Choosing Between Closed-Cell and Open-Cell Spray Foam
Closed-cell spray foam is the better pick for basement walls, rim joists, crawl spaces and any spot where you need the most R-value in a thin cavity or where moisture is a concern. Open-cell foam costs less per inch and works well in roof lines and interior cavities where there is room for thickness, but in cold climates it needs a separate vapour retarder. Many houses end up with both, each in the part of the house that suits it.
Below is how the two compare, where each one tends to go, and what to look for on a quote before anyone sprays.
What makes the two foams different
Both products start as two liquids mixed at the spray gun. They expand on contact and set into foam within seconds. The difference is in the cells.
Closed-cell foam sets into tightly packed cells that stay sealed. It is dense (often around 2 pounds per cubic foot), rigid once cured and adds some stiffness to the wall or roof it is sprayed against. It is usually rated at roughly R-6 to R-7 per inch.
Open-cell foam sets into cells that are broken open, so the foam is soft and spongy, a bit like a firm cushion. It is much lighter (often around half a pound per cubic foot), expands a lot more, and is usually rated at roughly R-3.5 to R-3.8 per inch.
Both stop air movement through the area they cover, which is the main reason people choose spray foam over batts in the first place. Where they part ways is moisture, strength, thickness and price.
Moisture and vapour control
This is the point that matters most in Canada and the northern US.
At about 2 inches, closed-cell foam slows water vapour enough to act as a vapour retarder on its own. That is why it is the usual choice against concrete foundation walls: warm, humid basement air cannot reach the cold concrete and condense behind the insulation. It also shrugs off occasional wetting far better than fibreglass or open-cell foam.
Open-cell foam lets vapour pass through fairly easily. In a cold climate, that means warm indoor air carrying moisture can move into the foam and reach a cold surface on the other side, such as roof sheathing or the back of exterior siding. To prevent that, open-cell foam in exterior assemblies normally needs a vapour retarder on the warm side (a membrane or a vapour-retarder paint, depending on what local code allows). Skipping that step is one of the more common spray foam problems found in older retrofit jobs.
Open-cell foam also holds water if it gets soaked, so it is a poor fit for below-grade walls or anywhere a leak is likely.
Where each foam tends to go
| Area of the house | Usual choice | Why |
|---|---|---|
| Basement concrete walls | Closed-cell | Vapour control against cold concrete, high R-value in a thin layer |
| Rim joists | Closed-cell | Seals the leakiest joint in many houses and controls condensation |
| Crawl space walls | Closed-cell | Handles damp conditions, works with a sealed ground vapour barrier |
| Cathedral or flat ceilings with shallow rafters | Closed-cell | Gets the most R-value into limited depth |
| Unvented roof lines with deep rafters | Either, depending on climate and code | Open-cell needs proper vapour control in cold zones |
| Interior walls and floors (sound control) | Open-cell | Softer foam absorbs sound, lower cost |
| Knee walls and bonus rooms over garages | Often closed-cell | Air seal plus strong R-value where space is tight |
Open attic floors are a different story. For a regular vented attic, blown-in cellulose or fibreglass on top of proper air sealing usually delivers the same or better result for much less money than spraying foam. Spray foam earns its cost in the tight, awkward or damp spots.
How cost compares
Spray foam is priced by the board foot (one square foot, one inch thick). As rough, typical ranges in many North American markets, closed-cell foam often runs about $1 to $2 per board foot installed and open-cell foam about $0.40 to $0.80. These are ballpark figures only, and your quote may land outside them.
Per inch, open-cell is clearly cheaper. Per unit of R-value, the gap narrows, because you need roughly twice the thickness of open-cell to match closed-cell. In a 2x4 wall, you cannot fit enough open-cell to match what 2 to 3 inches of closed-cell provides, so the comparison is often decided by the space you have.
Things that push the price up or down on a spray foam job:
- Total area and thickness. Small jobs, like rim joists alone, often carry a minimum charge because the rig and crew cost the same to bring out.
- Access. Low crawl spaces, finished basements with partial access and tall cathedral ceilings take more time.
- Prep work. Removing old batts, cleaning walls, fixing water issues and masking finished surfaces add hours.
- Thermal or ignition barrier. In living spaces, building codes generally require foam to be covered by a thermal barrier such as half-inch drywall. In some attics and crawl spaces an ignition barrier coating is required instead. Either one adds cost if it is not already planned.
- Region and season. Labour rates and demand vary widely, and fall is a busy time for basement and rim joist work.
Safety and the curing period
Spray foam is applied as a chemical reaction, and the fumes during spraying are a real hazard. Occupants are kept out of the house while it is sprayed and for a period afterward while it cures and the space is ventilated. Many manufacturers call for at least 24 hours before re-entry, and some products or conditions call for longer. Your quote should state the re-entry time and how the space will be ventilated.
In Canada, spray polyurethane foam is installed under a national standard (CAN/ULC-S705.2) that sets requirements for the installer and the job site, and there are quality assurance programs that track installations. In the US, the rules come from the building code your state or city has adopted and the product's own evaluation report. Either way, it is fair to ask which product is being used, how thick it will be in each area, and how the installer checks the depth.
Questions to settle before you sign
A good spray foam quote spells out:
- Which foam (closed-cell or open-cell) goes in each area, and the product name.
- The thickness and resulting R-value for each area.
- Whether a vapour retarder is needed and who provides it.
- Whether a thermal or ignition barrier is needed and whether it is included.
- Any prep: removal of old insulation, cleanup, protection of finishes.
- The re-entry time after spraying.
- Warranty terms, in writing.
Frequently asked questions
Can open-cell spray foam be used in a basement?
It is generally not recommended against basement concrete in cold climates. Open-cell foam lets vapour reach the cold concrete, where it can condense, and it absorbs water if the wall ever gets damp. Closed-cell foam or rigid foam board are the usual choices for below-grade walls.
Is closed-cell spray foam worth the extra cost?
It is worth it where its strengths matter: basement walls, rim joists, crawl spaces and shallow roof cavities where you need high R-value in little space and vapour control in one step. In an open attic floor or an interior wall, the extra cost usually buys little that cheaper materials cannot provide.
How thick should spray foam be on a rim joist?
Many installers apply about 2 to 3 inches of closed-cell foam on rim joists, which seals air leaks and gives roughly R-12 to R-20 depending on thickness and product. Local energy code may call for a specific minimum, so the quote should give the thickness and R-value for your area.
Does spray foam need to be covered with drywall?
In most living spaces, yes. Building codes in Canada and the US generally require foam plastic insulation to be covered by a thermal barrier, commonly half-inch drywall. Some unfinished attics and crawl spaces can use a thinner ignition barrier coating instead, depending on the product and local rules.
Get a quote for spray foam work
If you are planning to finish a basement, chase down a cold floor over a crawl space or tighten up a drafty rim joist, send us the details through the quote form. Mention which areas you have in mind and what you are noticing, and a local specialist will put the foam type, thickness and R-value for each area in a written quote. Homeowners in cities such as Montréal, Minneapolis and Calgary can request one the same way.