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

Foam at the roof line moves the insulation plane

When foam goes on the roof deck the attic becomes part of the conditioned space, which is the decision that matters far more than which foam you pick.

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

Open-cell spray polyurethane foam puffs up into a soft, light layer. Its cells connect to each other. That structure explains four things. Water vapor passes through it. Water runs through it. It soaks up sound better than its sibling. And it costs less per inch, because less chemical fills the same space. It is the usual pick at the roofline in mild and hot climates, where the sheathing still has a way to dry and letting vapor through does not hurt you.

Closed-cell spray polyurethane foam cures dense and hard, with sealed cells. It has the highest R per inch of any material we track. At normal thickness it blocks water vapor on its own. It shrugs off liquid water. And it stiffens whatever it is sprayed into. Pick it in three cases: when you do not have the inches to spare, when the code route for an unvented attic calls for insulation that air cannot pass through right against the deck, and in the tight triangle at the eaves where nothing else will fit.

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 math, not a spec. A contractor works out a thickness and sprays it in passes, each one limited in depth. Nobody orders foam in inches off a table. The bay depth row uses 9.25 inches. That is the real depth of a nominal 2x10 under American Softwood Lumber Standard PS 20. Framing varies at the eaves and from house to house, so measure yours before you assume a bay will hold your target. The R per inch ranges, where they come from, and why the FTC R-value Rule (16 CFR Part 460) makes a product's own label beat any general table, are all in the method note on the types hub.

Here is the number worth taking away from that table. For the same R-value, open-cell needs about 1.7 to 1.9 times the thickness of closed-cell. That spread is the two ends of the published R per inch ranges, not 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 inside the framing or has to be built down into the room below. And at the eaves, where the rafter meets the wall plate, it often decides whether full depth is reachable at all.

The rest of the comparison is behavior, and behavior 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 vapor 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.

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 that boundary changes how the whole building works. It is not swapping out a part. The list below is what a good contractor will raise before quoting. If none of it comes up in the sales call, that is your clearest sign 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 way of drying out. With no air moving above the insulation, the sheathing dries inward, or it does not dry at all. This is why how each foam handles water vapor stops being a row in a table and becomes the whole decision. Codes do allow unvented attics. That route generally calls for insulation air cannot pass through, pressed right against the underside of the deck. In colder zones a minimum share of it has to be that type, so the deck stays warm enough that water does not condense on it. The exact numbers come from the edition your area enforces. They belong on the drawings, not on a national page. Energy codes cover new building and work done under a permit. They do not usually cover adding insulation to an attic you already have. Which edition applies where you live depends on your state, and often on your city. Your local building department is the only authority on it.
• Gas and oil appliances get rechecked. Say a furnace or water heater vents on its own natural draft, and it now sits inside the newly sealed space. After tightening a house this much, someone has to check that it still gets enough air and still drafts.
• 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 is a model code. It keeps foam plastic in a lived-in building separated from the inside by a thermal barrier. In some attics it allows a lighter ignition barrier instead. Which edition your area adopted, and how it changed it, varies. So what your building department will accept, often a paint-on coating that swells in a fire, is a line on your quote. It is not a national rule.

None of that makes a sealed attic wrong. If your duct losses are already landing outside the insulated shell, moving the boundary fixes the real problem. The six items above are the scope of work, not arguments against it. What they are not is a choice you settle by reading two columns of R-values. Someone designs the assembly, and the contractor puts it in writing before anything gets sprayed.

Foam is sold by the board foot. One board foot is one square foot sprayed one inch thick. Because that unit already includes the thickness, board feet per square foot of surface is just the designed thickness in inches. That 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. R-60 takes 16.2 to 17.1 against 8.6 to 10. Multiply by the area actually being sprayed, then compare your 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 labor method with its estimates named, are at the cost guide.

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 vapor openness lets indoor humidity reach cold sheathing, which is the mechanism behind damp or moldy roof decks in unvented attics that were built vapor-open without an interior retarder. That is a design failure rather than a product failure, and it is repaired by design: a vapor 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 weak points are cost and stiffness. Of the materials we track, it is the most expensive installed for each unit of R-value. The reason is physical, not a markup. More chemical goes into the same space than with 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 odor, and the only repair is cutting it out and starting again.
• It is a pro job, and there is no honest amateur route at attic scale. The chemistry is isocyanate-based. The EPA, the CDC and NIOSH all publish guidance on exposure to spray polyurethane foam. Crews work under OSHA rules for breathing protection. And you have to leave the house during spraying, and for a set time afterward. Ask how long that is and plan around it.
• Blowing agents are changing. Closed-cell products in the United States have been switching to blowing agents that do less damage to the climate. That is under the EPA's phase-down of hydrofluorocarbons. Ask which product and which blowing agent you are 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.

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.

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What does spray foam cost?

No installed price for spray foam is published here, because none has been measured. The retail material has been read, and at R-30 it runs about 9 to 10 times blown fiberglass. The installed price comes from the contractor, so ask a bid for its board feet. See the material figures and how they were read.

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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 normal vented attic with a floor you can reach, loose fill blown to the target depth costs a fraction of foam and holds heat just as well. The spray foam against blown-in comparison makes that case in full. R-49 is R-49. A ceiling insulated to R-49 with blown fiberglass or cellulose slows heat exactly as well as one insulated to R-49 with foam. Cellulose is made to standards such as ASTM C739. There is no extra performance hiding inside the expensive material.

What foam really buys you on a floor is air sealing. And you can buy that on its own for far less. Canned foam, caulk and rigid board around the tops of walls, the recessed lights, the plumbing pipes, the wire holes and the hatch. All done before the loose fill goes over it. DOE guidance treats sealing the ceiling as the partner job to insulating it. On a leaky ceiling it is the better first buy 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.

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