This is the question we get asked more than any other, and the honest answer is that neither one is better — they are different materials that solve different problems. Putting open cell foam in a wet crawl space is a mistake. So is paying closed cell prices to insulate an interior partition wall for sound. Knowing which is which saves real money.
The short version
Closed cell foam is dense and rigid. Its cells are sealed, so water cannot pass through it. It delivers roughly R-7 per inch, acts as its own vapor retarder at 1.5 inches and above, and adds structural strength to the assembly it is sprayed into. Open cell foam is light and soft. Its cells are open, so it is vapor-permeable, it delivers about R-3.7 per inch, and it expands far more aggressively — which makes it excellent at filling irregular cavities and absorbing sound.
Where closed cell is the right call
- Basements and crawl space walls, where ground moisture is constant
- Rim and band joists, which sit right at the moisture and air transition
- Metal buildings, where condensation on steel is the actual problem
- Shallow 2x4 cavities, where you need maximum R-value per inch
- Anywhere a vapor retarder is required and you would rather not add poly
Where open cell is the right call
- Interior partition walls where sound control is the goal
- Floor cavities between units in two-family homes
- Attic slopes and roof decks above grade that benefit from drying inward
- Large areas where cost per square foot matters and moisture does not
- Irregular framing with wiring and plumbing that batts would bridge over
The R-value comparison is misleading on its own
R-7 per inch sounds twice as good as R-3.7, and per inch it is. But cavity depth is fixed. A 2x6 wall gives you 5.5 inches. Filled with open cell that is roughly R-20; you would need about 3 inches of closed cell to match it, and 5.5 inches of closed cell would be significantly more expensive for a gain you may not need above grade. The right question is not which has the higher R-value per inch, it is which gets your assembly to the required R-value at the lowest cost while handling the moisture conditions correctly.
What actually drives the decision
Moisture. That is the single factor that settles most jobs. If the assembly touches the ground, sits below grade, or involves a cold steel or concrete surface where condensation will form, we specify closed cell and we do not negotiate on it. Above grade, dry, and interior — open cell is usually the better value, and the money saved is often better spent extending the scope to cover the rim joist or the attic hatch that nobody budgeted for.
A note on air sealing
Both products air seal, and that is the point most homeowners miss. Air leakage moves more heat out of a typical Tri-State house than conduction through the walls does. This is why a house with an unsealed attic and R-49 of batts can still feel drafty and expensive to heat — the insulation is there, but the air is still moving. Either foam solves that. Which one you choose is about moisture, sound and budget, not about whether it will seal.
If you are weighing the two on a specific project, we will look at the assembly and tell you plainly which one it needs. If the honest answer is fiberglass because the cavity is open, regular and dry, we will tell you that too.