Is Spray Foam Waterproof? Can It Get Wet?

Is spray foam waterproof? Closed-cell vs open-cell

"Waterproof" means water can't get through, under pressure, for as long as it's there. Insulation foam isn't sold that way. The useful question is how each type behaves when it gets wet, and the two types behave very differently.

Closed-cell foam weighs about 2 pounds per cubic foot, and 90% or more of its cells are sealed bubbles. Water can wet the surface and fill the cells cut open there, but it has a hard time getting deeper. Open-cell foam weighs about half a pound per cubic foot, and nearly all of its cells connect to each other. That's why it's soft and lets vapor through easily.

What the data sheets say

Manufacturers test water absorption with ASTM D2842, which soaks a sample under 2 inches of water for 96 hours. Vapor permeance comes from ASTM E96. Here is what four current data sheets list:

Read the absorption column with care. ASTM says the results only describe real conditions that look like the test: 96 hours under 2 inches of water. Some foams keep absorbing after 96 hours, and deeper water pushes more in. These are comparison numbers, not a promise about a basement that sat flooded for a week.

The open-cell number surprises people. In the same four-day soak, that open-cell foam took on 6.7%, not far above the highest closed-cell value. In a soak test, at least, open-cell foam doesn't fill up like a kitchen sponge. Where the two types really split is in how they're rated for floods and what the industry says to do with them afterward, both covered below.

The vapor column matters for everyday moisture rather than floods. Closed-cell becomes a Class II vapor retarder (1 perm or less) at about 1½ to 2 inches, depending on the product, while open-cell stays vapor-open. Does spray foam trap moisture? explains what that means for walls and roofs.

What FEMA's flood rating actually says

FEMA's Technical Bulletin 2 lists which materials are acceptable below the flood elevation in mapped flood hazard areas. The current edition, from January 2025, defines a flood damage-resistant material as one that can take direct contact with floodwater for at least 72 hours without damage that needs more than cosmetic repair. Cleaning and sanitizing count as cosmetic.

Its insulation ratings:

• Acceptable: sprayed polyurethane foam or closed-cell plastic foams.
• Unacceptable: fiberglass and mineral wool (batts, blankets or blown), and all other types, including cellulose, cotton and open-cell plastic foams.

Some data sheets, including one revised a month after that bulletin came out, still advertise "FEMA Class 5." That label comes from the 2008 edition, which graded materials from Class 1 to Class 5. The 2025 edition dropped the classes and simply rates materials acceptable or unacceptable. Closed-cell foam passes either way.

Two limits are easy to miss. The rating covers the foam, not the whole wall: the same bulletin rates paper-faced drywall unacceptable. And FEMA says its ratings don't account for very long floods or for contaminants such as home heating oil.

Spray foam is not a roof, flashing or waterproofing

The closed-cell foam sprayed in an attic or a pole barn is insulation. Roofing foam is a different system. A roof guide published by the Insurance Institute for Business & Home Safety describes spray foam roofs as 2.5 to 4.0 lb per cubic foot foam, covered with a UV-protective coating or aggregate because sunlight breaks foam down. The coating has to be renewed every 10 to 20 years. One major manufacturer's data sheet for its closed-cell insulation foam is blunt: it "is NOT designed for use as an EXTERIOR roofing system."

So foam under a roof deck doesn't fix a leaking roof. It doesn't stand in for shingles, underlayment, or the flashing at chimneys, valleys, skylights and vents. Water that gets past those finds the deck before it ever reaches the foam.

The same goes below grade. Foam on the inside of a basement or crawl space wall is insulation and an air seal, not a cure for water coming through the foundation. Fix gutters, grading, drainage and cracks first. See basement wall insulation and crawl space spray foam.

How spray foam can hide a roof leak

This is the risk worth planning for. Foam on the underside of a roof deck covers the sheathing, so wet wood behind it can't be seen. The two types hide a leak in different ways:

• Closed-cell won't soak up the water, so the water can run along the joint between foam and sheathing, then down a rafter, and drip out well away from the hole. Or it can sit against the wood and show nothing at all.
• Open-cell can take water into the foam around the leak. Depending on how much and for how long, that may show up as a damp or stained patch, or not at all.

Neither type is a leak detector, so the roof has to be sound before anyone foams under it:

1. Don't foam under a roof near the end of its life. Re-roof first.
2. Have the flashing at chimneys, valleys, skylights and vents checked before spraying.
3. Look over the attic once a year and after big storms. Stains, drips, or foam that looks wet or discolored are the signs to chase.

For the full picture on roof decks, read sealed attic with spray foam.

What to do after a leak or a flood

1. Stop the water. Fix the roof or flashing, or get standing water out.
2. Find the wet wood. Foam hides it. An installer can cut a small plug of foam, check the wood behind it with a moisture meter, and patch the hole once things are dry.
3. Remove what can't be saved. The spray foam industry's flood guidance says to strip wet drywall, fiberglass and other fibrous insulation, and open-cell foam.
4. Keep closed-cell foam that is still bonded. In one manufacturer's 2017 case study, a basement sat in 4 feet of water for 5 days. Lab tests on cut-out samples found the closed-cell foam's properties intact. The drywall came off, dehumidifiers and fans ran for 5 days, and studs embedded in the foam got extra drying time before new drywall went up.
5. Dry before you close up or re-foam. New foam needs a clean, dry surface to stick. One major data sheet caps wood at 19% moisture content for spraying.
6. Treat dirty water as a different job. FEMA's ratings don't cover contaminants such as heating oil. If sewage or oil got in, bring in a cleanup professional.
7. Put the cover back. Foam facing living space needs a thermal barrier, such as ½-inch drywall. See thermal barrier vs ignition barrier.

Questions about foam that got wet, or a new project? Call (814) 208-3478, or Price My Project.