Closed-Cell vs Open-Cell Spray Foam
Short version: closed-cell where moisture and strength matter, open-cell where you need to fill an awkward space and want to spend less. Most houses end up using both.
Both are spray polyurethane foam. Both go on as a liquid, expand, and harden into place. The difference is what happens inside the foam as it cures.
In closed-cell, the bubbles seal against each other. Air and water vapor cannot move through it. That makes it dense, rigid, and roughly twice the R-value per inch.
In open-cell, the bubbles stay connected. It stays soft and light, expands much further, and costs less per square foot - but it does not stop moisture.
Bear in mind that comparing the two on price per square foot is not like for like, since open-cell needs roughly twice the thickness to reach the same R-value. How foam is priced, and why thickness drives the number.
Closed-Cell
Roughly R-6 to R-7 per inch.
Blocks moisture. Adds real stiffness to the frame. Costs more per square foot and does not expand as far, so it takes more material to fill a deep cavity.
Where we use it: crawl spaces, rim joists, basement and foundation walls, roof decks, anywhere damp is in play.
Open-Cell
Roughly R-3.5 to R-4 per inch.
Expands to many times its sprayed volume, so it fills awkward geometry that a batt never packs properly. Noticeably better at deadening sound. Does not block moisture.
Where we use it: attics, cathedral ceilings, interior walls where you want the quiet as much as the heat.
How we actually decide
The honest answer is that it is rarely one or the other. On a typical Maine house we will put closed-cell in the crawl space and against the rim joists, because that is where the damp and the cold floors come from, and open-cell overhead in the attic or cathedral ceiling, because that is where you need coverage more than density.
Spending closed-cell money on a dry attic usually is not worth it. We will say so.
The number on the bag is only half the story. A wall packed with R-21 fiberglass that leaks air at every seam performs worse in February than a properly sealed wall with a lower nominal R-value. Foam insulates and air-seals in one pass, which is the part the R-value alone never tells you. More on R-value and what Maine actually needs.
Does closed-cell really make the building stronger?
Yes, measurably. Because it adheres to the framing rather than sitting between it, a stud wall filled with closed-cell is stiffer than the same wall empty - the effect is a bit like gluing every piece of the frame together. The detail on that is here.
What about a house that is already finished?
Neither one, in a closed wall. Spray foam has to go into an open cavity - it expands hard enough that filling a finished wall with it can bow your drywall. For finished walls we use injection foam or dense pack, which go in through small holes we patch afterwards. See all services.
Not Sure Which One You Need?
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