Open-Cell vs. Closed-Cell Spray Foam: Which Should You Choose?

Published August 15, 2026 · 9 min read · By NorthStar Spray Foam & Insulation Inc.

The basic difference

Both products are sprayed polyurethane foam, applied as two liquid components that react and expand on contact. The difference is what happens to the cells as the foam rises.

In open-cell foam, the cell walls break during expansion, leaving a network of interconnected voids. The result is soft, spongy and light — you can press it with a thumb.

In closed-cell foam, the cells stay intact and sealed, each one holding gas. The result is rigid and hard — it does not compress under thumb pressure.

Every practical difference between the two follows from that one structural distinction.

Density

Closed-cell foam is substantially denser than open-cell. More material occupies the same volume, which is the root of its higher cost per inch, its rigidity, and its resistance to vapour and water movement.

Open-cell foam is light. That matters in a few places — a large area of foam applied to the underside of a roof deck adds load, and lightness can be an advantage. It also expands more during application, which fills irregular cavities readily but means more trimming where it overfills.

Thermal performance

Closed-cell delivers more thermal resistance per inch than open-cell. In a cavity where depth is limited, that is decisive: you may simply not be able to reach the required performance with open-cell in the space available.

Where depth is not constrained — an attic, for instance — the comparison shifts. You can apply more inches of open-cell to reach the same total, often for less money, because open-cell costs less per inch.

Air control

Both foams, applied at sufficient thickness, form a continuous layer that resists air movement. This is the main advantage of foam over batt or loose-fill: it addresses insulation and air sealing in one application, with no joints to tape and no seams to miss.

The critical word is sufficient. A thin skim of either product does not reliably control air. The thickness at which it does differs between products, which is one of the things a specification exists to establish.

Air control frequently matters more than raw thermal performance. Air leaking through an assembly carries heat and moisture with it, and in older GTA houses that leakage often costs more than the conduction through the insulation does.

Vapour and moisture

This is the difference that changes assembly decisions, and the one most often skipped.

Open-cell foam is vapour permeable. Water vapour can move through it. In an assembly that needs to be able to dry through that layer, that is exactly what you want.

Closed-cell foam restricts vapour movement at sufficient thickness. On the interior of a basement wall, that is precisely the point — it keeps interior humidity away from a cold concrete surface where it would condense.

But a layer that restricts drying in one direction has to be considered against what the rest of the assembly does. If there is already a vapour retarder on the other side, adding a second restrictive layer can leave the space between them unable to dry in either direction. Moisture that gets in — and some always does — then has nowhere to go.

Sound performance

Open-cell foam is the better acoustic performer of the two within a cavity. Its soft, porous, interconnected structure absorbs sound energy; closed-cell is dense and rigid, which makes it a poorer absorber.

Neither soundproofs a wall on its own. Sound isolation between spaces comes from mass, structural separation between the two faces, and sealing — insulation contributes but is not the mechanism. See our soundproofing page for the full picture, including why fibrous batts are often the better cavity absorber than either foam.

Rigidity

Cured closed-cell foam is hard and rigid, and it bonds strongly to the substrate. That adds stiffness to the assembly it is sprayed into — a genuine structural contribution in some applications, though one that should be assessed by an engineer rather than assumed.

The practical trade-off is later access. Closed-cell foam bonded to sheathing and framing is difficult to remove without damaging what it is stuck to. Worth thinking about if the cavity may need opening for services in future.

Open-cell is soft and easier to cut, which makes later access more straightforward.

Common applications

Where closed-cell is typically used

• Basement and foundation walls
• Crawl space walls and rim joists
• Cantilevered floors, bay window overhangs and rooms over garages
• Assemblies where depth is limited and performance is required
• Below-grade and exterior applications
• Metal roof decks in industrial buildings, for condensation control
• Anywhere resistance to bulk water matters

Where open-cell is typically used

• Attic roof lines in assemblies designed to dry inward
• Interior walls where depth is available
• Interior partitions where cavity sound absorption is wanted
• Filling irregular interior cavities where its expansion helps
• Larger areas where budget is a real constraint and depth is not

See our spray foam page for how we work through these decisions on site.

Cost factors

Open-cell costs less per inch installed. Closed-cell costs more per inch but may need fewer inches. Which is cheaper for your project depends on:

• The performance target
• The depth available — the constraint that often decides it outright
• Whether vapour control is needed from the insulation layer
• Whether rigidity is a requirement
• Total area, which affects how fixed set-up costs spread

Our guide on spray foam insulation cost in Mississauga covers every factor in detail.

Why the whole assembly matters

A wall or roof is a system. Cladding, sheathing, weather barrier, framing, insulation, vapour control and interior finish all interact, and inserting a new layer changes how the rest behaves — particularly where moisture can go.

Questions worth asking before either foam is specified:

• What is on the exterior side, and how permeable is it?
• Is there already a vapour retarder, and where?
• Which direction can this assembly currently dry in?
• Will it still be able to dry in that direction afterwards?
• Is the space vented, and does it need to stay vented?
• How is the space used — high humidity, or normal occupancy?
• Is there an existing moisture source that has to be fixed first?

This is why we assess before quoting. Choosing between open-cell and closed-cell from a description of a room is guessing at the one decision that most affects whether the assembly works in ten years.

Summary table

Frequently asked questions

Where this leaves you

Choosing between the two foams only matters once foam is the right family of product at all. In an open, accessible attic floor it usually is not — blown-in loose fill builds depth far more cheaply — and in a square, unobstructed stud bay a properly fitted fibreglass batt does the job for less again. Foam earns its price where the cavity is irregular, depth-limited, or has to be air sealed in the same operation.

Two related decisions come up constantly alongside this one. Whether the attic’s existing material can stay is covered in when old attic insulation should be removed; whether to insulate the attic floor or the roofline at all is covered in blown-in versus spray foam for attics. If the question is money rather than chemistry, what drives spray foam cost is the relevant guide.

One thing neither foam is: fire protection. Spray polyurethane is a combustible plastic that needs a thermal barrier where it is left exposed, and applied fire protection is a separate discipline entirely. Open-cell’s sound-absorbing quality is real but partial, so a genuine noise complaint belongs with sound-control work. We install both foams for homeowners and industrial operators across the GTA, including Concord. If you want the choice made against your actual assembly, ask us to assess it, or read how we approach spray foam first.

Last reviewed: August 15, 2026