Spray Foam and Moisture: What Every Homeowner Needs to Know

Why moisture matters with spray foam

Water gets into a house as liquid (roof leaks, rain, groundwater) and as vapor carried in the air. A U.S. Department of Energy (DOE) Building America guide notes that air leaks can move water vapor through a house at a rate one or more orders of magnitude greater than vapor diffusing through materials. That’s one reason spray foam’s air sealing is a real moisture benefit.

The catch is drying. Building Science Corporation (BSC) points out that layers meant to keep walls from getting wet often keep them from drying, too, and spray foam can be one of them. The goal is to make sure anything that gets wet can dry out.

Open cell vs closed cell: how each handles vapor and water

Vapor control is measured in perms. Under the International Building Code, a material counts as a vapor retarder at 1.0 perm or less.

• Open cell is vapor open. DOE’s basement insulation guideline puts low-density foam at roughly 15 to 40 perm-inches, and DOE’s spray foam guide notes it can absorb and hold liquid water, up to about one-third of its volume depending on the product.
• Closed cell is far less permeable. DOE’s guideline lists high-density foam at about 1 perm-inch, and its closed cells keep liquid water out.

Neither one is “better” everywhere. Open cell can work well above ground where the assembly can dry. Closed cell is the safer pick anywhere the foam could get wet. Our open cell vs closed cell guide compares cost and R-value, and the spray foam thickness chart shows how many inches of closed cell it takes to act as a vapor retarder.

Condensation and dew point, in plain terms

Warm air holds more moisture than cool air. When moist air touches a surface colder than its dew point, water condenses on it, the way a cold glass sweats in summer. With spray foam, the question is whether the inside face of the foam stays warm enough.

• Too little foam can cause condensation. DOE’s Building America Solution Center shows a roof where closed cell foam wasn’t thick enough to meet code, so the foam surface stayed colder than the dew point of the indoor air and condensation formed on it.
• Thickness matters in cold-climate walls. For “flash and batt” walls (closed cell foam plus batts), DOE’s spray foam guide calls for at least 1½ inches of closed cell, and at least 2 inches in climates above 6,000 heating degree days, to keep the foam’s inner surface above the dew point.
• Cold ducts sweat in humid attics. DOE notes that cooled air leaves the air handler at about 50 to 55°F. If a duct’s outer surface drops below the dew point of humid attic air, it sweats, which is why DOE rates closed cell, not open cell, for insulating ducts.

Can spray foam hide a roof leak?

It can delay when you notice one. DOE’s spray foam guide says a leak under closed cell foam may not be apparent right away, because water won’t pass through the insulation, which makes a leak in a worn roof harder to find. Open cell will get wet but can dry with time and dry conditions.

BSC’s Joseph Lstiburek, who has worked with spray foam under roof decks since the early 1980s, says there’s no compelling evidence that either foam type makes leaks easier to spot. Big leaks always show up; small ones don’t always, whatever the foam. His rule: never spray foam onto a wet roof deck.

In BSC modeling cited by DOE, roofs with either foam dried out seasonally with rain leaks of up to 1% through the sheathing, or with starting wood moisture up to 18%. Wetter wood or repeated, prolonged leaks could cause damage. DOE says to measure the wood before spraying and make sure it has dried below 19%, or the manufacturer’s limit. It also warns against plastic sheeting (a Class I vapor retarder) on the ceiling side of roof-deck foam in any climate, because it blocks drying to the inside after a leak.

Before foam goes under your roof, have known leaks repaired and ask how you’ll inspect the deck later. Our spray foam attic insulation guide covers roof deck versus attic floor.

Humid climates vs cold climates

DOE’s spray foam guide puts it simply: in warm climates, moisture mostly pushes in from outside; in cold climates, it mostly comes from the warm, humid air inside.

Hot and humid (the Gulf Coast, Florida, the Southeast). Air conditioning keeps the inside drier than the outside, so DOE says to avoid interior vapor retarders such as plastic sheeting with spray foam in these climates. The bigger issue is sealed attics with open cell foam. BSC has documented humid unvented attics where moisture from indoor air passes through open cell foam, gets stored in the roof sheathing, and collects near the peak. BSC’s fix isn’t to avoid open cell. It’s to make sure the attic is actually conditioned, with some way to remove moisture.

Cold (climate zones 5 through 8). The code requires foam under the roof deck to be a Class II vapor retarder or to be covered with one, which in practice means enough closed cell, or open cell with a vapor-retarder coating. BSC has written that installers often skip that coating, so check that it’s in your quote. BSC also recommends keeping winter indoor humidity below 35% in climate zones 6, 7 and 8 for spray-foamed roofs.

Crawl spaces and basements

Crawl spaces. A DOE Building America guide says vented crawl spaces in mixed and hot-humid climates tend to get wetter, not drier, and the moisture condenses on cooler floor framing, promoting fungal growth and wood damage. DOE warns that just sealing the vents can make moisture worse, and can create a deadly situation if naturally vented fuel-burning equipment sits in the crawl space. DOE’s checklist calls for:

• checking for standing water and drainage problems first
• covering the entire ground with at least 6-mil polyethylene, seams taped, run at least 6 inches up the walls
• closed cell foam (or sealed rigid foam) on the rim joists
• insulating the foundation walls with spray foam or rigid foam

Basements. DOE’s spray foam guide rates open cell “not acceptable” for foundations and below-grade spaces and closed cell “preferred,” applied directly against the foundation wall so moist indoor air can’t reach the cold concrete. DOE’s basement guideline is blunt: moisture management must be addressed before insulating a basement, because insulation can hide water problems or trap water. Foam in a basement usually needs a thermal barrier such as ½-inch drywall.

Ventilation and dehumidification after the foam goes in

A foamed house is tighter, so moisture made inside has fewer ways out. For sealed attics, DOE’s Building America Solution Center says a way to remove moisture is necessary:

• an exhaust fan at the attic peak, with balanced fresh air
• a heat or energy recovery ventilator (HRV or ERV)
• a dehumidifier in the attic
• supply and return air from the home’s HVAC system

Inside the house, EPA recommends keeping humidity below 60%, ideally between 30% and 50%. A humidity meter costs about $10 to $50 at many hardware stores, according to EPA. Vent clothes dryers and stoves outdoors, and use air conditioning or a dehumidifier when needed. DOE also advises that any fuel-burning appliance in a sealed attic be a direct-vent, sealed-combustion model.

Signs of a moisture problem

• A musty smell with no visible source. EPA says that can point to hidden mold.
• Condensation on windows, walls or pipes, or rust on drain pipes. EPA calls condensation a sign of high humidity.
• Indoor humidity that stays above 60% on a humidity meter.
• Water stains on ceilings or drywall below the roof, especially after rain.
• Damp sheathing or condensation near the peak of a sealed attic.
• Wet ground cover, standing water or damp floor framing in a crawl space.

EPA says wet materials dried within 24 to 48 hours after a leak usually won’t grow mold, so act fast.

Questions to ask a contractor about moisture

1. Open cell or closed cell here, and why, for my climate?
2. Will you check the moisture content of the wood before you spray?
3. Is there a leak, drainage or plumbing problem we should fix first?
4. Is the foam thick enough to keep condensation off its inside surface?
5. Do I need a vapor-retarder coating, and is it in the quote?
6. If you’re sealing my attic or crawl space, how will moisture be removed, and who handles the HVAC side?
7. Are there gas appliances in that space that need attention first?
8. How will I check the roof deck for leaks later?

For the rest of the vetting checklist, see how to choose a spray foam contractor. If you’re weighing a kit for a small area, see DIY vs professional spray foam.

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