Closed-Cell vs Open-Cell Spray Foam: Which One for Your Job?

Open-cell is not the cheap version of closed-cell. They are different materials with different jobs, and in climate zone 6, which is Broome County, the code makes the choice for you in at least one location.

Anyone who tells you open-cell is fine everywhere because it costs less has not looked at your assembly. Anyone who tells you closed-cell is always better is selling you the more expensive product. The correct answer depends on where it is going and what the assembly needs to do.

The physical difference

Both are polyurethane, made from the same two-component chemistry. The difference is what happens to the cells during expansion.

Closed-cell: the cells are sealed and filled with a blowing agent. The result is rigid, dense, and largely impermeable to both air and water vapor.

Open-cell: the cells are ruptured and filled with air. The result is soft, low-density, and permeable to water vapor while still being air-impermeable at adequate thickness.

Everything below follows from that.

The comparison

	Closed-cell	Open-cell
R per inch	5.0 – 7.0	3.5 – 3.7
Installed cost/sq ft	$3.00 – $5.00	$1.50 – $3.50
vapor permeability	Low. Generally acts as its own vapor retarder at adequate thickness	High. Roughly 2 to 15 perms depending on thickness
Density	Rigid, adds structural racking strength	Soft, no structural contribution
Expansion	Modest, roughly 1 inch per pass	Large, expands substantially
Water	Does not absorb water; will not hold it against a substrate	Absorbs and holds water
Sound	Poor absorber; it is rigid	Good absorber; commonly used in interior partitions
Yield per unit of material	Lower	Much higher, which is where the cost difference mostly comes from

R-value ranges are typical across the category. The 40% spread on closed-cell is real, and the figure for your job comes from the technical data sheet of the specific product being sprayed. We publish that sheet with every quote.

The code point that decides one of these for you

Broome County is in IECC climate zone 6. Under the residential code adopted in New York, air-impermeable insulation applied at a roof deck in climate zones 5 through 8 must be a Class II vapor retarder or carry a Class II coating.

In practice: open-cell spray foam at a Binghamton roofline requires a separate Class II vapor retarder as part of the assembly. Closed-cell at adequate thickness generally satisfies the requirement on its own.

That is not a preference, a sales position, or a regional custom. It is the code, and it exists because of the failure mechanism described below.

Why the code says this. Water vapor from inside the house diffuses through vapor-permeable foam, reaches cold roof sheathing, condenses there, and over sustained winters rots the deck. Field studies confirm the moisture comes from inside the house, not from rain above. The vapor retarder requirement in cold zones exists specifically to interrupt that path.

This is the single best question to ask any foam contractor: “Open-cell or closed-cell at the roof deck, and if open-cell, what is the vapor retarder?” Someone who has read that section of the code answers immediately. Someone who has not will tell you it does not matter.

Where each one belongs

Closed-cell belongs

At a roof deck in this climate. Both the code position and the moisture physics point the same way, and this is the application where getting it wrong is most expensive.

On foundation and crawl space walls. The point of insulating below grade is keeping the wall face above the dew point with nothing air-permeable between the foam and the cold masonry. Closed-cell does that and does not absorb water if the wall ever gets wet.

In rim joist bays. It bonds to the rim board, subfloor, sill, and both joists as one sealed piece, and it does not hold water against wood.

On the underside of steel roofing in a pole barn or metal building. The whole point there is eliminating a condensing surface, and only an impermeable material bonded directly to the steel does that.

Anywhere the assembly needs vapor control as well as insulation, or where rigidity and racking strength are useful.

Open-cell belongs

In interior partitions for sound. Genuinely good at it, and closed-cell is not.

In wall cavities in the right assembly, where the wall can dry to at least one side and the sheathing is not at risk.

At a roof deck only with a separate Class II vapor retarder in the specification, as above.

Where the assembly benefits from being able to dry, which is a real design consideration and not always the wrong answer. Some building scientists argue that a vapor-permeable roof assembly which can dry inward is more forgiving of a future roof leak than a sealed one, precisely because water that gets in can leave. That is a legitimate position and it is the reason open-cell at a roof deck is permitted at all in cold zones, with the vapor retarder.

The cost comparison people get wrong

Closed-cell costs roughly double per square foot. But it delivers roughly 1.6 times the R per inch.

Compare on cost per unit of R-value at a given cavity depth and the gap narrows substantially. Compare in a situation where cavity depth is constrained, which is most retrofits, and closed-cell often wins outright, because you physically cannot fit enough open-cell in the space to reach the target R-value.

A worked example. A 2x6 rafter bay gives you 5.5 inches of depth.

Open-cell filling the bay: 5.5 × 3.6 = R-19.8
Closed-cell at 4 inches: 4 × 6.0 = R-24, with 1.5 inches of bay to spare

If the target is R-30 or above, open-cell in that bay cannot get there at any price and closed-cell can with additional depth. If the target is R-20, open-cell is cheaper and adequate.

Where open-cell genuinely wins on cost: open spaces with unlimited depth available, such as an attic floor plane or a large unfinished area, where you can simply put more of it in.

The decision, in five questions

1. Is this at a roof deck? In climate zone 6, closed-cell, or open-cell with a Class II vapor retarder in the specification. There is no third option.

2. Is this below grade or against masonry? Closed-cell. It does not absorb water and it keeps the surface above the dew point.

3. Is this against steel? Closed-cell. The whole point is eliminating the condensing surface.

4. Is cavity depth constrained? If you cannot fit enough open-cell to reach the target R-value, that decides it.

5. Is this an interior partition where the goal is sound? Open-cell.

Anything not covered by those five is genuinely a judgement call, and it should be made by somebody who has looked at the assembly rather than by a default.

What we do

We specify per assembly, we say which and why in the written scope, and we publish the technical data sheet for the product so you can check the R per inch and the permeability yourself rather than taking our word for the range.

If open-cell is adequate for your job, we will tell you and charge you less. If closed-cell is required, the reason will be in writing.

Get a foam insulation quote with the product data sheet attached.

Attic spray foam insulation, where this decision matters most.

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