Spray foam vs blown-in, on the same attic

An attic hands you two surfaces. The floor, which is the top of the ceiling below. And the roof deck above your head. Loose fill can only go on the floor. Foam can go on either, and it is nearly always sold for the roof.

Put loose fill on the floor and the attic stays vented. Outside air comes in at the soffits and leaves near the ridge, so the attic runs close to outdoor temperature and damp air gets carried away. Spray the roof deck instead and the insulated shell of the house moves up. The attic is now indoor space, near indoor temperature, and the vents have to close.

Depths are computed from the aged R per inch ranges in this site's reference data, best case to worst case. Loose fill does not scale cleanly with depth, because it packs down under its own weight, so treat those inches as an estimate and read the bag label for the product you buy. Material prices were read at one retailer on 2026-08-11 and are shelf prices with no labor in them.

Moving the insulated shell up to the roof pulls four other things along with it. The roof can no longer dry by being washed with outside air, so it has to dry inward. A furnace or water heater that vents on its own draft is now inside a tighter house and has to be rechecked. Foam plastic is combustible, so the model code asks for a covering over it, and that covering is a real line on a real bill. And the old insulation lying on the attic floor now sits inside the shell, doing nothing. Remove or add over runs that test. The spray foam page carries the code detail.

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, take the first row. Seal the ceiling, then blow loose fill to the level for your climate zone. 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. Those levels are ENERGY STAR retrofit guidance, which is based 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.

Material is the part with a receipt, so start there. Every price below was read off a live product page at one retailer on 2026-08-11, then turned into dollars per square foot of ceiling at R-30. That target is used because it is the one R-value each product publishes its own coverage at. A package price on its own says nothing, because a bag of loose fiber and a foam kit do not cover the same area.

So foam material runs about 8.7 to 10.2 times blown fiberglass for the same R-value, before anybody is paid to spray it. The kit maker itself calls its yield "theoretical", which is a maker saying real yield is lower. No two-part open-cell kit was found at retail, so open-cell has no price here at all.

Installed, by a crew

One of these two jobs can be priced from parts rather than guessed at. Blowing fiberglass into a 1,200 square foot attic to R-49 comes out at $2,622 to $3,190, or $2.18 to $2.66 a square foot. That is built from the BLS mean wage for insulation workers, May 2025, times a burden multiplier of 2 to 3, plus 9 to 12 crew-hours per 1,000 square feet, plus material at shelf price. The multiplier and the crew hours are estimates, not measurements. The blown-in cost page takes that apart.

There is no matching installed figure for foam here, and none is invented to fill the gap. Foam is quoted job by job and this site holds no measured installed price for it. What you can do is check a bid on its own terms. Foam is sold by the board foot, which is one square foot sprayed one inch thick. Multiply the surface area by the inches. Closed-cell at R-49 is 7 to 8.2 inches, so the 1,200 square foot reference attic would be roughly 8,400 to 9,840 board feet. A sloped roof plane is more area than the floor under it, so a roof deck job is quoted on the larger number. Compare foam bids against each other on thickness and R-value, not against a national average.

Here is the part that decides the money. Foam seals air because it sticks to whatever it hits and fills the gap around a wire or at a rafter. Loose fill lies over a hole and the hole keeps working. That is a genuine advantage, and it is also the one thing you can buy separately.

The U.S. Department of Energy's Building America Solution Center puts attic work in a fixed order: seal the ceiling plane, insulate, then ventilate. Its guidance says the most important step is to seal all of the air leaks from the heated space into the attic. Sealing costs little in materials and a lot in patience. Caulk, cans of foam and rigid board at the top plates, the hatch, the plumbing stack, light cans, wire holes and duct boots.

Do that, then blow loose fill over the top, and you have bought both halves of what foam does. Most of the benefit, for a fraction of the price. The air sealing guide lists what leaks and in what order. This site publishes no total price for air sealing, because it holds no measured one. What it can say is that the materials are shelf items and the cost is mostly hours.

Two things still have to be right when the loose fill goes in. Keep at least 1 inch of clear space between the insulation and the roof sheathing at the eaves, which is what baffles are for. And leave the vents alone. 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.

Every roof leaks eventually, and the two approaches behave very differently on the day yours does. This is the difference people find out about years later, so it is worth reading before you sign anything.

• Loose fill in a vented attic. Water drips through and the ceiling stains, so you find out. Wet loose fill mats down and stops working. Cellulose holds water longer than fiberglass. Either way it comes out with a vacuum and the deck can be looked at.
• Open-cell foam at the roof deck. Water passes through it and shows below, which at least tells you. The soaked foam then stays wet for a long time.
• Closed-cell foam at the roof deck. It sheds water. A leak can run along the top of the foam and appear far from its source, or stay out of sight while the sheathing takes damage.

There is a second moisture effect and it is easy to miss. A vented attic dries because outside air washes the underside of the roof. A sealed, foamed roof cannot do that. It has to dry inward, which is why an unvented attic is a designed assembly with conditions attached rather than a product choice. 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. If what is up there now is already wet or has mold on it, neither material goes in until that is fixed. Start at mold in attic insulation.

This is the case where foam earns its price on the numbers rather than on the shape of the roof. If the ducts or the air handler are in the attic, they sit outside the insulated shell of your house. Every foot of duct is losing heat to a space that runs near outdoor temperature. Every leaky joint is either blowing air you paid to heat into the attic, or pulling attic air into the system.

Blowing more depth on the attic floor does nothing about that. It makes the attic colder in winter and hotter in summer, which makes the duct losses slightly worse. Foam at the roof deck moves the boundary above the equipment, so those losses land inside the house instead of outside it. That is a real change and depth cannot copy it at any thickness.

The cheaper things to try first are sealing the duct joints properly, insulating the ducts themselves, or moving the equipment. None of those is free and the last one rarely is cheap. If the ducts stay in the attic and the losses are large, the roof line is where the insulation belongs. Ask for the assembly to be designed, not improvised at the hatch. What insulation does to a bill explains why leaks and ducts often outweigh depth.

Reversibility is the quietest difference and one of the largest. Blown-in comes out with an insulation vacuum: a truck-mounted machine, a long hose through the hatch or a gable vent, and bags at the other end. It is a morning's work on a clean attic. How insulation vacuums work covers it, and what removal costs prices it.

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. Treat it as permanent. This site publishes no dollar figure for foam removal, because it holds no measured price for that work.

That carries into your next roof. Loose fill on the floor is untouched by a reroof, because a roofer works from above and never meets it. Foam bonded to the underside of the sheathing is a different situation, and a roofer may end up replacing decking rather than lifting it. Ask a roofer that question before you spray, not after. If the roof is near the end of its life, reroof first either way.

We do not sell insulation and we do not install it, which is why this section can say what it says. Most people asking this question have an ordinary vented attic with an open floor, and for them the honest answer is the cheaper one. Not because foam is bad. Because the expensive half of what foam does is buyable on its own.

Before you buy anything, measure. If your attic is already within about R-10 of the level for your climate zone, more depth pays back slowly and the money is better spent on sealing. That R-10 mark is this site's estimate rather than a measurement, set at roughly one step of the ENERGY STAR table. The depth checker turns inches of what you have into an R-value range, so you know where you stand before you shop.

Three more cases where the answer is to spend nothing on either material this year. The attic is dry, clean and simply thin, in which case more of what is already up there is the cheapest fix. Or the ceiling has never been sealed, in which case sealing alone may be the whole job. Or you have one cold room and no diagnosis yet, in which case find the cause first. The signs guide starts that.

The two jobs are quoted in different units, which is why they are so hard to hold side by side. Ask for these and the comparison gets easy.

A blown-in bid

• The target R-value and the settled depth in inches, both written down.
• The material, the product name, and the coverage per bag it is priced on.
• Air sealing as its own line, or a plain statement that it is not included.
• Baffles at the eaves, and what happens at the hatch.
• Whether anything is being taken out first, and what the disposal costs.

A foam bid

• Open-cell or closed-cell, the product name, and the blowing agent.
• Thickness in inches and the target R-value, for every surface being sprayed.
• The surface area in square feet, which on a roof job is the roof plane and the gables.
• Which vents are being closed, and which code route the assembly follows.
• The fire covering: which barrier, which product, and who inspects it.
• How long the house has to be empty, and what the maker of that product says.
• What happens to the insulation already on the attic floor.

A foam quote with no thickness on it cannot be checked, and thin spots are invisible from the hatch once the crew has gone. Photographs during the work are worth more than a promise after it.

Blown-in

• Blown over an unsealed ceiling it underperforms its label, in both seasons.
• Blown carelessly it blocks the eaves, which starves the vents and can start a moisture problem the insulation then gets blamed for.
• It packs down over the years, so an old attic is usually shallower than it was.
• It cannot follow a sloped ceiling, and it does nothing for a knee wall.

Spray foam

• Sprayed thin in spots, and you cannot push a ruler into it to check.
• Sprayed off-ratio or too cold it can shrink, hold a smell, or never fully cure.
• It buries the things you should have dealt with first. Old wiring, light cans not rated for contact with insulation, and bath fans that vent into the attic.
• The fire covering turns up as a change order after the foam is in, if it was not on the contract.

Neither list is a reason to rule a material out, and both are reasons to put the scope in writing before a deposit changes hands. Which attic jobs you can do yourself covers who does what.

• Spray foam attic insulation, on its own
• Open-cell against closed-cell foam
• Blown-in against batts, for an attic floor
• Cellulose against fiberglass
• Air sealing an attic, which comes first either way
• Every attic insulation material, compared