Both foams insulate and air-seal. The difference is density, and density decides everything else: R-value per inch, how much water vapor gets through, how strong the cured foam is, and how far a set goes.
The numbers side by side
| Property | Thermoseal HFO (closed cell) | Thermoseal 500 (open cell) | Thermoseal 800 (open cell) |
|---|---|---|---|
| Core density | 1.9 to 2.1 lb/ft³ | 0.5 lb/ft³ | 0.75 lb/ft³ |
| R-value per inch (ASTM C518) | 7.4 (R-25 at 3.5 in) | 4.1 | 4.5 |
| Cell structure | over 92 percent closed | over 97 percent open | open |
| Water vapor permeance (ASTM E96) | 0.86 perms at 1.37 in | 6.15 perms at 3.5 in | see data sheet |
| Air-impermeable at (ASTM E283 / E2178) | 2.0 in | 3.5 in | see data sheet |
| Compressive strength (ASTM D1621) | 27.4 psi | not rated for load | not rated for load |
| Sound (STC, ASTM E413) | not the strength | STC 39 | see data sheet |
| Blowing agent | HFO, GWP of 1 | water | water |
| Surface burning (ASTM E84) | Class 1, flame spread 10, smoke under 450 | Class 1, flame spread under 25, smoke under 450 | Class 1 |
Where closed cell goes
- Crawlspace walls and band joists. It wants a vapor retarder, it may get wet, and 2 inches gives an air barrier and R-15 in one pass of work.
- Rim and band joists anywhere. Thin cavity, high R, stops the air that batts never did.
- Metal buildings and pole barns. Bonds to the skin, stops condensation on the inside of the metal, adds stiffness.
- Below grade and basement walls. Water resistance and the vapor retarder in one product.
- Exterior walls in cold climates where the assembly needs the vapor retarder on the warm side.
- Roofing and anywhere strength matters. Thermoseal HFO passes wind uplift at 210 psf and the Fortified roof test on its data sheet.
- Flash and fill. Two inches of closed cell for the air and vapor control, then open cell or batt to fill the cavity at a lower cost per inch.
Where open cell goes
- Attic roof decks. Depth is there (5.5 to 10 inches in a rafter bay), the vapor retarder is usually not wanted on the roof deck, and the softer foam is forgiving over sheathing seams.
- Interior walls, between floors, media rooms. STC 39 and a noise reduction coefficient of 0.75 on the Thermoseal 500 sheet.
- Any deep cavity in a mild or hot climate where the assembly does not call for a vapor retarder.
- Big, irregular spaces where yield matters more than inches: an open-cell set covers roughly three and a half times the board feet of a closed-cell set.
The two questions that decide it
- Does the assembly need a vapor retarder on this side? Cold climate, warm side of the wall, crawlspace, basement: closed cell. Roof deck in a vented-to-unvented attic conversion, interior partitions: open cell.
- How much depth is there? Under 3 inches of cavity and an R-value to meet: closed cell. Six inches or more and a budget: open cell.
Strength, water, thin cavities and code vapor retarders go to closed cell. Depth, sound and cost per cavity go to open cell. Most houses get both.
Both need a barrier over them
Spray foam inside a building needs a thermal barrier (usually half-inch drywall) or, in attics and crawlspaces that are only entered for service, an ignition barrier or a listed coating. The ignition and thermal barrier guide has the coating rates from the Thermoseal evaluation reports.
Quick answers
What is the R-value of closed-cell spray foam per inch?
Thermoseal HFO is R-7.4 per inch and R-25 at 3.5 inches (ASTM C518).
What is the R-value of open-cell spray foam per inch?
Thermoseal 500 is R-4.1 per inch; Thermoseal 800, the 0.75 lb open cell, is R-4.5 per inch (ASTM C518).
Is closed-cell spray foam a vapor barrier?
Thermoseal HFO measures 0.86 perms at 1.37 inches, which puts it under 1 perm, the usual line for a vapor retarder. Open cell at 3.5 inches is 6.15 perms and is not a vapor retarder.
At what thickness is spray foam an air barrier?
Thermoseal HFO closed cell is air-impermeable at 2 inches and Thermoseal 500 open cell at 3.5 inches, by the ASTM E283 and E2178 tests on the data sheets.