The Core Difference at a Glance
Open-cell and closed-cell spray foam are both polyurethane-based products, but they behave very differently once applied. The distinction comes down to cellular structure: open-cell foam has cells that break during expansion, leaving an interconnected, sponge-like matrix. Closed-cell foam retains fully intact cells filled with a blowing agent, creating a dense, rigid material.
Choosing between them isn't just about cost — it's about matching the product's properties to your specific application.
Head-to-Head Comparison
| Property | Open-Cell Foam | Closed-Cell Foam |
|---|---|---|
| R-Value per Inch | ~R-3.5 to R-3.8 | ~R-6.0 to R-7.0 |
| Density | 0.4–0.5 lb/ft³ | 1.7–2.0 lb/ft³ |
| Vapor Permeance | High (vapor permeable) | Low (vapor barrier) |
| Water Resistance | Absorbs water | Repels water |
| Structural Rigidity | Flexible/soft | Rigid/hard |
| Sound Damping | Excellent | Moderate |
| Relative Cost | Lower | Higher |
| Typical Expansion | 100x original volume | 30–40x original volume |
When to Choose Open-Cell Foam
Open-cell foam excels in specific situations where its flexibility and vapor permeability are advantages rather than drawbacks:
- Interior wall cavities — fills cavities completely, excellent for soundproofing between rooms
- Attic roof decks in warm, humid climates — allows the roof deck to dry inward if moisture enters
- Budget-conscious projects — significantly lower material cost when high R-value per inch isn't critical
- Irregular shapes and hard-to-reach spaces — its high expansion rate fills complex cavities thoroughly
Note: Open-cell foam should not be used in below-grade applications, crawl spaces, or anywhere standing water or high moisture is a concern.
When to Choose Closed-Cell Foam
Closed-cell foam is the premium choice when performance demands are higher:
- Exterior walls — high R-value per inch in limited cavity space
- Crawl spaces and basements — moisture resistance and vapor barrier in one product
- Roofing and commercial flat roofs — structural rigidity adds to roof deck strength
- Cold climates — better performance in extreme temperature differentials
- Flood-prone areas — water resistance prevents insulation failure after water intrusion
Can You Use Both Together?
Yes — and in some applications, a hybrid approach is optimal. A common technique is to apply a thin layer of closed-cell foam first (to provide a vapor barrier and high-density air seal), then fill the remainder of the cavity with open-cell foam for cost efficiency. This is particularly effective in climate zones that require both a vapor retarder and high total R-values.
Code Considerations
Building codes in certain climate zones mandate specific vapor retarder requirements. Closed-cell foam at sufficient thickness can serve as the required vapor retarder, eliminating the need for a separate membrane. Always verify local code requirements with your contractor or building inspector before specifying a product.
The Bottom Line
Neither product is universally "better" — they serve different purposes. Use this simple decision framework:
- Is moisture a concern? → Closed-cell
- Is space limited and high R-value per inch critical? → Closed-cell
- Is soundproofing the primary goal? → Open-cell
- Is this an interior application in a conditioned space? → Open-cell is often sufficient
- Is budget tight for a large area? → Open-cell for cost efficiency