Spray Foam Insulation Pros and Cons: An Honest List

Short answer

The real pros of spray foam insulation: it air-seals and insulates in one layer, closed-cell foam gives about R-6 to R-7.5 per inch and doubles as a vapor retarder, and it fills odd-shaped spaces that batts can't. The real cons: it costs far more than blown insulation, its quality depends on the crew mixing it on site, you have to leave the house, it's hard to remove, it can hide a roof leak, and it needs fire-code coverings. It does not keep pests out.

Key points

Unlike batts or blown insulation, spray foam is an air barrier on its own. Closed-cell foam also controls vapor at about 1.5 to 2 inches and stiffens wood framing.
On an open, vented attic floor, blown cellulose or fiberglass reaches the same R-value for a fraction of the price.
Foam is made on site from two liquids. A bad install is the biggest risk, and it doesn't get better with time.
Closed-cell foam on a roof deck can hide a leak. Fix the roof first and keep it maintained.
It's a combustible plastic that needs a code-approved covering, and it is not a pest barrier.

Spray foam insulation pros and cons at a glance

Topic	The pro	The con
Air leakage	Fills gaps and bonds to framing; an air barrier by itself	Only works where it's sprayed
R-value	Closed-cell about R-6 to R-7.5 per inch; open-cell about R-3.6	Open-cell needs about twice the depth
Moisture	Closed-cell sheds water and is a Class II vapor retarder at about 1.5–2 in.	Closed-cell slows drying; open-cell can soak up water
Cost	Insulation, air sealing and (closed-cell) vapor control in one trip	Many times the cost of blown insulation on an open attic floor
Installation	Fills rim joists, irregular cavities and metal buildings	Quality depends on the crew; occupants leave for at least 24 hours
Years later	Doesn't settle or sag	Hard to remove; covers the roof deck and framing
Fire	Made with fire retardants	Combustible; needs a thermal or ignition barrier
Climate	Today's HFO blowing agents have GWPs of 2 to 4	Older HFC agents had GWPs of 794 and 1,030
Pests	No real advantage	Not a pest barrier; pests can chew through it

The real pros

It air-seals and insulates in one step

R-value is measured on the material in a lab. It says nothing about air moving through gaps at top plates, rim joists and wire holes. Spray foam expands into those gaps and bonds to wood or steel. The Department of Energy notes it "can fill even the smallest cavities, creating an effective air barrier."

Fiberglass doesn't stop air, so a batt job needs separate air sealing to perform as rated. See air sealing before insulation.

More R-value per inch

The HFO closed-cell data sheets we checked list aged values of about R-7.2 to R-7.5 per inch, and one drops to R-6.3 for a 1-inch layer. A DOE Building America guide used R-6 for typical closed-cell foam of its day and about R-3.6 for open-cell. That matters where depth is limited: cathedral ceilings, 2×4 walls, rim joists. More in spray foam R-value per inch.

Closed-cell handles water and vapor

The same DOE guide calls closed-cell foam hydrophobic, says it "will not absorb water," and lists it under 1 perm at 2 inches. Manufacturers' data sheets put the Class II vapor retarder point at about 1.5 to 1.9 inches. In climate zone 5 that's the vapor retarder framed walls need (closed-cell vs open-cell in zone 5). Open-cell is vapor-open and can hold liquid water, so DOE says to keep it above ground level.

It adds strength

DOE's guide puts closed-cell at about ten times the compressive strength of open-cell and says "the racking strength of frame walls is substantially increased" when closed-cell fills the cavities. Open-cell is soft and adds little.

It fits odd spaces and allows an unvented attic

Foam flows around wiring, pipes and irregular framing, and it follows the ribs of metal siding. With enough foam against the roof deck, the IRC's unvented attic section (R806.5) lets the attic and its ductwork become part of the heated house. See sealed attic with spray foam.

The real cons

It costs more, sometimes much more

Take R-49 over 1,000 square feet. At R-7 per inch, a typical aged value for HFO closed-cell, that's 7 inches, or 7,000 board feet: $10,500–$15,750 at FoamGuard's public guidance of $1.50–$2.25 per board foot installed. Blowing cellulose or fiberglass to R-49 on a 1,000-square-foot attic floor costs a small fraction of that. The numbers are in what spray foam insulation costs.

DOE says it plainly: "Foam insulation products and installation usually cost more than traditional batt insulation."

The crew is making the product in your building

Batts arrive finished. Spray foam is mixed from two liquids at the gun, and how it turns out depends on the mix ratio, temperatures, the surface and how thick each pass goes on. Off-ratio or misapplied foam can stay sticky or crumbly, shrink away from framing and keep smelling (off-ratio spray foam).

A seven-year Arizona State University study of 37 spray foam roofs found that "the contractor and the season when the roof was installed did affect the quality of the roofs."

Choosing the contractor is how you control this risk. Our contractor guide lists what to check.

You have to leave, and odor is possible

CPSC staff guidance says residents should leave for at least 24 hours, and that the right time depends on the product. A smell that fades in a day or two is normal. One that lingers is a sign something went wrong (spray foam smell, is spray foam insulation safe).

It's hard to take back out

Foam bonds to everything it touches. Taking it out means cutting and scraping by hand, and CPSC staff note there are no standard processes for removing misapplied foam. Running a new wire or pipe through a foamed wall is slower than through batts. If you plan to gut a room soon, foam it after.

It covers the roof deck

DOE's guide warns that "when leaks occur in ccSPF they may not be immediately apparent as the water will not penetrate the insulation," and that on a worn roof foam "will make the leak more difficult to locate." From the attic you see foam, not sheathing, and so does a home inspector, which can raise questions when you sell (selling a house with spray foam).

One layer of closed-cell foam on a roof deck: what it does well, and what it costs you.

Fix leaks before foaming, and if the shingles are near the end of their life, reroof first.

It's a combustible plastic

DOE notes that both open-cell and closed-cell foam "are combustible" and are made with fire retardants. In living space, code requires a thermal barrier equal to 1/2-inch gypsum board, and attics and crawl spaces have their own ignition-barrier rules. Make sure the covering is priced with the foam. See thermal barrier vs ignition barrier and is spray foam flammable.

The blowing agent's climate footprint

Closed-cell foam is expanded by a gas that stays in its cells. EPA lists two older HFC blowing agents at global warming potentials (GWP) of 1,030 (HFC-245fa) and 794 (HFC-365mfc), and the HFOs that replaced them at 2 and 4. Federal rules now limit new foam systems to a GWP of 150. Ask which product is in the drums (HFO vs HFC spray foam).

It does not keep pests out

Spray foam is not a pest barrier, and anyone selling it as one is overselling it. The polyurethane industry's own homeowner FAQ says "some pests may chew through the foam over time." Mice, insects and termites need their own fix: openings closed with metal, mesh or mortar, and pest control where it's needed.

Spray foam insulation problems: where they come from

Most of the reasons people give for why spray foam is bad trace back to four things:

A bad install. Off-ratio foam, passes sprayed too thick, or foam sprayed on wet or frosty surfaces.
The wrong place. Foam where blown insulation would have done the job for less, or open-cell foam where it can get wet.
Water behind the foam. A roof or plumbing leak that closed-cell foam keeps out of sight.
Rushed re-entry. Skipped ventilation or people back inside too soon.

None of these means the material can't work. Each is a reason to be choosy about where foam goes and who sprays it. To check an existing job, see how to spot bad spray foam.

When spray foam is worth it, and when it isn't

Usually worth it	Usually not worth it
Rim joists	An open, vented attic floor with nothing in the attic
Basement and crawl space walls	Interior walls, where the goal is sound
Roof decks with ducts or equipment in the attic	A roof that leaks or is due for replacement (fix it first)
Cathedral ceilings and other shallow cavities	Walls you plan to open up in a year or two
Metal buildings and pole barns	Spaces with an active water problem (fix the water first)

For right-column jobs, put the money into air sealing and cheaper insulation, or into fixing the water. For how long a good install lasts, see how long does spray foam insulation last.

Want a number for your own project? Price My Project or call (814) 208-3478.

FAQ

Is spray foam better than fiberglass?

Where you need air sealing, vapor control or a high R-value in a shallow space, usually yes. On an open attic floor, usually not. The full comparison is in spray foam vs fiberglass vs cellulose.

Does spray foam cause rot or mold?

Correctly installed foam in a sound assembly doesn't. Rot starts with water that gets in and can't get out, such as a roof leak behind closed-cell foam or open-cell foam in a wet spot. See does spray foam trap moisture.