What Is a Vapor Barrier?
A vapor barrier (more precisely, a vapor retarder) is a material that slows the movement of water vapor through the building assembly. In cold climates like Connecticut, warm interior air carries moisture that can condense on cold surfaces — causing mold, rot, and insulation degradation. Vapor control prevents this.
| Class | Permeance | Examples | CT Use Cases |
|---|---|---|---|
| Class I (vapor barrier) | <0.1 perm | Polyethylene sheet, foil | Ground cover in crawl spaces |
| Class II (vapor retarder) | 0.1–1.0 perm | Closed cell foam (2"+) | Basement walls, crawl space walls |
| Class III (vapor retarder) | 1.0–10 perm | Latex paint, open cell foam | Interior walls in some assemblies |
Key insight: Closed cell spray foam at 2 inches achieves a permeance of 0.8–1.0 perm — Class II vapor retarder performance. This eliminates the need for a separate poly vapor barrier in most basement and crawl space applications.
Closed Cell Spray Foam as a Vapor Retarder
The major advantage of closed cell spray foam over other insulation types is that it handles both thermal resistance and vapor control in one material. This simplifies the assembly and eliminates the risk of trapping moisture between a separate vapor barrier and the insulation.
- 2 inches of closed cell spray foam = R-12 to R-14 + Class II vapor retarder
- No separate poly sheet needed on basement walls
- No risk of moisture trapping between layers
- Adheres directly to the substrate with no air gaps
Vapor Control Requirements by Location
| Location | CT Requirement | Best Solution |
|---|---|---|
| Basement walls | Class II vapor retarder (interior) | 2" closed cell spray foam |
| Crawl space walls | Class II or ground cover | 2" closed cell + poly ground barrier |
| Exterior walls (new const.) | Class III (latex paint suffices) | Open or closed cell + latex paint |
| Attic floor (vented) | Class II on warm side | Kraft-faced batts or spray foam at bypasses |
| Crawl space floor (ground) | Class I (6 mil poly minimum) | 10–20 mil poly vapor barrier on soil |
Avoiding Common Moisture Mistakes
- Don't add poly over closed cell foam — you'll trap moisture between layers in the event of any leakage.
- Don't use open cell foam in below-grade applications — it will absorb moisture from the foundation.
- Don't insulate over wet surfaces — fix any active water intrusion before installing insulation.
- Don't leave a gap between insulation and concrete — air can carry moisture into this gap and condense. Closed cell foam eliminates gaps by adhering directly to the concrete.
Frequently Asked Questions
Is open cell spray foam a vapor barrier?
No. Open cell foam is Class III — highly vapor-permeable. It is not a vapor barrier or retarder and should not be used in applications where vapor control is required (below-grade, cold exterior walls without other vapor control).
Do I need a vapor barrier on a basement floor?
If you're finishing the basement floor, a vapor barrier beneath the flooring system is highly recommended in Connecticut — 6 mil poly minimum. Moisture migrates up through concrete slabs and can damage flooring without it.