What Is Open Cell Spray Foam?
Open cell spray foam (also called ocSPF or 0.5-lb foam) is a low-density spray polyurethane foam where the cells remain open after expansion. This gives it a soft, spongy texture and allows it to fill wall and ceiling cavities completely — getting into every corner that rigid insulation would miss.
Because the cells are open, air and moisture vapor can pass through open cell foam. This makes it an excellent air barrier but not a vapor barrier — a crucial distinction for certain applications.
Best use case: Open cell spray foam is ideal for interior walls and soundproofing, conditioned attic rafters, and any application where you need to fill large cavities affordably and vapor permeability is acceptable.
R-Value and Performance
Open cell spray foam delivers R-3.5 per inch — lower than closed cell (R-6 to R-7/inch) but still effective for large applications where thickness is not a constraint.
| Thickness | R-Value | Common Use |
|---|---|---|
| 3.5 inches | R-12 | 2×4 wall cavity (fills completely) |
| 5.5 inches | R-19 | 2×6 wall cavity |
| 6 inches | R-21 | CT exterior wall code minimum |
| 10 inches | R-35 | Attic rafter bays |
| 14–17 inches | R-49 to R-60 | Full attic code R-value (CT) |
Open Cell vs. Closed Cell: At a Glance
Open Cell Advantages
Lower cost per sq ft. Better soundproofing. Fills complex cavities more easily. Vapor-permeable (good for some attic applications).
Open Cell Limitations
Lower R-value per inch. Not a vapor barrier. Cannot be used below grade or in wet environments. Requires more thickness to achieve same R-value.
When to use closed cell →Best Applications for Open Cell Spray Foam
- Interior wall soundproofing — Open cell fills wall cavities completely, dramatically reducing sound transmission between rooms and floors.
- Attic rafter bays (conditioned attic) — Applied to the underside of roof decking in conditioned attic assemblies where vapor permeability is desired by the designer.
- 2×4 and 2×6 wall cavities — Fills the full cavity depth, eliminating thermal bridging and air infiltration.
- Cathedral ceilings — Fills rafter bays where access is limited and depth is sufficient for adequate R-value.
Moisture Considerations: When NOT to Use Open Cell
Because open cell foam is vapor-permeable, it is not appropriate for below-grade or high-moisture applications. Avoid open cell foam in:
- Basement walls — Use closed cell instead. Open cell can trap moisture against the foundation wall, leading to mold.
- Crawl spaces — Moisture from soil can saturate open cell foam in crawl space applications.
- Rim joists — Closed cell's vapor barrier properties make it far superior here.
- Exterior applications — Open cell is not weather-resistant and cannot be left exposed.
Rule of thumb: If the surface you're insulating is in contact with the ground, outside air, or a moisture source — use closed cell. For interior applications where moisture is controlled — open cell is a cost-effective choice.
Frequently Asked Questions
How much does open cell spray foam cost in Connecticut?
Open cell spray foam typically costs $1.00–$2.00 per square foot installed — about 30–40% less than closed cell. A 2×6 wall cavity in a typical CT home runs $1,500–$3,500.
Does open cell spray foam off-gas?
Like all spray foam, open cell off-gases during application. Occupants must leave during spraying and for 24 hours after. Once cured, it is inert and safe for occupied spaces.
Can open cell foam be used in an attic?
Yes, with design consideration. Open cell works well in conditioned attic assemblies when applied to the underside of the roof deck at sufficient thickness. Consult EcoFoam to confirm the right assembly for your specific attic.