Spray foam insulation: open-cell vs closed-cell, where each works and how to stay safe

What spray foam insulation is and the product types

Spray foam starts as two liquids. The A side contains isocyanates (in building foam, usually polymeric MDI). The B side is a polyol blend with catalysts, flame retardants, blowing agents and surfactants. They meet at the spray gun, react with heat, expand and set into foam that fills gaps, cracks and voids.

Open-cell vs closed-cell spray foam

The two foams are chemically similar but behave so differently that DOE's Building America guidance treats them as separate products that are not interchangeable.

A vapor retarder is defined as 1 perm or less, so open-cell foam is not one. That single fact drives most of the decisions below.

Where each type of spray foam works best

DOE's Building America guide rates each foam for common locations. A summary:

For the floor of a vented attic, blowing loose-fill over a well-sealed ceiling is the usual approach: see blown-in insulation and attic insulation. For crawl spaces, read crawl space encapsulation.

Flash-and-batt is a common hybrid: a thin layer of closed-cell foam against the sheathing for air and vapor control, with batts or loose-fill filling the rest of the cavity. DOE's guide recommends at least 1½ inches of closed-cell in this setup in cold climates, and at least 2 inches in climates above 6,000 heating degree days, so the inner face of the foam stays warm enough to avoid condensation.

Air sealing, moisture and vapor: getting it right

Spray foam's strength is stopping air leaks, which other insulation lets through. Its risk is moisture trapped in the wrong place. Points from DOE's guide:

• Open-cell in cold climates needs interior air sealing and a vapor retarder at the finished surface, such as vapor-retarder primer. DOE cites a reported failure in northern Vermont where open-cell walls had neither, behind an exterior that could not dry.
• Closed-cell in hot-humid climates: avoid adding a plastic vapor barrier on the inside, or moisture can be trapped between two barriers.
• Roofs: with closed-cell foam on the roof deck, a slow roof leak may go unnoticed because water cannot pass through the foam. Fix any roof leak before foaming and keep an eye on the roof afterwards.
• Open-cell under a roof gets wet when the roof leaks, then dries out slowly once the leak is fixed.

In the UK, spray foam in lofts has caused difficulties for homeowners: a Scottish Parliament briefing notes that some lenders refuse mortgages or equity release where it is present, and that foam can hide problems from surveyors and block ventilation if installed badly.

Fire safety and building code

Both foams are combustible. Building codes in the US generally require spray foam in living spaces to be covered by a 15-minute thermal barrier, which DOE's guide equates to ½-inch gypsum board. In attics and crawl spaces entered only for servicing, an ignition barrier may be enough, or exposed foam may be allowed if that exact product has been tested for it. Ask the contractor for the product's evaluation report and how they will meet the barrier rule in your space.

Health and safety during and after installation

EPA's guidance for homeowners, step by step:

1. 01

   Before the day

   Ask the contractor and the manufacturer for the ventilation time, re-entry time and re-occupancy time for this product. Some manufacturers recommend 24 hours for professional two-component foam, but times vary.

2. 02

   Plan to be away

   Everyone without protective gear leaves during spraying, including children, pets and other trades. Children are especially vulnerable because they play close to floors where dust and residue settle.

3. 03

   Seal the work area

   The crew should shut down the heating and cooling system and seal vents so vapors stay out of the rest of the house.

4. 04

   Let it cure fully

   Foam can look hard while still curing. Temperature, humidity, thickness and mixing all affect cure time.

5. 05

   Ventilate and clean

   Re-entry should wait until the foam has cured and the area has been ventilated and thoroughly cleaned.

6. 06

   Check for problems

   Strong lingering odors or symptoms are a reason to call the contractor back. If that does not resolve it, contact an independent indoor air quality consultant, your state consumer protection office or contractor licensing board, and report it to the CPSC at SaferProducts.gov.

Cured foam is considered relatively inert. The risks come back when it is drilled, sanded, ground, heated or burned, so tell any electrician, plumber or builder working near it, and find out whether a house has spray foam before renovating.

DIY spray foam kits vs hiring a professional

• Cans of one-component foam are fine for sealing small gaps around pipes, wires and window frames. Use eye protection, nitrile gloves and good ventilation, keep cans away from children, and let the foam cure before trimming it.
• Low-pressure two-component kits are sold for small jobs such as a few rim joist bays. EPA calls for full skin coverage, eye protection, chemical-resistant gloves and a properly fitted respirator, and warns that DIY users often underestimate these hazards. Mixing errors also slow curing and can leave odors.
• High-pressure rigs are for trained professionals only.

For whole attics, roof decks and walls, hire a professional and ask:

• Which product, open- or closed-cell, and what thickness and R-value will I get?
• How will you handle vapor control for my climate and this location?
• How will the foam meet the thermal or ignition barrier requirement?
• What is the re-occupancy time, and how will you ventilate and clean before we return?
• What training has the crew completed, and what paperwork will you leave? In the UK, the briefing advises keeping test certificates, contracts and guarantees for when you sell.

When spray foam is a poor choice

• Open-cell foam below grade, in basements or crawl spaces.
• Anyone in the home has asthma or a known isocyanate sensitivity, and alternatives would do the job.
• The roof leaks or the timbers need checking: fix and inspect first, because foam hides the deck.
• A simple attic top-up: loose-fill over a sealed attic floor is usually the straightforward answer.
• You may need to remove it later: EPA notes that removal may fail to solve a problem and can create new ones.
• The attic has vermiculite: leave it undisturbed and read asbestos testing first.

Sources

Official guidance on spray foam, checked on 27 October 2026.

• US DOE Building America · Which spray foam is right for you? ↗Density, R-value per inch, expansion, vapor permeance, water absorption, fire barriers and where each foam fits.
• US EPA · Spray polyurethane foam product types ↗High-pressure, low-pressure and one-component products, protective equipment and chemistry.
• US EPA · Vacate and safe re-entry time for spray foam application ↗Leaving during installation, re-occupancy, cure times, children and where to report problems.
• US EPA · Health concerns about spray polyurethane foam ↗Isocyanates, sensitization, asthma and who is most at risk.
• US EPA · Potential chemical exposures from spray polyurethane foam ↗Exposure during and after installation, heating or sanding cured foam, odors and removal.
• Scottish Parliament Information Centre · The use of spray foam insulation in Scotland ↗UK lender concerns, roof inspection, ventilation and keeping paperwork.