Open-Cell vs Closed-Cell Spray Foam: Which One Does Your Attic Actually Need?

Most homeowners who ask about spray foam insulation have no idea there are two distinct types with very different properties, cost profiles, and appropriate applications. They hear “spray foam” and assume it is all the same product. It is not — and choosing the wrong type for your attic can mean paying two to three times more than necessary, or selecting a product that does not perform as expected in your specific assembly.

This is a question I address regularly with homeowners across Toronto, Mississauga, Brampton, Oakville, Hamilton, and the rest of the GTA. Let me explain what I have learned through years of spray foam installations alongside blown-in cellulose work.

Close-up of spray foam applied to an attic rim joist — a common and highly effective application for closed-cell foam.

The Fundamental Difference: Cell Structure

Both open-cell and closed-cell spray foam start with the same two-component chemical reaction. The difference is what happens at the microscopic level as the foam expands:

• Open-cell foam expands to a spongy, flexible structure where the cell walls rupture during curing, leaving the cells open. Air fills the space between the foam matrix.
• Closed-cell foam cures with intact, sealed cells containing a gas (the blowing agent) rather than air. The result is a rigid, dense material.

This structural difference drives almost every other performance distinction between the two products.

Open-Cell Spray Foam: Properties and Applications

Performance Characteristics

• R-value: Approximately R-3.7 per inch — similar to blown-in cellulose and standard fiberglass batts
• Density: 0.5 lb/cubic foot (very light)
• Air barrier: Excellent — expands to fill every gap and void
• Vapour permeability: Permeable — allows moisture vapour to pass through
• Rigidity: Soft and flexible after curing
• Cost: Generally $1.50–$3.00/sqft depending on depth

Best Applications for Open-Cell Foam

Open-cell foam excels where you need a combined insulation and air barrier in a single application and where vapour permeability is acceptable or desired:

• Underside of roof decks in unvented (“hot roof”) assemblies, where it needs to be thick enough to keep the deck warm in winter (typically 5.5 inches or more in Ontario’s climate)
• Interior walls where soundproofing is a benefit and vapour permeability is acceptable
• Attic floor applications where a thick, air-sealing layer is needed but you are not concerned about vapour drive through the foam

Limitations

Open-cell foam requires a vapour barrier on the warm side of the assembly in most Ontario applications because it does not restrict vapour movement on its own. It also requires greater thickness to achieve equivalent R-values compared to closed-cell foam, which matters in depth-constrained situations.

Closed-Cell Spray Foam: Properties and Applications

Performance Characteristics

• R-value: R-6.0 to R-6.5 per inch — roughly 1.7 times higher than open-cell
• Density: 2.0 lb/cubic foot (much denser)
• Air barrier: Excellent
• Vapour permeability: Near-zero above 2 inches — acts as a vapour retarder
• Rigidity: Rigid and structural — can add stiffness to framing
• Cost: $3.00–$7.00/sqft depending on thickness

Best Applications for Closed-Cell Foam

• Rim joists: This is where closed-cell foam is most consistently worth the premium. A 2-inch layer controls both air and moisture while providing meaningful R-value in a compact space.
• Crawlspace walls: Moisture resistance makes it ideal for below-grade or semi-below-grade applications
• Cathedral ceiling undersides: In shallow rafter bays where every inch of R-value counts
• Exterior sheathing applications: Structural rigidity makes it suitable for some continuous insulation assemblies
• Flood-prone or high-humidity areas: Closed-cell resists water infiltration at low exposure levels

Side-by-side: key property differences between open-cell and closed-cell spray foam, and when each is appropriate.

Side-by-Side Comparison for Ontario Attics

The Ontario Building Code Context

Ontario Building Code requires a minimum of R-50 for attic assemblies in most residential construction. We recommend upgrading to R-60 for optimal energy performance and comfort, particularly given the GTA’s climate — cold winters and hot summers place significant demand on your home’s thermal envelope year-round.

Both open-cell and closed-cell foam can achieve these targets, but the thickness and cost required differ significantly. For a vented attic aiming for R-60:

• Open-cell foam on the attic floor: approximately 16 inches — comparable to blown-in cellulose in thickness and significantly more expensive
• Closed-cell foam on the attic floor: approximately 9 inches — a real advantage only in depth-constrained spaces

For most standard vented attics, the cost premium of spray foam over blown-in cellulose is not justified by a meaningful performance advantage. The exception is specific problem areas where air sealing or moisture control is critical.

As Natural Resources Canada’s attic insulation guide explains, the combination of air sealing and insulation depth is what drives thermal performance — the material choice matters less than doing both steps correctly.

Spray Foam and Ontario Rebate Programs

Both open-cell and closed-cell spray foam installations can be eligible for Ontario’s Home Renovation Savings (HRS) program, provided:

• A pre-retrofit energy audit is completed by a registered energy advisor
• The work is performed by a licensed, participating contractor
• The installation meets minimum code requirements

For standalone attic insulation upgrades, the rebate can reach up to $1,250 (subject to eligibility and program availability). Combined with other home upgrades, the total rebate available under HRS can reach $5,000–$10,000.

For complete program details, visit homerenovationsavings.ca.

Because spray foam costs significantly more than blown-in cellulose, the rebate represents a smaller percentage of the total project cost. However, in applications where spray foam is genuinely the right product — rim joists, cathedral ceilings, crawlspaces — the rebate meaningfully reduces the out-of-pocket investment.

Before and after: rim joist insulation with closed-cell spray foam combined with blown-in cellulose on the attic floor — a hybrid approach that maximizes performance and cost-efficiency.

A Practical Decision Guide for GTA Homeowners

Choose Open-Cell Foam If:

• You have a cathedral ceiling or unvented roof assembly where foam is required at adequate thickness
• You want excellent air sealing combined with vapour-permeable behaviour
• Your assembly will have a vapour retarder on the warm side

Choose Closed-Cell Foam If:

• You are insulating rim joists — this is the single most common and cost-effective spray foam application
• You need vapour control and air sealing in the same thin application
• Space is severely constrained and you need maximum R-value per inch
• Moisture exposure is a concern (crawlspaces, below-grade walls)

Consider Blown-In Cellulose Instead If:

• You have a standard vented attic
• Your primary goal is achieving R-50 or R-60 at the lowest cost per unit of R-value
• Maximizing rebate return on investment is a priority
• Environmental impact matters to you (cellulose has significantly lower embodied carbon)

As Energy Star notes, proper air sealing paired with insulation delivers the best outcome regardless of material — the combination matters more than either element alone.

The Hybrid Approach: Getting the Best of Both

The most cost-effective attic insulation strategy in many GTA homes combines materials strategically:

1. Closed-cell spray foam on all rim joists (typically 2 inches, creating both a vapour retarder and air barrier)
2. Manual air sealing of attic floor penetrations using fire-rated sealants
3. Blown-in cellulose to R-60 over the attic floor

This hybrid approach gives you the moisture control and air sealing of spray foam where it matters most, and the cost efficiency of cellulose over the large attic floor area. It is what we recommend in a significant percentage of our GTA projects.

Frequently Asked Questions

Is spray foam always better than cellulose for air sealing?

Spray foam creates a more continuous air barrier on its own. However, cellulose with proper pre-installation air sealing achieves comparable air tightness results in most vented attic applications. The performance gap is smaller than foam marketing suggests.

Can spray foam be added over existing cellulose or fiberglass?

In some cases, yes — typically for targeted air sealing in specific problem areas. Adding spray foam over existing insulation for its thermal properties alone is rarely cost-effective.

How long does spray foam last?

Properly applied spray foam has an indefinite lifespan as long as it remains undisturbed. It does not settle like loose-fill materials. However, UV exposure will degrade exposed foam, so any exposed surfaces in unconditioned spaces should be covered.

Will spray foam affect my rebate eligibility?

Both types of spray foam are eligible under Ontario’s HRS program when installed by a participating contractor following an energy audit. Many homeowners save 70–85% through government rebates. Your actual cost depends on your home’s specific needs and rebate eligibility.

Making the Right Call for Your Home

Open-cell versus closed-cell spray foam is not a question that has a universal answer — it depends on where in your home the foam is going, what moisture and air sealing conditions exist, and how the choice fits your overall project budget and rebate strategy.

What I can tell you from 18 years of field experience is that the most expensive option is rarely the most appropriate one for standard attic applications. A well-executed blown-in cellulose installation with proper air sealing outperforms a poorly-applied spray foam job every time.

Canada Energy Solution installs all three spray foam types (open-cell, closed-cell) as well as blown-in cellulose (starting at $0.99/sqft) and batt insulation across Toronto, Mississauga, Brampton, Oakville, Hamilton, Vaughan, Burlington, North York, Scarborough, Etobicoke, and the GTA. We are TSSA certified, Energy Star partners, hold a BBB A+ rating and a 4.9-star Google rating from 438+ reviews. We offer 24/7 emergency service.

Learn more at our attic insulation page or call (647) 812-5200 for a free home assessment. Actual cost depends on your home — book a free assessment for an exact quote.

Our work is independently rated through our BBB Business Profile. For information about spray foam insulation types, Owens Corning’s insulation resource centre is also a useful reference.