What’s the difference between open cell and closed cell spray foam?
Both start as liquid that expands into foam on the wall or roof deck. The difference is what happens inside the bubbles. In open cell foam the tiny cells are broken open, so it cures soft and spongy, at about half a pound per cubic foot. In closed cell foam the cells stay sealed, so it cures hard and dense, at about two pounds per cubic foot.
That one difference drives everything else: how well it insulates, how it handles water, and what it costs.
| Open cell | Closed cell | |
|---|---|---|
| R-value per inch | About R-3.7 | Up to about R-6.5 |
| Density | About 0.5 lb per cubic foot | About 2 lb per cubic foot |
| Thickness to stop air leaks | 3 to 5.5 inches | 1 to 1.5 inches |
| Slows water vapor | No | Yes, at enough thickness |
| FEMA flood rating | Unacceptable | Acceptable |
| Installed cost per board foot | $0.40 to $0.65 | $1.00 to $1.50 |
| Cost per R-value, per square foot | About $0.11 to $0.18 | About $0.15 to $0.23 |
How much better does closed cell insulate?
Inch for inch, closed cell insulates about 75% better: roughly R-6.5 per inch against R-3.7 for open cell. A U.S. Department of Energy example shows the gap in practice — three inches of closed cell reached R-19 under a roof deck, while open cell needed seven and a half inches to reach R-28.
The gap shrinks inside an ordinary 2x4 wall. The wood studs leak heat no matter what foam sits between them, and closed cell is usually sprayed about three inches thick rather than filling the cavity, because it is hard to trim. So the whole wall ends up performing closer to open cell than the per-inch numbers suggest.
Which one handles water and moisture?
Closed cell, clearly. Its sealed cells don’t soak up water, and at enough thickness it also slows water vapor moving through a wall. Open cell does not block vapor, and like a sponge it can hold water.
FEMA makes the same call. In its flood-resistant materials bulletin, sprayed polyurethane or closed-cell foam is rated acceptable below flood level, while open-cell foam is grouped with materials rated unacceptable. If you are in a flood zone, or insulating anywhere that could flood, that settles it.
Whether your climate also needs a separate vapor retarder depends on where you live and how the wall or roof is built. That is a good question to ask any installer before they start.
How much does each cost?
Spray foam is priced by the board foot: one square foot, one inch thick. As of August 2026, installed prices run about $0.40 to $0.65 per board foot for open cell and $1.00 to $1.50 for closed cell — around two and a half times as much per inch.
Per inch is the wrong comparison, though, because closed cell insulates more per inch. Divide the price by the R-value and the gap narrows: open cell works out to roughly $0.11 to $0.18 per square foot for each point of R-value, and closed cell to roughly $0.15 to $0.23. Closed cell still costs more for the same insulation — just not two and a half times more. What the extra buys is water resistance and a lot more insulation in a thin space.
Where should you use closed cell?
- Anywhere that could get wet — basements, crawl spaces and anything below flood level.
- Where space is tight and you need the most insulation from the fewest inches.
- Where you need to stop air and slow vapor with a thin layer, since closed cell seals air at an inch or so.
Where does open cell make sense?
- Attic rooflines and cathedral ceilings with room for the thickness it needs. More on that in our spray foam attic insulation guide.
- Dry, above-ground spaces where the lower cost goes further.
- Projects where budget matters most and there is space to reach the R-value you need.
Do spray foam contractors install both?
Most do. Of the 29 contractors in the FoamMatch directory who list which type they spray, 21 install both open and closed cell. So you usually don’t have to pick the contractor by foam type — pick a contractor you trust, then ask which foam they would use in each part of your home, and why.