varies at the eaves and between houses, so measure yours before assuming a bay will take a target. R per inch ranges, their sources, and why the FTC R-value Rule (16 CFR Part 460) makes a product's own label the authority over any generic table, are set out in the method note on the types hub.

The number worth carrying out of that table: for the same R-value, open-cell needs about 1.7 to 1.9 times the thickness of closed-cell, that spread being the two ends of the published R per inch ranges rather than a tolerance. At R-49 it is 13.2 to 14 inches against 7 to 8.2. Between rafters, that difference decides whether the foam fits in the framing or has to be furred down into the room below, and at the eaves, where the rafter meets the top plate, it often decides whether full depth is reachable at all.

The rest of the comparison is behaviour, and behaviour is what actually gets people into trouble. These rows are qualitative on purpose: there is no honest single number for most of them.

Read those rows as a pair rather than a scorecard. Closed-cell's vapour tightness is an advantage against a cold deck and a liability in an assembly that needs to dry inward. Open-cell letting water through is a defect while the roof is leaking and a diagnostic advantage afterwards, because you find out. Rigidity buys stiffness and costs you crack tolerance. There is no row where one foam is simply better.

The decision that actually matters: where the insulation plane goes

Almost everyone searching this comparison is standing one step past the decision that determines their outcome. In an attic, the question is not which foam. It is which surface gets insulated: the floor of the attic, or the underside of the roof deck. Those are two different buildings when you are finished, and only one of them involves spray foam at all.

Plane one: the attic floor

Insulation on the floor, ventilation above it. Outside air moves in at the soffit vents, across the underside of the deck and out at the ridge, keeping the roof close to outdoor temperature and carrying away any moisture that gets up there. The attic is outside the thermal boundary of the house and is meant to be. This is how most American houses with pitched roofs are built, it is cheap to improve, and loose-fill blown to depth is the normal material for it. Blowing the floor changes nothing about how the building handles moisture: you are adding depth to a system that already works.

Plane two: the underside of the roof deck

Insulation sprayed against the sheathing, between and often across the rafters. The attic is now inside the thermal boundary. It stays near indoor temperature, ducts and an air handler up there are no longer sitting in a space that swings with the weather, and their leakage and conduction losses land inside the house instead of outside it. This is the conditioned, or unvented, attic, and foam at the roofline is what creates it. That is the real reason spray foam is in the attic conversation at all. It is not that foam is a better blanket than loose-fill. It is that foam is the material that can insulate and air seal a sloped, irregular surface overhead in one pass.

What changes when the plane moves up

Moving the boundary is a change to the physics of the building, not an upgrade to a component. The list below is what a competent contractor will raise before quoting, and its absence from a sales conversation is the clearest signal you are talking to the wrong one.

• The vents close. Soffit, ridge, gable and any powered attic fan. Leaving them open pumps outside air into a space you just paid to insulate. That is not optional tidying, it is the definition of the assembly.
• The roof loses its ventilated drying path. With no moving air above the insulation, the sheathing dries inward, or it does not dry. This is exactly why the vapour behaviour of the two foams stops being a table row and becomes the decision. Codes recognise unvented attic assemblies, and the path generally requires air-impermeable insulation in contact with the underside of the deck, with a minimum share of it required in colder zones so the deck stays above the temperature where condensation forms. The specific figures come from the edition your jurisdiction enforces and belong on the drawings, not on a national page. Energy codes apply to new construction and permitted work, not usually to voluntarily adding insulation to an attic you already have. Which edition applies where you live depends on your state and often your city, and your local building department is the only authority on it.
• Combustion appliances get re-examined. A furnace or water heater that vents atmospherically inside the new conditioned space needs its combustion air and draft checked after a tightening job of this scale.
• The old floor insulation has to be decided. It is now inside the envelope and no longer doing anything useful in most designs, which raises whether it comes out. Remove or add over runs that test.
• The roof itself should be near the start of its life. Spraying to the underside of sheathing that is already due for replacement puts the foam in the demolition path, and some roofing warranties treat an unvented deck differently. Sequence it: reroof first, insulate second.
• The foam gets covered. The International Residential Code, a model code, separates foam plastic in a habitable building from the interior with a thermal barrier, or in some attics an ignition barrier. Which edition your jurisdiction has adopted, and how it has amended it, varies, so what your building department accepts, often an intumescent coating sprayed over the foam, is a line item on the quote rather than a national rule.

None of that makes a conditioned attic wrong. Where the duct losses are already landing outside the envelope, moving the boundary is the fix for the actual problem, and the six items above are the scope of work rather than objections to it. What they are not is a material preference to be settled by reading two columns of R-values: the assembly gets designed, and the contractor puts it in writing before anything is sprayed.

How to read a foam quote

Foam is sold by the board foot, meaning one square foot sprayed one inch thick. Because the unit already carries the thickness, board feet per square foot of surface is simply the designed thickness in inches, which makes a quote checkable: reaching R-49 takes 13.2 to 14 board feet of open-cell for every square foot of surface, against 7 to 8.2 of closed-cell, and R-60 takes 16.2 to 17.1 against 8.6 to 10. Multiply by the area actually being sprayed and compare the answer with the board feet on the quote. A quote that gives board feet but no thickness per surface cannot be checked that way, and thin spots are invisible from the hatch once the crew has gone.

What the money turns on:

• Both foams sit at the top of the installed cost ranking among attic materials, closed-cell above open-cell and the blown loose-fills at the bottom. On a floor that could simply be blown, that is a jump between cost tiers for the same R-value on the same ceiling, not a small premium.
• Closed-cell costs more per inch and needs fewer inches. Those pull in opposite directions, so a per inch comparison is not the comparison. Ask both quotes for a target R-value and a price, and compare those.
• A roofline job is priced on the roof plane, not the floor. A pitched roof has more surface than the floor beneath it, and the gable ends usually get sprayed too, so the square footage on a roofline quote is legitimately larger than your attic floor area. Use that area, not your floor area, in the board foot check above.
• The limit on pass thickness shows up in the schedule. The cure gives off heat, so each pass is capped and the next waits for the last to cool. A deep specification is more time on site as well as more chemical.

Installed dollar figures, built up from a measured material price and a stated labour method with its estimates named, are at the cost guide.

Where each one fails

Open-cell

Its failures are moisture failures. It holds bulk water rather than repelling it, so a soaked layer stays wet against wood and takes a long time to give the water back. In a cold climate its vapour openness lets indoor humidity reach cold sheathing, which is the mechanism behind damp or mouldy roof decks in unvented attics that were built vapour-open without an interior retarder. That is a design failure rather than a product failure, and it is repaired by design: a vapour retarder coating on the interior face, or closed-cell against the deck instead. Its other limitation is simple geometry. At 16.2 to 17.1 inches for R-60, it needs depth that many rafter bays do not have.

Closed-cell

Its failures are cost and rigidity. Of the materials this site tracks, it sits at the top of the installed cost ranking per unit of R-value, and the reason is physical rather than commercial: more chemical goes into the same volume than in open-cell, and the work needs a rig, a trained crew and site controls that a bag of loose-fill does not. Sprayed onto framing that moves seasonally, or onto a substrate that was dirty, damp or too cold at the time, it can crack or lose adhesion, and a hairline separation at the rafter edge is an air path you cannot see. Because it sheds water, a leak above it can run a long way before it announces itself. And it buries what it covers: wiring, the top of the deck, and any future roof diagnosis. It is effectively permanent, so treat the decision as permanent.

Both

• It is chemistry, not a product out of a bag. Two components mixed at the gun in a set ratio, at a controlled temperature, in passes of limited thickness. Off-ratio or cold-sprayed foam can shrink, fail to cure, or hold an odour, and the only repair is cutting it out and starting again.
• It is a professional installation, with no honest amateur route at attic scale. The chemistry is isocyanate-based. The EPA, the CDC and NIOSH publish exposure guidance on spray polyurethane foam, crews work under OSHA respiratory protection rules, and the house is vacated during spraying and for a stated period afterward. Ask what that period is and plan around it.
• Blowing agents are changing. Closed-cell formulations in the United States have been moving to lower global-warming-potential blowing agents under the EPA's phase-down of hydrofluorocarbons. Ask which product and which blowing agent is being quoted.
• Inspection after the fact is hard. You cannot measure foam depth with a ruler in the way you can push one into loose-fill. Get the designed thickness per surface in the contract, and take photographs during, not after.

Which one, given your attic

Find the row that describes your situation and take the recommendation. If two rows apply, the one describing a constraint on the building wins over the one describing a preference.

If no row moved you off the attic floor, the recommendation is the first row. Air seal, then blow loose-fill to the ENERGY STAR level for your climate zone, and skip both foams. The R-value guide sets your target, and the attic calculator takes your own square footage and current depth and returns what is left to add.

You may not need either

We do not sell insulation and we do not install it, which is why this section can say what it says. Most people who arrive comparing open-cell against closed-cell should buy neither.

For a standard vented attic with a floor you can reach, loose-fill blown to the target depth costs a fraction of foam and holds heat identically. R-49 is R-49: a ceiling insulated to R-49 with blown fibreglass or cellulose, made to specifications such as ASTM C739 in the case of cellulose, resists conducted heat the same as a ceiling insulated to R-49 with foam. There is no premium performance hiding inside the expensive material. What foam actually buys on a floor is air sealing, and that is work you can buy on its own for far less: canned foam, caulk and rigid material around the top plates, recessed lights, plumbing stacks, wire penetrations and the hatch, done before the loose-fill goes over it. DOE guidance treats sealing the ceiling plane as the partner job to insulating it, and on a leaky ceiling it is the better first spend either way.

Two more cases where the answer is to spend nothing this year. If you are already within about R-10 of your zone's recommendation, more depth pays back slowly. That R-10 threshold is an estimate rather than a measurement, set at roughly one step of the ENERGY STAR table. Measure what you have before shopping: the depth checker turns inches into an R-value range. And if the attic is dry, clean and simply thin, the cheapest correct job is more of whatever is already there, not a different material over the top of it.

Related

• Every attic insulation material, compared
• Blown-in insulation: fibreglass and cellulose
• What attic insulation costs, built up from parts
• How much insulation does my attic need?