Open-cell vs closed-cell, picking the right foam for the assembly

It is mid-August. Attic season is winding down, the worst of the heat is behind us, and the fall calls are already starting: rim joists, crawl spaces, a pole barn somebody wants done before deer season. This is the time of year when the open-cell versus closed-cell question shows up on every other estimate.

The honest answer is that neither foam is "better." They are different tools. We watch contractors lose bids by quoting closed-cell where open-cell would have won the job, and we watch others get callbacks because they put open-cell where the assembly needed closed-cell. The material choice should come from the assembly, not from habit or from whatever is sitting in the trailer.

Here is how we would sort it out, with the numbers that actually drive the decision.

The core differences in plain terms

Open-cell foam runs around 0.5 pounds per cubic foot. It is water-blown, soft, and vapor-open. Figure roughly R-3.5 to R-3.8 per inch. A set yields in the neighborhood of 15,000 to 16,000 board feet, and most products let you spray full depth in one pass. Check the TDS, but that is the usual picture.

Closed-cell runs around 2 pounds per cubic foot. It is rigid, adds some racking strength, and acts as a Class II vapor retarder at roughly 1.5 to 2 inches. Figure R-6 to R-7 per inch, with aged values closer to R-6. A set yields roughly 4,000 to 4,500 board feet, and most TDS sheets cap you around 2 inches per lift, so thick applications mean multiple passes and more gun time.

That yield gap is the whole economic story. One set of open-cell covers three to four times the board footage of one set of closed-cell. Closed-cell gives you more R per inch, but open-cell usually gives you more R per dollar when you have the depth to work with.

Where open-cell earns its keep

• Vented and unvented attics where you have full rafter depth. In a 2x10 or 2x12 roofline you have room to stack inches, and open-cell fills it in one pass. Remember that in cold-climate unvented roof assemblies, code and most TDS sheets want a vapor retarder over open-cell — plan for that coating or membrane in the bid.
• Interior walls and floors where the job is sound, not thermal. Open-cell's soft structure absorbs airborne sound far better than closed-cell. Bedroom walls, home offices, media rooms — open-cell every time.
• Big open framing where speed matters. Full-depth passes and high yield mean your crew sprays more square footage per day per set.

The worked math: say you need R-38 in a roofline. Open-cell at R-3.7 per inch means about 10.5 inches — fine in a 2x12 bay. That depth at open-cell yield costs you far less material than getting there with closed-cell, and the labor is one pass instead of three lifts with cure time between them.

Where closed-cell is the only right answer

• Rim joists. You want air seal, vapor control, and R-value in a shallow cavity, and 2 to 3 inches of closed-cell does all three in one product.
• Crawl spaces and anywhere near ground moisture. Closed-cell does not absorb water the way open-cell can, and the vapor retarder function matters against damp block and soil.
• Below-grade walls. Same logic — moisture drive from the soil side means you want the Class II retarder in the foam itself.
• Metal buildings. Closed-cell bonds to the panel, controls condensation on cold steel, and stiffens the skin. Open-cell against single-skin metal in our climate is asking for sweating panels.
• Flood-prone assemblies. FEMA's guidance on flood-resistant materials points to closed-cell; open-cell holds water.
• Anywhere depth is limited. If you have a 2x4 wall and need to hit a number, R-6-plus per inch is how you get there.

The economics, honestly

Cost per R-value per square foot usually favors open-cell when depth is available, because of the yield difference. Cost per inch installed favors nobody — closed-cell material costs more per board foot and sprays slower in lifts.

So the estimating question is never "which foam is cheaper." It is "what does this assembly need, and what is the cheapest way to deliver that?" A hybrid answer is often the winner: two inches of closed-cell against the deck or rim for air seal and vapor control, then cheaper open-cell or even fiberglass over it for the rest of the R. That flash-and-batt approach can beat a straight closed-cell bid by a wide margin and still perform.

One more August-specific note: if you are quoting fall work now, quote the foam the assembly needs, not the foam you have on hand. Sets keep fine when stored right, and switching material mid-job because the bid was built around your leftover inventory is how off-ratio days and callbacks happen.

Pick the foam from the assembly's moisture and depth conditions first. Let R-value and price sort themselves out after that.

Quick decision list to tape inside the trailer

1. Is the surface below grade, in a crawl space, on metal, at a rim joist, or in a flood zone? Closed-cell.
2. Is the cavity shallow and the R target high? Closed-cell, or flash-and-batt.
3. Is it an attic or roofline with full rafter depth? Open-cell, with a vapor retarder where code and the TDS require it.
4. Is the customer buying quiet, not warmth? Open-cell.
5. Torn between them on price? Run cost per R for the actual depth available, not cost per set.

And in every case: the manufacturer's TDS beats anything in this post. Pass thickness, minimum substrate temperature, vapor retarder requirements — the sheet for your specific product is the rulebook.

Lining up fall material

If you are planning your fall rim-joist and crawl-space season now, this is a good month to lock in material. Our job estimator can run the open-versus-closed math on your actual assemblies, and we deliver the sets to the job or your shop — we are delivery only, so nobody has to detour through Saratoga Springs on the way to work. Send us the assembly and we will help you pick the right drum.

Code details vary by jurisdiction and cycle — confirm assembly requirements with your local code official and the product's TDS.