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Insulation, Fire and Thermal Barriers in Ontario: What Burns and What Does Not, the OBC 9.10.17 Rule That Puts Drywall Over Every Foam, Fire-Rated Assemblies, Chimney Clearances and the Substitutions That Void a Rating
Insulation & Air Sealing
Foam Burns, Rock Does Not, and the Code Has a Rule for Every Square Foot of Foam Facing a Room. It Is Missing From Nearly Every Foam Quote
All of the foam insulations, spray foam of both kinds, EPS, XPS and polyiso, are petroleum plastics, and they burn the way plastic burns: fast, hot and with a black smoke that fills a basement in a minute. The code does not ban them. It asks that wherever foam faces a living space there be a thermal barrier over it, which in practice is 1/2 in. drywall or a tested intumescent coating, so that in a fire the foam is the last thing to get involved rather than the first. That one line, OBC 9.10.17, is the most commonly missed item in insulation work in Ontario, because it is not the foam company’s trade and nobody else was asked. Here is what burns, what does not, what the barrier is, and what the whole thing costs.
What burns and what does not
| Insulation | In a fire | Barrier needed where exposed to a room? | Notes |
|---|---|---|---|
| Closed cell spray foam | Burns, fast, dense black smoke | Yes, thermal barrier | 2 lb polyurethane. The foam of rims, basements, cathedrals. |
| Open cell spray foam | Burns | Yes, thermal barrier | 0.5 lb; more air, same chemistry. |
| EPS, XPS board | Burns, melts and drips | Yes, thermal barrier | Polystyrene. ICF forms, basement walls, under slabs. |
| Polyiso board | Burns, chars somewhat better than polystyrene | Yes, thermal barrier | Foil facers do not change the answer. |
| Cellulose, borate treated | Chars and self-extinguishes; smoulders if untreated or wet | No barrier rule, but it is not left exposed either | Flame spread under 25 when tested to CAN/ULC S703. |
| Fibreglass batt or blown | Glass does not burn; softens and slumps at about 540 C; kraft facing burns | No; faced batts need the facing covered | Non-combustible as a loose product. |
| Mineral wool batt or board | Does not burn; melts above about 1,000 C | No | The insulation inside fire-rated assemblies and firestopping. |
Combustible does not mean banned. It means the foam has to be inside the wall, behind the barrier, which is where insulation belongs anyway. Which batt goes where for fire and sound is on the batt comparison page.
The thermal barrier rule
OBC 9.10.17 says that foamed plastic insulation in a wall or ceiling of a building has to be protected from the interior by a thermal barrier. The accepted barriers in Part 9 are 12.7 mm, which is 1/2 in., gypsum board; 11 mm plywood, OSB or waferboard; lath and plaster; sheet metal; or any material that passes the CAN/ULC S124 test. The reason is time. Drywall holds the fire off the foam for the minutes it takes a family to get out and a truck to arrive. Bare foam gives the fire a head start and a smoke that will stop you before the heat does.
In practice, in a house: the foam on the basement walls gets drywall when the basement is finished. The foam on the garage ceiling under a bonus room gets drywall. The ICF walls I build have foam on both faces, and the inside face is drywalled like any other wall, which is why an ICF house needs no further thought; the exterior foam face is behind the cladding and faces nobody. Where drywall is not practical, a mechanical room ceiling full of pipes, a knee wall nobody will finish, the alternative is an intumescent coating sprayed over the foam, a paint that swells into a char when heated. $1.25 to $2.00 a square foot, with the product’s own listing to show the inspector. On 1,000 sq ft of exposed basement wall that is $1,250 to $2,000; drywall on the same wall, at the drywall rate, costs more but you were going to want it anyway.
The line everybody misses: the foam company quotes the foam. The drywaller quotes the drywall if he is asked. Nobody quotes the coating on the rim joists, the knee wall or the garage ceiling until the inspector asks where it is. Put it on the list before the foam truck comes.
Attics, crawlspaces and the rim: the ignition barrier
The code eases up where nobody lives. In an attic or a crawlspace that is entered only to service equipment, with no storage and no finished floor, the full thermal barrier is not demanded; what most Ontario inspectors look for is a thinner protective coat called an ignition barrier over the foam, at about $1.25 a square foot, or foam that carries its own listing for exposed use in that space. It is still a coating, still a line, still missing from most quotes.
The rim joist in an unfinished basement is the argument every inspector has had. The foam is 2 in. thick, in a strip 10 in. high, at the top of a wall in a space with a concrete floor and a furnace. Many building departments accept it bare in an unfinished basement; some want the intumescent coat; all of them want drywall over it once the basement is finished. Ask your own building department before the foam goes on, and if the answer is a coating, have the foam contractor include it: a $150 to $250 extra on a rim job and a $400 call-back later.
Fire-rated assemblies and the substitutions that void them
Some walls and ceilings in a house are not just drywalled; they are rated. The wall and ceiling between an attached garage and the house has to be gas-tight, taped and sealed so exhaust and a car fire stay on the garage side. A duplex or a house with a secondary suite has a 45-minute or 1-hour fire separation between the units, built to a listed assembly: a specific drywall, a specific number of layers, a specific stud spacing and, often, a specific insulation. Penetrations through those assemblies get firestopped with mineral wool packing and a rated sealant, not with a can of foam from the hardware store.
The insulation in a listed assembly is part of the listing. Where the sheet says mineral wool, mineral wool goes in; swap in fibreglass because it was on sale and the assembly is no longer the one that was tested, and the inspector is right to fail it. Swap in spray foam and you have put a combustible into a wall whose job was to not burn for an hour. The listing is a recipe, and nobody gets to substitute ingredients.
Clearances: chimneys, pot lights, old wiring
- Masonry chimneys and B-vents: 3 in. (75 mm) clear of any insulation, combustible or not, with a sheet-metal dam to hold loose fill back. Non-combustible does not mean allowed to touch; the clearance is about the flue temperature, not the insulation.
- Non-IC pot lights are not rated for contact with insulation and have burned down attics. They get boxed with a clearance or, better, replaced with IC-rated airtight fixtures at $45 a cover or the price of a new light. Insulation blown over an uncovered non-IC fixture is the first thing an inspector pulls back.
- Knob-and-tube wiring: the Ontario Electrical Safety Code does not allow thermal insulation in contact with it, and most insurers refuse the house anyway. The wiring is replaced before the attic is insulated, not after.
- Furnace and water heater flues, exhaust fans, heat recovery vents: the manufacturer’s clearance, printed on the appliance, and sealed with a rated material where they pass through the air barrier.
- Foam against a fireplace chase or a wood stove wall: no. Mineral wool, with the clearance.
What to specify and what it costs
For a spray foam job, the quote carries the foam, the masking and the minimum, and it should carry a line that says where the thermal barrier is coming from: drywall by the finishing trade, or intumescent coating by the foam contractor at $1.25 to $2.00 a square foot, or ignition barrier in the attic or crawlspace at $1.25. The spray foam cost page has the rest of the bill. For board foam on a basement wall, the same: the board is $2.75 to $4.75 a square foot taped, and the drywall over the stud wall in front of it is the barrier; the rigid foam page covers which board. For an ICF house, nothing extra: the inside is drywalled, the outside is clad, and the foam never sees a room.
Checking it is the easiest inspection on this site. Stand in every room with foam in the wall or ceiling and ask whether you can see foam. If you can, something is owed. A house with bare foam in the rec room passed no final inspection, and what else that contractor skipped is on the installation problems page.
Questions people ask
Does spray foam need a thermal barrier in Ontario?
Yes, wherever it faces a living space: 1/2 in. gypsum board, 11 mm plywood or OSB, or a coating tested to CAN/ULC S124, under OBC 9.10.17. In an attic or crawlspace entered only for service, an ignition barrier coat is the usual acceptance. Ask the building department about bare foam at the rim in an unfinished basement.
Is spray foam insulation flammable?
Yes. Both closed and open cell foam, and all the foam boards, are combustible plastics that burn fast with dense smoke. The code allows them inside walls and ceilings behind a thermal barrier, which is why exposed foam in a finished room is a code failure, not a style choice.
What is an intumescent coating and what does it cost?
A paint applied over foam that swells into an insulating char when heated, listed as a thermal barrier or ignition barrier for specific foams. $1.25 to $2.00 a square foot applied, $1.25 for the ignition-barrier grade in attics and crawlspaces. It is the alternative where drywall is not practical.
Is mineral wool fireproof?
It is non-combustible and keeps its shape past about 1,000 C, which is why fire-rated wall and floor assemblies and firestopping around penetrations specify it. It still has to keep the 3 in. clearance from chimneys and flues, because the clearance is about the flue, not the insulation.
Can I use fibreglass instead of mineral wool in a fire-rated wall?
Only if the listed assembly names it. The insulation is part of the tested recipe; substituting another product means the wall is no longer the one that was tested, and the inspector can fail it. Where the listing says mineral wool, that is what goes in.
Get the barrier into the foam number
The spray foam cost calculator prices the foam surface by surface and lets you add the coating or the drywall over each one, so the thermal barrier is in the total before the truck arrives instead of in a change order after the inspector leaves. If you have foam quotes already and none of them mention a barrier, the $99 review will say which rooms are going to fail.
Sources and further reading
- Ontario’s Building Code: the official page with the current regulation; vapour barriers are 9.25, attic ventilation 9.19, thermal barriers over foam 9.10.17, radon 9.13.
- Electrical Safety Authority: Ontario: knob-and-tube wiring, the inspection rules, and why thermal insulation cannot be placed around it.
- Rockwool: mineral wool batts and boards: R-value, fire rating, sound ratings and installation.
