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Spray Foam vs Mineral Wool in Ontario: Where Foam Wins (Air Sealing, Below Grade, Rim Joists) and Where Wool Wins (Cost, Fire, Sound, Repairs), With the Cost Per R-Point Worked Out
Insulation & Air Sealing
Spray Foam vs Mineral Wool: Foam Seals and Wool Does Not, Wool Burns Last and Foam Does Not, and the Price Is Double. Pick by the Wall, Not the Brochure
Closed cell spray foam at 3.5 in. gives you R-20, an air barrier and a vapour retarder in one pass for about $4.40 a square foot. A mineral wool batt gives you R-22 nominal, no air barrier at all and nothing stopping vapour for $2.40, and it will still be sitting there unburned after the foam on the other side of the house is gone. Both are good products. Each is the wrong one in about half the places I see it specified. The decision is about the surface the insulation goes against, what has to stop air in that assembly, and who will want to open it up later. Here is where each one wins, with the money.
The numbers side by side
| Closed cell spray foam | Mineral wool batt | |
|---|---|---|
| Installed cost, 2×6 wall | $4.40 per sq ft at 3.5 in. (R-20); $6.90 full depth (R-31) | $2.40 per sq ft for R-22, $2.00 to $2.75 |
| Cost per R-point per sq ft | about $0.22 | about $0.11 |
| R per inch | R-6 nominal, R-5.6 aged | R-3.7 to R-4.3 |
| Effective R in a 2×6 wall | about R-24 full depth; R-19 as 2 in. flash plus R-14 batt | about R-17 |
| Air barrier | Yes, at 1 in. and up | No. Needs sealed poly or a taped sheathing |
| Vapour retarder | Yes, at 2 in. and up; no poly needed | No. Needs 6 mil poly on the warm side, $0.50 |
| Against concrete | Yes, directly | Only with a foam layer or an airspace and the moisture detail right |
| Fire | Combustible; thermal barrier required where exposed | Non-combustible; melts above about 1,000 C |
| Sound, 2×6 wall | Poor to fair; rigid foam carries sound | STC 45 to 48 |
| Next renovation | The electrician curses it | Pull it out, put it back |
| Masking, minimum charge | $300, and $900 to $1,500 minimum | None, and you can do it yourself |
The foam rates and what moves them are on the spray foam cost page. Mineral wool against fibreglass, the cheaper batt, is on its own page.
The one thing foam does that wool cannot
Mineral wool, like fibreglass, is a filter. Air goes through it. A batt in a leaky wall is a wool sweater in the wind: warm on a still day, useless on a windy one. The code knows this and asks for an air barrier as a separate layer, which in a frame wall is the poly, sealed at every lap, box and plate. The good contractors do it and the rest tape the easy bits.
Closed cell foam is its own air barrier at 1 in. and its own vapour retarder at 2 in. It fills the shape of whatever it is sprayed into, bonds to the wood and the concrete, and the first time a blower door runs on the house it shows. That is the whole case for foam, and it is a strong one in three places: on a concrete wall, where a batt against cold concrete grows mould if the detail is wrong; at the rim joist, where there are fourteen odd-shaped bays per wall and no way to seal poly; and against roof sheathing in an unvented cathedral, where nothing else is allowed to be that close to the deck. If someone offers you mineral wool in any of those three spots without a foam layer first, the wall is being built for the brochure, not the climate. The basement section has the concrete detail in full.
Where wool wins, which is more places than people think
Cost. R-22 for $2.40 against R-20 for $4.40. Per R-point, wool is less than half the price, and if the frame wall has a decent air barrier the foam’s extra tightness in that wall buys less than you would hope. On 2,400 sq ft of new wall that is $5,760 against $10,560, a $4,800 difference that would pay for the whole air sealing package twice.
Fire. Mineral wool is spun rock. It does not burn, it does not feed a fire, and it is what goes into fire-rated assemblies and around chimneys for a reason. Foam is a petroleum product that burns fast with black smoke and needs 1/2 in. of drywall over it anywhere it faces a room, which is a line on the budget that foam quotes never carry. What burns and what does not.
Sound. A 2×6 wall with mineral wool tests at STC 45 to 48. The same wall with rigid closed cell foam is worse than empty for some frequencies, because the foam couples the two faces together. Bedroom walls, the wall against the garage, the laundry: wool.
Repairs. In fifteen years someone will want a new circuit or a bathroom fan run through that wall. In wool they pull a batt, do the work and put it back. In foam they chisel. I have had electricians add a line to a quote for working in a foamed wall, and I did not argue.
No truck. No masking, no minimum, no re-entry time, no odour. You can hang the batts on a Saturday with a bread knife and a dust mask.
The basement, where the two meet
Assembly B on the basement pages is 3 in. of closed cell on the concrete, R-18, with a 2×4 wall in front of it and no poly: $4.25 plus $4.50, $8.75 a square foot, about R-19 to R-21 effective and the best air seal there is. Assembly C is a 2×6 wall with an R-22 mineral wool batt and poly: $5.50 plus $2.40 plus $0.50, $8.40, R-20 nominal. On the 984 sq ft of wall in a 36 x 28 ft house that is $8,610 against $8,266, a saving of $344 for the batt, which is not much to pay for the better air seal.
And the batt wall is the one I would not build on a poured foundation. A batt against cold concrete needs an airspace off the wall, a foam sill under the plate, a moisture barrier on the concrete and the poly inside, and if any of those is missed it grows mould behind the drywall where nobody sees it until the smell arrives. Mineral wool handles water better than fibreglass, it drains and dries and does not slump, but it is still a filter against a wall that sweats in July. Foam first on concrete, then whatever you like in front of it. In the ICF basements I build the question does not come up, because the wall is foam both sides of the concrete and nothing is added.
Rule of thumb: anything that touches concrete or roof sheathing gets foam. Anything inside a frame wall with a real air barrier, or anything you want to be quiet or fire-safe, gets wool. Flash and batt, 2 in. of foam and an R-14 batt at about $3.60, is the honest compromise in a 2×6 wall and runs about R-19 effective.
What I use in my own houses
The houses I build are ICF, so the exterior walls above and below grade already have 2.5 in. of EPS on each face of the concrete and need neither product. That leaves the places foam is still the answer: 2 in. of closed cell under a bonus room floor over the garage, against the sheathing in a cathedral, at any rim that is framed rather than formed. Mineral wool goes into the interior partitions around bedrooms and bathrooms at $1.80 a square foot for R-14, where it is bought for quiet, not for heat. Over the ceiling, blown cellulose to R-60 with the poly below it, because nothing beats loose fill per dollar on a flat attic floor.
Where a client has a frame wall, I price it both ways in the wall assembly comparison calculator and let the effective R and the installed cost argue it out. The result is usually flash and batt or a batt with exterior foam, and almost never full-depth foam, because R-24 effective for $6.90 is a poor way to spend $6.90. A stamped heat-loss calculation run on the effective numbers is what settles it for the furnace or heat pump, and it does not care which product got you there.
Questions people ask
Is spray foam better than mineral wool?
On concrete, at the rim joist and against roof sheathing, yes, because foam is an air barrier and vapour retarder and the batt is a filter. In a frame wall with a sealed air barrier, in a sound wall or a fire separation, mineral wool does the job for less than half the price per R-point and comes out again when the wall is opened.
How much more does spray foam cost than mineral wool?
About double for the same R: $4.40 a square foot for 3.5 in. of closed cell at R-20 against $2.40 for an R-22 mineral wool batt, before masking and the $900 to $1,500 minimum that foam jobs carry.
Which is safer in a fire?
Mineral wool. It is non-combustible and melts above about 1,000 C. Spray foam burns and must be covered with a thermal barrier, usually 1/2 in. drywall, wherever it faces a living space.
Is mineral wool a vapour barrier or air barrier?
Neither. It needs 6 mil poly or a smart retarder on the warm side and a sealed air barrier somewhere in the wall. Closed cell foam is both on its own at 2 in.
Can I put mineral wool against a basement wall?
Only behind a foam layer or with an airspace, a foam sill and a moisture barrier done exactly right. Foam on the concrete first, then the batt in a stud wall, is the assembly that does not grow mould.
Price the foam side of the wall
The spray foam cost calculator prices every surface you are thinking of foaming at its real thickness with the masking and the minimum, so you can hold the number next to the batt price and decide wall by wall. If a quote has foam in the bedroom partitions and batts on the basement concrete, the $99 review will say which surfaces got the wrong product.
Sources and further reading
- Rockwool: mineral wool batts and boards: R-value, fire rating, sound ratings and installation.
- CUFCA Quality Assurance Program: Canadian Urethane Foam Contractors Association: certified installers with photo ID, licensed contractors, daily work sheets and the CAN/ULC S705 standards a spray foam job is held to.
- Keeping the Heat In: Natural Resources Canada: the plain-language Canadian guide to air sealing, insulation materials, basements, walls and ventilation.
