Open Cell vs Closed Cell Spray Foam: Complete Comparison Guide
A technical reference for contractors, builders, and homeowners selecting spray foam insulation for Ontario projects.
Cellular Structure and Chemistry
The fundamental difference between open-cell and closed-cell spray foam lies at the microscopic level. Both are polyurethane foams created by mixing an isocyanate (A-side) with a polyol resin blend (B-side), but the formulations and blowing agents produce dramatically different cellular structures.
Closed-cell spray foam (ccSPF) uses hydrofluoroolefin (HFO) blowing agents such as Honeywell Solstice or Chemours Opteon, which have a global warming potential (GWP) of less than 1. The cells are completely sealed, trapping the low-conductivity blowing agent gas within each cell. This produces a dense, rigid material with densities of 1.7 to 2.0 pounds per cubic foot and R-values of 6.0 to 7.0 per inch.
Open-cell spray foam (ocSPF) uses water as its blowing agent. The reaction between water and isocyanate produces carbon dioxide gas, which creates the cells. However, the cell walls rupture during formation, creating an interconnected network of open cells filled with air. The result is a lightweight, flexible material with densities of 0.4 to 0.6 pounds per cubic foot and R-values of 3.6 to 3.8 per inch.
Comprehensive Property Comparison
| Property | Closed-Cell (ccSPF) | Open-Cell (ocSPF) |
|---|---|---|
| R-Value per Inch | R-6.0 – R-7.0 | R-3.6 – R-3.8 |
| Density (pcf) | 1.7 – 2.0 | 0.4 – 0.6 |
| Air Permeance (L/s/m² at 75 Pa) | < 0.02 at 25 mm | < 0.02 at 89 mm |
| Water Vapour Permeance | Class II (<60 ng) at 50 mm | Vapour permeable (>600 ng) |
| Water Absorption (% by volume) | < 2% | Up to 30%+ (absorbs then dries) |
| Compressive Strength (kPa) | 170 – 240 | 5 – 10 |
| Tensile Strength (kPa) | 200 – 350 | 10 – 20 |
| Dimensional Stability | Excellent (rigid) | Good (flexible) |
| Sound Transmission Class (STC) | STC 34 – 37 | STC 37 – 39+ |
| Expansion Ratio | ~35:1 (1" per pass) | ~100:1 (3"+ per pass) |
| Blowing Agent | HFO (low GWP) | Water / CO&sub2; |
| Yield per Board Foot | Lower (less expansion) | Higher (more expansion) |
| Cost per Sq Ft (installed, 2") | $2.50 – $4.00 | $1.50 – $2.50 (at 3.5") |
Applications by Location
Below-Grade (Basements, Crawl Spaces)
Recommended: Closed-cell. Below-grade applications face constant moisture exposure from ground water, soil vapour, and concrete wicking. Closed-cell foam's low water absorption and vapour retarder properties protect the wall assembly from moisture damage. At 2 inches, closed-cell provides R-13 plus a Class II vapour retarder, eliminating the need for polyethylene sheets on basement walls. Open-cell can be used in below-grade applications but requires a vapour retarder coating (latex paint or membrane) and should not be used in basements with active water intrusion history.
Above-Grade Walls
Both suitable depending on budget and performance goals. In 2x6 walls, open-cell filling the full cavity provides excellent thermal and air-sealing performance at lower cost. Closed-cell at 2 to 3 inches is preferred when maximizing R-value in limited cavity depth (2x4 walls) or when the wall assembly requires a vapour retarder within the cavity. For cold-climate wall assemblies following the OBC SB-12 prescriptive path, the ratio of exterior to interior insulation must meet dew point control requirements — closed-cell's vapour retarder properties simplify compliance.
Attic and Roof Assemblies
Recommended: Open-cell for unvented roof decks. Open-cell's high expansion ratio makes it ideal for filling deep rafter bays efficiently. At 5.5 inches between 2x6 rafters, it provides R-21 and excellent air sealing. Open-cell also allows roof leaks to be detected quickly, as water passes through it and becomes visible on the ceiling below. Closed-cell can be used in roof assemblies but is significantly more expensive for the depth required and makes leak detection more difficult.
Rim Joists and Headers
Recommended: Closed-cell. Rim joists are the most vulnerable point in the building envelope for condensation in Ontario's climate. The rim joist sits at the junction of the warm interior and cold exterior, and winter temperatures drive the dew point directly through this area. Closed-cell at 2 inches provides R-13, air sealing, and vapour retardance — all critical for preventing condensation and mold at this location.
Interior Partition Walls (Sound)
Recommended: Open-cell. Open-cell spray foam provides superior sound attenuation compared to closed-cell, with STC ratings of 37 to 39 or higher. The soft, flexible cellular structure absorbs sound vibrations rather than transmitting them. For party walls between units, basement theatre rooms, and home offices, open-cell is the preferred choice for noise reduction.
Pros and Cons Summary
Closed-Cell Pros
- + Highest R-value per inch available
- + Built-in vapour retarder at 2 inches
- + Water-resistant (less than 2% absorption)
- + Adds structural rigidity (racking strength)
- + Flood resistant — does not lose R-value when wet
- + Ideal for moisture-prone locations
Closed-Cell Cons
- – Higher material cost per square foot
- – Multiple passes needed for thick applications (1" per pass max)
- – Higher GWP blowing agent (though HFO variants are near-zero)
- – Can mask roof leaks
Open-Cell Pros
- + Lower cost per square foot
- + Excellent sound attenuation (STC 37+)
- + Water-blown (lowest environmental impact)
- + High expansion ratio — fills cavities efficiently
- + Allows roof leak detection
- + Breathable — allows assemblies to dry
Open-Cell Cons
- – Lower R-value per inch (nearly half of closed-cell)
- – Not a vapour retarder (requires additional VR in some assemblies)
- – Absorbs water (not suitable for flood-prone areas)
- – No structural contribution
Recommendation Matrix
| Application | Closed-Cell | Open-Cell | Notes |
|---|---|---|---|
| Basement Walls | Best | OK | Moisture protection critical |
| Crawl Space | Best | No | High moisture exposure |
| Rim Joist | Best | OK | Condensation zone in winter |
| Above-Grade Walls (2x6) | Good | Best Value | Open-cell fills cavity efficiently |
| Above-Grade Walls (2x4) | Best | OK | Max R-value in limited space |
| Attic (unvented roof deck) | OK | Best Value | Cost-effective for deep cavities |
| Interior Walls (sound) | OK | Best | Superior sound absorption |
| Metal Building / Pole Barn | Best | No | Condensation control essential |
| Walk-In Cooler / Freezer | Best | No | Moisture and structural needs |
Cost Analysis: Equivalent Performance
A common mistake is comparing closed-cell and open-cell solely on per-board-foot cost. The more accurate comparison considers the total installed cost to achieve the same thermal performance, including any additional materials required (vapour retarders, supplemental insulation).
For a typical Toronto basement (1,000 sq ft of wall area) insulated to R-20:
- Closed-cell at 3": $3,500 to $5,500 installed. No additional materials needed.
- Open-cell at 5.5": $2,000 to $3,500 installed, plus $500 to $1,000 for vapour retarder application. Total: $2,500 to $4,500.
The cost difference narrows to approximately $500 to $1,500 for comparable performance. When you factor in closed-cell's moisture protection in below-grade applications, the premium is often worthwhile insurance against future moisture problems.