Does Spray Foam Trap Moisture? The Honest Answer

How moisture gets into walls and roofs

Water reaches a building assembly four ways, and spray foam changes each one differently. A DOE Building America study of spray-foamed roofs, led by Building Science Corporation, looked at the same list of risks.

So the honest answer to "does spray foam trap moisture" is: it traps what is already there, and it stops most of what would have arrived by air.

Vapor permeance: closed-cell vs open-cell

Vapor retarders are graded by permeance, in perms, using the ASTM E96 dry cup test:

• Class I: 0.1 perm or less (a vapor barrier)
• Class II: more than 0.1, up to 1.0 perm
• Class III: more than 1.0, up to 10 perms
• Vapor-open: more than 10 perms

These ranges come from data sheets from three manufacturers, listed under Sources. The exact thickness where a given foam reaches Class II is on its technical data sheet (TDS).

Two things follow. Closed-cell at normal thicknesses is a retarder, not a barrier: it slows vapor a great deal but does not stop it completely. Open-cell lets vapor through easily, which is why it needs a separate vapor retarder in Climate Zone 5 wherever winter vapor drive matters. Closed-cell vs open-cell in Zone 5 covers where each belongs.

Why foam usually lowers condensation risk

Take a January night in Erie: 20°F outside, 70°F and 35% relative humidity inside. That indoor air has a dew point of about 41°F. Any surface it touches that is colder than 41°F collects condensation or frost.

In a 2×6 wall with R-19 fiberglass batts, the inside face of the sheathing sits around 23°F in that weather. Fiberglass doesn't stop air, so indoor air leaking in around outlets and top plates reaches that 23°F sheathing and leaves frost on it. In spring the frost melts into the wood.

Now spray 3 inches of closed-cell foam against the sheathing instead, at about R-7 per inch, a typical aged value for HFO closed-cell (R-value per inch explains the range). The sheathing is just as cold, but no indoor air reaches it, because the foam is bonded to it with no gap. The room-side face of the foam sits around 66°F, far above the dew point. The 41°F line falls about 1.7 inches inside the foam, where there is no air movement and vapor moves at under 1 perm.

These are rough steady-state numbers, but the pattern holds. It is also why about 2 inches of closed-cell foam provides the Class II interior vapor retarder that framed walls in Zone 5 need under IRC R702.7.

When spray foam really can trap moisture

Wet framing

Foam sprayed over wet lumber seals the water in, and the wood then dries slowly through the foam. Manufacturers' limits for wood are typically 18 to 19% moisture content, and the DOE and BSC roof study recommended sheathing and framing below 18% before spraying. A house framed through a wet spring can be wetter than that. A pin-type moisture meter settles it: test several spots, especially low on walls, at plates and anywhere that took rain.

Roof leaks under foam

This is the risk most worth taking seriously. Closed-cell foam on the underside of a roof deck leaves the sheathing between the foam and the roofing, and that wood can't dry much in either direction. A small leak can go unnoticed until the wood is damaged. Because closed-cell foam sheds water, the drip can also travel along the foam or a rafter and show up away from the actual leak.

The DOE and BSC study found no known risk for foam under plywood and OSB roof decks when the roof meets the IRC, the roof covering is leak-free and the sheathing is dry when sprayed. It also warned that bigger or more frequent leaks than they modeled could cause damage. To manage it:

1. Don't foam under a roof near the end of its life. Re-roof first.
2. Have flashing at chimneys, valleys, skylights and vents checked before spraying.
3. When re-roofing a foamed deck, consider a fully adhered underlayment over the whole roof, as the study recommends.
4. Look over the attic once a year and after big storms. Dark stains or drips on the foam are the signs to chase.

Open-cell under a cold roof deck

In Climate Zone 5 and colder, foam under an unvented roof deck has to be a Class II vapor retarder or be covered by one (IRC R806.5), and the DOE study makes the same call for cold climates. Closed-cell meets that by itself. Open-cell, at more than 10 perms, needs a vapor retarder coating on its room-side face. Skip it and winter vapor can build up in the roof sheathing.

Wood sandwiched between two vapor retarders

Closed-cell on the inside of a wall that also has a low-perm layer outside, such as foil-faced foam board, leaves the sheathing almost no way to dry. That can work if the wood starts dry and stays dry, but it leaves no margin. Metal buildings are different: the foam bonds directly to the steel, with no wood or air space between them.

Foam that has pulled away

Foam that shrinks or separates from studs and rafters opens a gap air can use, and that brings back the cold-surface problem. Good foam looks even, smooth and uniform. Rippled, cracked, shrunken or discolored foam is a warning sign. Our guide to choosing a spray foam contractor covers what to look for.

Drying direction in a cold climate

Vapor moves from warm to cold, so in a Western PA winter it pushes outward. Plan where each assembly dries:

• Walls with closed-cell on the sheathing: the sheathing dries outward. Keep the outside vapor-open with housewrap and siding that has some air space behind it.
• Basement walls: concrete dries slowly inward. Keep closed-cell to a moderate thickness and don't add plastic over it. See basement wall insulation.
• Roof decks: dry very slowly in either direction. They have to start dry and stay dry.

Before any foam goes on, run this short check:

• Framing and sheathing read at or below the foam's moisture limit, typically 18 to 19%, on a moisture meter.
• The roof is sound and the flashings have been checked.
• There are no active foundation leaks.
• The assembly has a drying path to at least one side.
• Any foam under a roof deck in Zone 5 is a Class II vapor retarder or gets coated.
• The surface is at least 5°F above the dew point when sprayed, as manufacturers' application guides require.

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