Spray foam attic insulation

Spray polyurethane foam, often shortened to SPF, is not a product you lift off a shelf. It arrives as two liquids in two barrels. A heated hose feeds both to a gun. They mix at the tip and react as they hit the surface. The foam swells many times over in seconds, then hardens in place. Two things follow, and both matter more than the R-value.

It takes the shape of whatever it hits. It fills the gap around a wire, the crack at a rafter, the awkward triangle at the eaves. That is why it seals air so well. Loose fill lies over a hole. Foam plugs it.

It is a chemical reaction, not a blanket. The mixing ratio, the hose temperature, the surface temperature and the thickness of each pass all have to be right. Foam sprayed off-ratio or too cold can shrink, hold a smell, or never fully cure. The repair is cutting it out. Two kinds are sold for attics.

R per inch ranges are settled, aged values from this site's reference data. Labeled R-values come from standard tests such as ASTM C518, under the FTC R-value Rule (16 CFR Part 460), so the product label beats any general table. Which foam suits your assembly, and how each one handles a roof leak, is on the open-cell versus closed-cell page.

An attic hands you two surfaces to insulate. Foam can go on either one. They are not two versions of one job. They leave you with two different buildings.

The attic floor, which keeps the attic vented

Insulation lies on top of the ceiling below, and the attic above it stays vented. Outside air comes in at the soffits and leaves near the ridge. The attic runs close to outdoor temperature all year, and damp air that gets up there is carried away. Nearly every American house with a pitched roof is built this way, and attic ventilation is what keeps it working.

Foam can be sprayed on that floor. It is rarely the right buy. You are paying for the most expensive material in the attic to do the job the cheapest material also does, which is fill depth. What foam adds on a floor is the air seal, and that is buyable on its own for a fraction of the price. If foam does go down there, the vents stay open, and the baffles at the eaves still have to hold 1 inch of clear space between the insulation and the roof sheathing.

The roof deck, which makes the attic part of the house

Here the foam goes against the underside of the sheathing, between the rafters and often over them. The insulated boundary is now the roof. The attic sits inside the house and stays near indoor temperature. Ducts and an air handler up there stop sitting outside the insulated shell, so their leaks and losses land in the house instead of in the yard. That is the real reason to spray a roof deck, and it is a good one.

And the vents come out of service. Soffit, ridge, gable, and any powered attic fan. Left open, they would pump outside air straight into space you just paid to insulate.

What changes when the insulation moves up

Moving that boundary changes how the building works. It is not swapping a part. A contractor worth hiring raises all of this before quoting.

• The roof loses its way of drying. No air washes the underside of the sheathing any more. It has to dry inward, or it does not dry.
• Gas and oil appliances get rechecked. A furnace or water heater that vents on its own draft is now inside a tighter house. Somebody has to check it still gets air.
• The old floor insulation has to be decided. It is now inside the shell and in most designs it is doing nothing. Remove or add over runs that test.
• The roof should be near the start of its life. Foam under sheathing that is due for replacement is wasted. Reroof first, insulate second.
• The foam has to be covered. That is a fire rule, it is a real cost, and it is the next section.

If you have an open vented attic, a floor you can reach and no ducts up there, you are in the first case. Most people are. The second case is a building decision, and it needs a contractor who works in that language.

A sealed attic with no vents is allowed. It is not a workaround. The model International Residential Code carries a route for it at Section R806.5, and that route comes with conditions. In plain terms:

• The whole attic has to sit inside the insulated shell of the house.
• Insulation that air cannot pass through goes in direct contact with the underside of the roof sheathing. Spray foam is the usual way. Rigid board above the deck is the other way, and the two can be combined.
• If batts or blown insulation are part of the assembly too, that air-tight layer carries a minimum R-value of its own, set by climate zone in Table R806.5. Its job is condensation control. It keeps the sheathing warm enough that moisture does not settle on it. Foam alone against the deck has no minimum in that section.
• Colder zones add conditions about which layer controls water vapor.

Those minimum R-values are not printed here on purpose. They depend on the edition your area enforces, and they belong on the drawings for your house. These come from the model International Residential Code. Your state, and often your city, decides which edition applies where you live and whether it was changed. Your local building department is the only authority on that. And these rules cover new building and permitted work, not a top-up you choose to do on an attic you already have.

So ask for two things in writing before anyone sprays. Which vents are being closed, and which code route the assembly follows. A contractor who cannot answer the second one is not the contractor for this job.

Foam plastic is combustible, and it gives off heavy smoke when it burns. So the model IRC does not let it sit exposed inside a house. Section R316.4 asks for a thermal barrier between the foam and the inside of the building. Gypsum board is the usual one.

Section R316.5.3 allows something lighter in an attic entered only to service utilities. That is an ignition barrier, and on sprayed foam it is often a coating rolled or sprayed over the cured surface, tested and approved for that product. It chars in a fire instead of feeding it.

Three things follow, and all three cost money:

• If your attic is entered only to reach a furnace, a fan or a wire, the lighter route is usually the one your inspector is looking for.
• If the attic is a room, or gets used for storage, the answer changes and the heavier barrier comes into play. Say out loud what you plan to use the space for.
• Which rule applies to your house is your building department's call, not a national one, and it turns on the code edition your area adopted. Ask them before the crew is booked.

Put the covering on the quote as its own line, with the product named. It is the line that goes missing when a bid comes in low.

Foam is mixed on site from chemicals that include isocyanates. That is why the crew wears breathing protection, and it is why you are asked to leave.

EPA guidance on spray polyurethane foam says building occupants and other workers should leave during installation. Re-entry stays restricted until the foam has finished curing, the building has been aired out and the space has been cleaned. On how long that takes, EPA is careful. Some manufacturers recommend 24 hours after application for the two-part high pressure professional system. That covers workers coming back without protective gear, and people moving back in. EPA says the recommended time varies by product, and it points you at the manufacturer or the contractor for your own job.

So ask three questions before a date is booked:

• How long do we have to be out, and what does the maker of this product say?
• How will the attic and the house be ventilated during the work and after it?
• What happens if there is still a smell when we come back?

That last one matters. A smell that hangs around for weeks is not something to wait out. It usually means the foam did not cure right, and the fix is cutting it out. Get that remedy written into the deal before the rig arrives.

The small cans of expanding foam sold at a hardware store are a different product, made for the gap around a pipe rather than for an attic. Some makers put their cure at roughly 8 to 24 hours.

There is no honest amateur route at attic scale. The reasons stack up:

• The rig. Two heated barrels, a pump that holds the ratio, a heated hose and a gun. The small kits sold in a box are not that machine.
• The chemistry. Ratio, hose temperature, surface temperature and a cap on how thick one pass can be. Curing gives off heat, so each pass waits for the last to cool.
• The cost of getting it wrong. Bad foam cannot be fixed by adding more. It comes out.
• The rules. Crews work under OSHA rules for breathing protection. EPA, CDC and NIOSH all publish guidance on exposure to these chemicals.
• The design. On a roof deck the assembly has to be worked out first, not improvised from the hatch.

A real install runs in this order, and you should be able to watch it happen:

1. The attic is walked and measured, on the roof plane if the deck is being sprayed.
2. Wiring, light fixtures, chimneys and flues are found and dealt with. Anything that gets hot needs clearance, and old wiring needs a decision before it is buried.
3. The vent plan is settled and written down, along with the fire covering.
4. The house is masked off, and the crew sets up containment and ventilation.
5. Foam goes on in passes, to a thickness that is on the contract.
6. It is trimmed where it needs to be, then the ignition or thermal barrier goes on.

Two of those steps are the ones that slip. The vent plan gets agreed out loud and never written down, and the fire covering turns up as a change order after the foam is in. Both belong on the contract before the deposit.

This is the part the sales call skips. Loose fill comes out with a vacuum and a long hose. Foam does not. Cured foam is stuck to the wood on purpose. Taking it off means cutting and scraping by hand, in a hot attic, with the debris falling on the person doing it. Closed-cell is the worse of the two, and some of it never leaves the framing at all. Treat it as permanent.

This page publishes no dollar figure for foam removal, because this site holds no measured price for that work. The removal prices it does publish are for blown-in and batts taken out with a vacuum. Read what removal costs as what it is, a price for a different job, and see the removal guide for how that work runs.

A second cost hides behind the first. Once foam is on the roof sheathing, a roofer working from above may have to replace decking rather than lift it. Ask a roofer that before you spray, not after. None of this means do not spray. It means the decision is close to one-way. Loose fill on a floor can be changed in an afternoon with rented gear. Foam on a deck is a change to the building.

Find the row that matches your house and take the answer. If two rows fit, a limit on the building beats a preference.

If no row moved you off the attic floor, the answer is the first row. Air seal, then blow loose fill to the level for your climate zone, and skip foam. How much R-value do I need sets the target, and the attic calculator takes your own square footage and current depth and returns what is left to add.

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Tell us about the attic and your ZIP. We pass it to a licensed contractor in your area who can quote the assembly properly.

A marketing service, not a licensed contractor. Insulation work is performed by independent licensed local contractors.

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.

What happens next?

Your request goes to a licensed insulation contractor near you, usually within about an hour during the day.

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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 land on this page should not buy foam.

R-value does not care what it is made of. A ceiling at R-49 in blown fiberglass slows heat just as well as a ceiling at R-49 in foam. What foam adds on an attic floor is the air seal, and you can buy that on its own for a small share of the price. Caulk, canned foam and rigid board around the tops of the walls, the light cans, the pipes, the wire holes and the hatch, all before the loose fill goes over the top. That is the air sealing job, and on a leaky ceiling it is the better first buy either way.

The money argument is plain. Foam is the highest cost tier of attic materials and blown loose fill is the lowest. On a floor you can reach, the same money spent on sealing plus loose fill usually buys a deeper job, with change left over.

Two more cases where the answer is to spend nothing this year. If you are already within about R-10 of the level for your zone, more depth pays back slowly. That R-10 mark is an estimate rather than a measurement, set at roughly one step of the ENERGY STAR table. And if the attic is dry, clean and simply thin, the cheapest right answer is more of what is already up there. The depth checker turns inches into an R-value range, so measure before you shop.

The R-49 and R-60 levels on this page are ENERGY STAR retrofit guidance for an attic with nothing in it. ENERGY STAR bases that guidance on the model 2021 IECC table. 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.

Foam is sold by the board foot, which is one square foot sprayed one inch thick. So a quote with no thickness on it cannot be checked, and thin spots are invisible from the hatch once the crew has gone. Ask for all of this in writing:

• The product name, whether it is open-cell or closed-cell, and the blowing agent.
• Thickness in inches and the target R-value, for every surface being sprayed.
• The area being sprayed, in square feet. On a roofline job that is the roof plane and the gable ends, which is legitimately more area than your attic floor.
• Which vents are being closed, and what replaces them.
• The fire covering: which barrier, which product, and who inspects it.
• How long the house has to be empty, and what the product maker says about that.
• What happens to the insulation already lying on the attic floor.
• What the contractor will do if the foam smells or shrinks after it cures.

Dollar figures for blowing an attic floor are at the cost guide. They are built up from a measured material price and a stated labor method. Foam is quoted job by job, and this site holds no measured price for it. So compare foam bids against each other, on thickness and R-value, rather than against a national average.

These are the failures that show up months or years later, when the crew is long gone. None of them is a reason to rule foam out. All of them are reasons to buy it carefully.

• Thin spots. You cannot push a ruler into foam the way you can into loose fill. Take photographs during the work, not after it.
• Hidden leaks. Closed-cell sheds water, so a roof leak can travel along the top of the foam and appear a long way from its source, or stay out of sight while the sheathing rots.
• Movement. Rigid foam sprayed onto framing that shifts with the seasons can crack or pull away, and a hairline gap at a rafter edge is an air path you cannot see.
• Buried surprises. Old wiring, recessed lights that are not rated for contact with insulation, and bath fans that vent into the attic instead of outside. All of these get harder to reach once foam is over them. Deal with them first.
• Blowing agents. Closed-cell products in the United States have been moving to blowing agents that do less damage to the climate, under EPA's phase-down of hydrofluorocarbons. Ask which product and which agent you are being quoted.

• Open-cell against closed-cell spray foam
• Spray foam against blown-in, on the same attic
• Every attic insulation material, compared
• Which attic jobs you can do yourself
• Remove the old insulation, or add over it