Open-Cell vs. Closed-Cell Spray Foam: Which Goes Where

“Spray foam” is two very different materials that happen to come out of the same gun. Choosing between them is the most important decision in most foam projects, and it is not about which one is “better.” It is about which one belongs in the spot you are insulating.

The short version

What “open” and “closed” actually mean

Both foams start as two liquids — an isocyanate “A side” and a resin “B side” — mixed at the spray gun. They react, expand and cure in seconds.

In open-cell foam, the tiny cells break open as they form. The result is light and flexible, with air filling the cells. Air and water vapor can move through the foam slowly, but it still stops air leakage when installed thick enough, because the cell structure is tortuous.

In closed-cell foam, the cells stay intact and hold a gas with better insulating properties than air. That gives closed-cell its higher R-value per inch, its rigidity, and its resistance to water and vapor. The U.S. Department of Energy sums it up: closed-cell has a greater R-value and stronger resistance to moisture and air leakage, but is denser and more expensive, and open-cell should not be used below ground level where it could absorb water.

Location by location

Moisture is usually the deciding factor

If the location can get wet, sits against concrete or soil, or has a cold surface that warm, humid air could reach, closed-cell is almost always the answer. Its low vapor permeance and rigid structure keep moisture out of the assembly.

Open-cell’s vapor-open nature is not a flaw; it is a feature in the right place. In walls and roofs that need to dry inward, it lets them. It just needs the assembly designed around it — which is why cold-climate roof decks get a vapor retarder coating over open-cell foam.

Space and budget come next

When you have room, open-cell reaches a given R-value for less money. A 2×6 wall filled with open-cell lands around R-20. To get the same in a 2×4 wall, you need closed-cell. Shallow rafters, cathedral ceilings and anywhere you cannot add depth push you toward closed-cell.

The hybrid option

“Flash and batt” or “flash and fill” pairs a thin layer of closed-cell — enough to air-seal and control condensation — with less expensive fiberglass, mineral wool or open-cell for the remaining R-value. It is popular in cold climates, where code sets a minimum foam R-value at the roof deck for condensation control and the rest can be air-permeable.

A quick decision path

Use the thickness calculator to see how many inches each foam needs for your target R-value — and whether it fits the framing you have.