Polymers

Open Cell vs Closed Cell Spray Foam

Gas-filled and rigid, or air-filled and spongy. The trade is R-value against cost.

Reviewed July 2026

Open cell vs closed cell spray foam is a question about one material in two forms. Both are polyurethane, a thermoset foam insulation whose cells hold a gas. The difference is what fills those cells and how tightly they are packed, and everything else, the R-value, the rigidity, the vapor behavior, and the price, follows from that single structural fact.

What the two words mean

The US Department of Energy describes it directly. In closed-cell foam, the high-density cells are closed and filled with a gas that helps the foam expand to fill the spaces around it. Open-cell foam cells are not as dense and are filled with air, which gives the insulation a spongy texture and a lower R-value. That is the entire distinction, and the rest of this guide is consequences.

Open cell vs closed cell at a glance

We have deliberately not printed an R-value per inch for either foam. Manufacturers publish tested figures for their own products, and those are the numbers to build with. What DOE establishes is the direction: closed cell is higher.

Thermal drift, the wrinkle nobody mentions

Closed-cell foam owes its high R-value to the low-conductivity gas trapped in its cells, and that gas does not stay put forever. As some of it escapes and air replaces it, the R-value falls. This is called thermal drift or ageing, and DOE notes that most of it happens within the first two years after the insulation is manufactured, after which the R-value remains unchanged unless the foam is damaged. Rigid foam boards can slow the process with foil or plastic facings, and a reflective foil facing an open air space acts as a radiant barrier that adds roughly R-2 on its own.

Open-cell foam has no such problem, and for a slightly funny reason: its cells are already full of air, so there is nothing to lose. Low-density open-cell foams use air as the blowing agent and their R-value does not change over time. Some low-density varieties use carbon dioxide instead. You buy a lower number, but the number you buy is the number you keep.

How each one behaves in a wall

Low-density open-cell foam is sprayed into open wall cavities and rapidly expands to seal and fill them. It is water vapor permeable, remains flexible, resists wicking of moisture, provides good air sealing, is fire resistant, and will not sustain a flame. A slow-expanding version exists for cavities in existing homes, where a fast expansion could damage the wall. Its softness is a feature in a wall that moves.

Closed-cell foam is rigid, denser, and more resistant to water-vapor diffusion, which is why it turns up where moisture control and structural rigidity matter and where cavity depth is limited. Both are also used as the insulating core of structural insulated panels, where liquid foam injected between two wood skins under pressure bonds strongly to them; polyurethane wall panels run about 3.5 inches thick and ceiling panels up to 7.5 inches, and they insulate 30 to 40 percent better than expanded polystyrene for a given thickness.

One environmental footnote worth knowing: all closed-cell polyurethane foam insulation made today is produced with a non-HCFC gas as the foaming agent. Soy-based polyurethane spray-foam products also exist and apply with the same equipment as petroleum-based ones.

Which to use, and the safety line

Choose closed cell when you need the most R-value in the least depth, added rigidity, or better resistance to vapor diffusion, and you can pay for it. Choose open cell when you have cavity depth to spare, want an air seal at lower cost, and prefer an R-value that never drifts. Many houses sensibly use both, in different assemblies.

What is not optional is the installation. Spray polyurethane foam mixes and reacts in place, and polyurethane is built from isocyanates, which OSHA identifies as respiratory sensitizers whose main hazardous effects are occupational asthma and other lung problems. That means ventilation, real respiratory protection, and honoring the manufacturer's re-occupancy waiting period before anyone returns to the space. Once fully cured, the foam is a stable solid; the hazard belongs to the spraying and curing window, as we explain in is polyurethane toxic.

Sources: US Department of Energy, Insulation Materials; OSHA, Isocyanates.

Materials Review is an independent educational resource. It is not affiliated with Pittsburg State University or the former Kansas Polymer Research Center, and it is not a substitute for a licensed engineer. Confirm structural, safety, and code questions with a qualified professional before acting.