The basic tradeoffs
Closed-cell (2lb)
- R-value
- Roughly R-6 to R-7 per inch.
- Air barrier at
- About 1 inch.
- Vapor control
- Becomes a Class II vapor retarder at about 1.5 inches; functions as insulation, air barrier, and vapor retarder in one material.
- Where it's the standard choice
- Foundation walls, slabs, rim joists, crawl spaces, radon barrier applications, roof decks in most cold-climate assemblies.
- Tradeoff
- Costs more per board-foot; higher embodied impact per inch than open-cell.
Open-cell (½ lb)
- R-value
- Roughly R-3.5 to R-4 per inch.
- Air barrier at
- About 3.75 inches or more.
- Vapor control
- Stays vapor-open even at full cavity depth; not a vapor barrier. Often paired with a separate interior vapor strategy in cold climates.
- Where it's a reasonable choice
- Above-grade wall cavities and interior sound-dampening applications, where a vapor-open assembly that can dry to the interior is actually preferred.
- Tradeoff
- Needs more thickness for the same air-sealing performance; a real (manageable) moisture consideration on unvented roof decks. See below.
The roof-deck question, answered honestly
This is the one still-studied controversy in spray foam, and it deserves a straight answer rather than either dismissal or vague hedging. Multi-year field research from Oak Ridge National Laboratory, working with the University of Florida, monitored homes with open-cell foam applied directly to the underside of roof sheathing in unvented (conditioned) attic assemblies. The finding: when sheathing moisture problems showed up, the dominant driver in most cases was humidity generated inside the conditioned space, carried upward and trapped against the cold roof deck, not moisture entering from outside or above. Researchers have noted that if outside-in moisture were the main mechanism, closed-cell foam would actually perform worse in that scenario, because it would block the assembly's one remaining drying path to the interior.
The practical response building scientists recommend isn't "avoid open-cell on roofs." It's controlling interior humidity, generally keeping winter indoor relative humidity below about 35% in cold climate zones, and sizing any ductwork in a conditioned attic correctly so it isn't adding uncontrolled moisture or pulling conditioned air where it shouldn't. In a Prairie winter, indoor humidity is already something worth managing carefully regardless of insulation choice (see our page on air barriers vs. vapor barriers), so this isn't a separate problem so much as the same discipline applied to the roof assembly.
Why we're telling you this instead of just selling one product
A technically credible contractor should be able to explain a real, active area of building science debate rather than pretend it doesn't exist or dismiss it as competitor scare-tactics. The honest answer is: it depends on the assembly, the climate zone, and how well interior humidity is controlled, not a blanket claim in either direction.
Cost and embodied impact
Open-cell costs less per board-foot and uses less material and blowing agent per inch of coverage, so its embodied environmental impact per inch is lower. Closed-cell costs more, but delivers roughly double the R-value per inch and does the job of three separate control layers (insulation, air barrier, vapor retarder) in one material, which can reduce total assembly complexity and the number of places workmanship can go wrong. Neither cost comparison is complete without accounting for what a fibrous-batt-plus-separate-air-barrier assembly would cost to achieve the same performance.
Frequently asked questions
What is the difference between open-cell and closed-cell spray foam?
Closed-cell foam is denser, roughly R-6 to R-7 per inch, needs only about 1 inch to act as an air barrier, and becomes a vapor retarder at about 1.5 inches. Open-cell foam is lighter and softer, roughly R-3.5 to R-4 per inch, needs about 3.75 inches or more to act as an air barrier, and stays vapor-open even at full cavity depth.
Does open-cell spray foam cause roof rot?
Field research from Oak Ridge National Laboratory and the University of Florida found that when moisture problems occur under open-cell foam on a roof deck, the dominant cause is usually humidity generated inside the conditioned space, not moisture entering from outside. The practical fix is controlling interior relative humidity, generally keeping winter indoor RH below about 35% in cold climates, and sizing ductwork correctly in a conditioned attic, rather than avoiding open-cell foam altogether.
Which spray foam type is better for a cold climate like Manitoba or Saskatchewan?
Neither is universally better; the right choice depends on the assembly and what job needs doing. Closed-cell foam is generally preferred for below-grade and slab applications because it also functions as a vapor retarder and air barrier in one step and resists moisture directly. Open-cell foam is a reasonable, lower-cost choice in above-grade wall cavities where a separate vapor strategy is already planned, and its lower embodied impact and better sound dampening are real advantages where vapor control isn't the deciding factor.
Not sure which foam fits your project?
We'll walk through the assembly with you before recommending either.
