What “open” and “closed” cell means
Spray foam is made of millions of tiny bubbles (cells) that form as the A-side and B-side react.
- In open cell foam, most of the cell walls break open as the foam cures. Air moves between the cells, which is why the foam is soft, light and lets water vapor pass through.
- In closed cell foam, the cells stay sealed and hold the blowing agent inside. That trapped gas is what gives closed cell its higher R-value, and the sealed structure is what makes it rigid and moisture-resistant.
The blowing agent also differs. Sprayco open cell foams are water-blown: water reacts with the A-side to release CO₂, which expands the foam. Sprayco HFO closed cell uses an HFO blowing agent, a newer-generation gas with a much lower global warming potential than the HFC agents it replaced.
Sprayco 500, 800 and HFO side by side
| Property | Sprayco 500 | Sprayco 800 | Sprayco HFO |
|---|---|---|---|
| Type | Open cell | Open cell | Closed cell |
| Core density | 0.5 lb/ft³ | 0.75 lb/ft³ | 1.9–2.1 lb/ft³ |
| R-value per inch | 4.1 | 4.5 | 7.4 (R-25 @ 3.5") |
| Cell content | > 97% open | > 75% open | ≥ 92% closed |
| Vapor permeance | 6.15 perms @ 3.5" | 5.92 perms @ 3.5" | 0.86 perms @ 1.37" |
| Sound (STC / NRC) | 39 / 0.75 | 38 / 0.65 | — |
| Compressive strength | — | — | 27.4 psi |
| Blowing agent | Water | Water | HFO |
| Set | 2 × 55 gal (A + B) | 2 × 55 gal (A + B) | 2 × 55 gal (A + B) |
Physical properties are from the formulator's technical data sheets (ASTM C518, D1622, D6226, D2856, E96, E413/C423, D1621). Actual performance varies with thickness, substrate and processing conditions.
Where open cell wins
Interior walls and partitions
A 2×4 or 2×6 wall cavity filled with open cell gives good insulation and excellent sound control at the lowest cost per square foot. The foam stays flexible, so it moves with the framing instead of cracking away from it.
Unvented attics and rooflines
Open cell is widely used on the underside of the roof deck to create a sealed, conditioned attic; the U.S. Department of Energy's Building America program documents open cell roof-deck applications at R-25 to R-28. Both Sprayco open cell sets work here. The difference is thickness: at R-4.5 per inch, Sprayco 800 reaches a given R-value in less depth than Sprayco 500 at R-4.1, which matters when rafter depth is limited.
One caution for colder climates: open cell is vapor-permeable, so under a roof deck it generally needs a separate Class II vapor retarder in climate zones 5 through 8. Closed cell doesn't. The installer should confirm what the local jurisdiction requires.
Sound control
Open cell absorbs sound; closed cell mostly reflects it. For media rooms, bedrooms over garages and multi-family party walls, Sprayco 500's STC 39 / NRC 0.75 is the number to quote.
Budget-driven jobs
One open cell set covers far more area than one closed cell set. When the customer is insulating a whole house and the walls are deep enough to hit the target R-value, open cell usually wins the bid.
Where closed cell wins
Tight cavities
At R-7.4 per inch, Sprayco HFO reaches R-25 in a 3.5" stud bay. Open cell can't get there in the same space. Think 2×4 walls that need to meet a high R-value target, rim joists, and anywhere the installer can't add framing depth.
Moisture and below grade
Foundations, crawlspaces, basements and slab edges. Closed cell doesn't absorb water the way open cell can, and at 0.86 perms at 1.37" it sits under the 1-perm mark, so a continuous lift acts as a vapor retarder. Whether a separate vapor retarder is still required is set by local code and climate zone; the installer should check.
Metal buildings and exterior walls
Closed cell bonds to metal, adds rigidity, and controls condensation on the inside of a metal skin. It is also the only spray foam used for continuous insulation on the exterior side of a wall, because it is the one that acts as its own vapor barrier. One thing it is not: a roofing foam. Spray foam roofing uses a separate 2.8–3.0 lb/ft³ roofing-grade product that is topcoated for UV; 2 lb wall and attic foam is not a substitute for it.
Structural strength
At 27.4 psi compressive strength, closed cell stiffens the assembly it's sprayed into. It's commonly used in walls and roofs for racking strength and in trailers and containers for both insulation and rigidity.
Air sealing in one pass
Both foams air-seal, but closed cell also handles vapor, which means one product does the job of two layers.
The cost conversation
Per set, closed cell costs more and yields less. Per inch of R-value, the gap narrows. Per job, it depends on whether the cavity has room. A useful way to frame it at the counter:
Deep cavity, interior, dry
Lowest cost to the R-value target.
Open cell
Shallow cavity or high R target
Open cell physically can't get there.
Closed cell
Any moisture exposure
Regardless of budget.
Closed cell
Sound is the priority
It absorbs; closed cell reflects.
Open cell
Running both on the same rig
Sprayco 500, 800 and HFO all run through standard plural-component proportioners and heated hose, and all three are available as 55-gallon A-side and B-side sets. Processing temperatures and pressures differ by product; your installer should run the parameters on the technical data sheet, and your Sprayco rep can send the full TDS for each set.
Switching between open and closed cell on one rig
It is done every day, and it is where most changeover problems start. The B-side resins are different chemistries, and if one is left in the system when the other arrives, the result is off-ratio foam and, in some cases, a reaction inside the gun. The procedure manufacturers publish comes down to four steps:
- 01
Pull the B-side transfer pump, wipe it down completely and drain the upper chamber before it goes into the new drum.
- 02
Purge the B-side recirculation line of the old material into a separate container before using it to heat the new drum.
- 03
Purge the hose to the gun: either into a container (expect two to five gallons) or by spraying off-target until the foam is clearly the new product. A rule of thumb from one formulator: each foot of hose holds enough material for about one square foot of closed cell at 1 inch, or one square foot of open cell at 3.5 inches.
- 04
Reset temperatures and pressures to the new product's parameters, then test-spray before touching the job.
Don't flush with water unless the foam manufacturer's written procedure calls for it; it introduces a contaminant into the B-side. Crews that switch constantly often run two rigs, or two B-side pumps with a changeover valve. Ask your Sprayco rep for the change-over procedure for each set, including whether the A-side is shared between products.
What to stock
For most dealers the practical answer is all three, in proportions that match the local market:
- Sprayco 500 is the volume product for residential walls and attics, where cavity depth is not the constraint and yield drives the bid.
- Sprayco 800 earns its place with crews doing rooflines and unvented attics who want more R per inch without going to closed cell.
- Sprayco HFO is the one installers can't substitute. If it's not on your shelf, the job goes to the competitor who has it.
Sprayco ships mixed pallets, so a first order can be one set of each. Orders ship within one business day.
Quick answers
Can open cell spray foam be used in a basement?
It can be sprayed there, but it absorbs water and lets vapor through, so most installers specify closed cell below grade.
Is closed cell spray foam always better than open cell?
No. It costs more and yields less, and it doesn't absorb sound as well. Where there's room in the cavity and no moisture risk, open cell is often the better value.
What's the difference between Sprayco 500 and Sprayco 800?
Density. Sprayco 800 is 0.75 lb/ft³ with a tighter cell, which gives R-4.5 per inch versus R-4.1 for Sprayco 500. The 500 is lighter, yields more, and is slightly better for sound.
Can you switch between open cell and closed cell on the same spray rig?
Yes, with a proper change-over: wipe down the B-side transfer pump, purge the recirculation line and the hose to the gun, reset temperatures and pressures, then test-spray. Don't flush with water unless the foam manufacturer's written procedure calls for it.