Why Spray Foam Installations Fail (and How to Avoid It)

How spray foam is supposed to work

The U.S. Environmental Protection Agency (EPA) describes professional two-component spray foam as isocyanates (the A side) and a polyol blend (the B side) fed through hoses to a spray gun, where they mix and react, expanding to form polyurethane foam. The polyol blend also carries blowing agents, amines, flame retardants and other chemicals.

That means the “factory” is the installer’s rig and the inside of your house. Ratio, temperature, pressure and how thick each layer goes on all happen on site, which is why installation quality matters so much more with spray foam than with batts or blown-in insulation.

The most common ways spray foam goes wrong

1. Off-ratio mixing

The A and B sides have to meet in the right proportion. Research done by the National Institute of Standards and Technology (NIST) for the U.S. Consumer Product Safety Commission (CPSC) lists “off-ratio (A-side to B-side) application” first among the non-ideal conditions suspected behind homeowner complaints, along with low substrate temperature and incorrect nozzle pressure and temperature.

A 2016 paper on misapplied spray foam (Building Dynamics, presented at an ASTM symposium) says foam may fail to fully cure when the contractor doesn’t follow the specified procedures for layer depth, time between layers, ratio, temperature and mixing. It describes misapplied foam as infrequent, but when it happens the telltale sign is a persistent “fishy” odor.

2. Passes sprayed too thick

Spray foam is put on in layers called passes or lifts, and each one gives off heat as it reacts. The Spray Polyurethane Foam Alliance (SPFA), the industry’s trade group, warns that spraying too much per pass without letting the foam cool can cause poor foam quality and even a fire hazard from heat buildup and spontaneous combustion. It notes that closed cell foam holds that heat longer than open cell.

Building Science Corporation (BSC) explains that too-thick passes can leave the core of the foam too low in density and dimensionally unstable, so it cracks or pulls away later under thermal stress, often in cold weather, months after the job. BSC stresses that “too thick” refers to each pass, not the total, and that the maximum per pass is set by the manufacturer, with typical lifts between 2 and 5 inches.

Going too thin causes problems too. SPFA notes that open cell foam sprayed too thin may not generate enough heat to form good foam.

3. Wrong temperatures

Each foam product has a temperature range for the chemicals, the air and the surface it’s sprayed on. SPFA’s best-practice guide tells installers to check ambient and substrate conditions against the manufacturer’s data sheet before spraying, and to run a small test “bun” each day to confirm the foam is coming out right. NIST’s list of non-ideal conditions includes spraying onto a substrate that is too cold. BSC adds that a too-hot surface, such as a sun-baked roof in summer, makes heat buildup worse.

4. Shrinkage and pull-away from framing

Foam that shrinks can open gaps along studs, rafters and window openings, which defeats the air sealing you paid for. BSC recommends “picture framing” each bay, spraying a fillet of foam around the edges before filling the middle, and says it is especially important around windows and flashing. SPFA’s tech tip on high-lift foams says passes thicker than the manufacturer’s maximum often shrink, and that the foam may have to be removed.

5. A wet or dirty surface

Foam needs a clean, dry surface to bond to, and SPFA tells installers to confirm substrate conditions before every job. The U.S. Department of Energy (DOE) says to measure roof sheathing before spraying and make sure it has dried below 19% moisture content, or the manufacturer’s limit. Our spray foam and moisture guide covers how wet wood and hidden roof leaks behave under foam.

6. Missing thermal or ignition barriers

Foam plastic insulation is combustible, so building codes require it to be covered. Under the International Residential Code (IRC, section R316.4), foam has to be separated from the living space by a thermal barrier such as ½-inch gypsum wallboard, unless an exception applies. In an attic or crawl space that people enter only for repairs and maintenance, the code allows a lighter ignition barrier instead, such as ⅜-inch gypsum board, ¼-inch wood structural panel or 1½ inches of mineral fiber insulation, or a coating or assembly that has been specifically tested and approved.

North Carolina’s State Fire Marshal has ruled that a floored storage area in the attic means the space is no longer “repairs and maintenance only,” so the lighter ignition barrier no longer qualifies there. If you store things in your attic or plan to finish it, tell your installer before you sign, and check with your local building department, since codes vary by state and city.

7. Moving back in too soon

EPA says nobody should go back into the work area until the foam has finished curing and the space has been ventilated and cleaned. It notes that some manufacturers recommend 24 hours for professional two-component foam, but the time varies by product, and it tells homeowners to get the specific re-occupancy time from the contractor or manufacturer. EPA’s ventilation guidance also calls for sealing off HVAC openings during spraying so fumes don’t spread through the house.

Warning signs after the job

• A strong or lasting odor. Some smell right after installation is normal. The Building Dynamics paper says an odor that starts strong or lasts beyond a few weeks suggests misapplied foam.
• Gaps, cracks or foam pulling away from studs, rafters or window frames.
• Dark, scorched or discolored foam in the middle when it’s cut open. A building inspector writing for the Journal of Light Construction (JLC) says this points to too much heat during the reaction and can lead to cracks and shrinkage.
• Uneven thickness. The same JLC inspector measures foam depth in at least 15 spots to confirm it meets the specified thickness.
• Breathing problems or other symptoms that start after the job.

What to do if you think your foam was installed wrong

EPA and CPSC give homeowners the same steps:

1. If anyone has breathing problems or other health effects, get medical help right away.
2. Contact the contractor and the foam manufacturer and ask for help finding the problem.
3. Consider hiring an independent indoor air quality (IAQ) consultant to diagnose it.
4. If that doesn’t resolve it, contact your state or local consumer protection office or the contractors’ licensing board.
5. File a report with CPSC at SaferProducts.gov.

Don’t rush to tear the foam out yourself. EPA says it isn’t aware of standard removal practices, that removal may not fix the problem because odors can migrate into other materials, and that cutting or heating foam can create new hazards.

How to avoid a failed install

Most of these failures come down to process, so ask about process before you hire:

• Which product will you use, and can I see its data sheet? It lists the per-pass thickness limit, temperature range and re-occupancy time.
• Who will be spraying, and what training do they have? Our guide to choosing a spray foam contractor covers licensing, insurance and certifications.
• How thick will each pass be, and how long do you wait between passes?
• Do you check the temperature and moisture of the surface, and do you spray a test bun each day?
• What thermal or ignition barrier does my space need, and is it in the quote?
• How will you ventilate the space, and when can we move back in?
• Will you show me the finished foam and measure its thickness before final payment?

If you’re still choosing between foam types, see open cell vs closed cell spray foam and the spray foam thickness chart. For roof decks and attic floors, see our spray foam attic insulation guide.

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