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Cathedral Ceiling Insulation in Ontario: Vented or Unvented, Foam Against the Sheathing or Dense-Pack Cellulose, the R-31 Rule, Cost per Square Foot of Roof and Why These Roofs Grow Ice Dams
Insulation & Air Sealing
Cathedral Ceilings: No Attic, No Room, and the Same Water Problem in a Space You Cannot Get Into
A cathedral ceiling puts the drywall on the underside of the rafters, so the whole insulation job has to happen in the 7 to 11 in. between the drywall and the roof sheathing. No attic to blow 17 in. into, no room to walk, no way to look at it after. Ontario asks for R-31 in that space, half the attic number, because that is what fits. There are two honest ways to do it, vented with a channel under the sheathing or unvented with foam sprayed to it, and one common wrong way: a batt stuffed full depth with no air gap and a poly that leaks. The wrong way is why the ice dam on so many Ontario houses starts over the great room.
The numbers
R-31OBC SB-12 cathedral or flat roof, Zone 1
$4.25dense-pack cellulose, per sq ft of roof
$6.905.5 in. closed cell to the sheathing, per sq ft
$4.002 in. closed cell plus R-22 batt, per sq ft
| Assembly | Per sq ft of roof | Nominal R | Vented? | Notes |
|---|---|---|---|---|
| Baffle plus R-28 or R-31 batt in a 2×10 or 2×12 rafter, 6 mil poly | $2.00 + $0.50 + baffle | R-28 to R-31 | Yes, 2 in. channel | The code-minimum new build. Needs a continuous soffit-to-ridge channel in every bay and a sealed poly. Only works in a 2×12 at R-31. |
| Dense-pack cellulose in the rafter bay | $3.50 to $5.00, default $4.25 | R-3.6 per in.: 9.25 in. is R-33 | Yes if a baffle was installed first; otherwise unvented and risky | The retrofit for an existing finished ceiling: holes drilled from below or above, patched after. |
| Closed cell foam to the sheathing, 5.5 in. | 5.5 x $1.25 = $6.90 | R-31 nominal | No, by design | Air seal and vapour retarder in one. Masking $300 a job. Thermal barrier required under it. The roof dries outward only. |
| Flash and batt: 2 in. closed cell plus R-22 batt | $2.50 + $1.50 = $4.00 | R-12 + R-22 = R-34 | No | Cheapest unvented. The foam must be thick enough to keep the batt side of it above the dew point. |
| Rigid foam above the sheathing at re-roof | Roofing rates | R-5 per in. plus what is below | Either | The best answer when the roof is coming off anyway. Roofing hub. |
Board-foot rates: closed cell $1.00 to $1.50 installed, $1.25 as the default, with a $900 to $1,500 minimum on a small ceiling. The spray foam cost calculator prices the foam options by surface and thickness; the closed vs open cell page covers why open cell does not go against Ontario sheathing without a vapour retarder.
Why R-31 and not R-60
Because the rafter is the ceiling. A 2×10 has 9.25 in. of depth, less a 2 in. vent channel, which leaves 7.25 in. for insulation; a 2×12 leaves 9.25 in. SB-12 lets the cathedral or flat portion go to R-31 where the attic above a flat ceiling has to be R-60, and the prescriptive packages trade a little elsewhere to cover the difference. On a new house where the cathedral is most of the roof, the SB-12 form on the permit says which package and what else has to be stronger; the SB-12 page explains the trading. R-31 loses about twice the heat per square foot of R-60, which is why the heat-loss calculation has to be told which roof is which.
Effective R is lower again. Rafters at 16 in. on centre are about 10% of the roof area and bridge whatever is between them, so an R-31 batt assembly is about R-26 effective; foam to the sheathing does not fix the bridging either, only continuous insulation above the deck does. The stamped heat-loss design that sizes the furnace or heat pump uses the effective number, and that is the job of OntarioHeatLoss.ca.
Vented: the channel is the whole design
A vented cathedral works like a vented attic with no room to walk: cold air comes in at the soffit, runs up a channel under the sheathing in every rafter bay, and leaves at the ridge vent. The channel keeps the sheathing cold so snow does not melt from below, and carries off any moisture that gets through the poly. It needs three things, and the absence of any one is the whole failure: a channel at least 2 in. deep from soffit to ridge in every bay with nothing blocking it, a soffit vent and a ridge vent actually connected to the bay, and a sealed vapour barrier below the insulation. OBC 9.19 wants the 1:300 ventilation with vents at both ends; a hip or a valley that cuts a bay off from the ridge is a dead bay and a wet one.
The commonest cathedral I open in a 1980s or 1990s house: 2×10 rafters, R-28 batts stuffed full depth, no baffle, poly stapled with gaps at every recessed light, and a ridge vent that connects to nothing. The sheathing is black on the underside, the top of the batt is matted, and the ice dam is on the eave below the great room every February. The fix is not more insulation. It is the channel or the foam, and the ice dam page goes through why the dam forms there first.
Unvented: foam to the sheathing, and the thermal barrier
The unvented roof gives up on the channel and makes the underside of the sheathing warm and airtight instead. Closed cell foam sprayed to the deck, 5.5 in. for R-31 on its own, is an air barrier, a vapour retarder and insulation in one lift, with no poly below it because the foam is the vapour control. Water that gets in from above now has to dry outward through the shingles, which is slow, so the roofing has to be good and stay good; this is not the assembly for a roof with leaks you have not found.
Flash and batt, 2 in. of closed cell with an R-22 batt under it, is the cheaper unvented version at $4.00 a square foot against $6.90 for full foam. The arithmetic that makes it safe: the foam has to be thick enough that the surface where the batt meets it stays above the dew point of the house air through January. For a Zone 1 roof, 2 in. of closed cell, R-12 of R-34, is the least I would spray. 1 in. of foam with an R-28 batt under it is a condensation plane with a warranty.
Thermal barrier, no exceptions. Spray foam exposed inside a living space needs 1/2 in. drywall or an approved intumescent coating over it (OBC 9.10.17); an attic or crawlspace nobody lives in gets an ignition barrier at about $1.25 a square foot. Foam left bare “until we get to the drywall” is a fire that has not started yet, and the spray foam page has the rest of the rules.
A 600 sq ft great-room ceiling, four ways
New build, vented, 2×12 rafters. Baffles, 38 bays at $3.50, $133; R-31 batts 600 at $2.00, $1,200; 6 mil poly sealed, 600 at $0.50, $300. $1,633 before HST. Cheapest, code-minimum, and it lives or dies on the poly being sealed at every light and the ridge actually venting.
Existing finished ceiling, dense-pack from above at re-roof. Sheathing holes drilled per bay, cellulose packed to 3.5 lb per cubic foot, patched before the new shingles: 600 at $4.25, $2,550. R-33 in a 2×10, unvented unless a baffle went in first, and with no new air barrier below. It is the right answer when the drywall is staying and the roof is coming off.
Existing ceiling opened from below, full closed cell. 600 at $6.90 is $4,140, masking $300, $4,440 before HST, $5,017 with. Then new drywall as the thermal barrier. Airtight, R-31, no channel to fail, and the ice dam stops. Flash and batt on the same opened ceiling: 600 at $4.00 is $2,400 plus masking $300, $2,700, R-34 nominal. At the bottom of the range, $1.00 a board foot on a big open job, the full foam drops to $3,300; two men and a truck do not come out for 600 sq ft at that rate, and the $900 to $1,500 minimum is the floor on any of these if the ceiling is small.
Questions people ask
What R-value does a cathedral ceiling need in Ontario?
R-31 nominal under OBC 2024 with SB-12 for Zone 1, against R-60 for an attic above a flat ceiling. The difference is the rafter depth: a 2×12 with a 2 in. vent channel holds about R-31 of batt, and foam to the sheathing reaches it in 5.5 in.
Does a cathedral ceiling need to be vented?
Either vented, with a continuous 2 in. channel from soffit to ridge in every bay and a sealed poly below the insulation, or unvented with closed cell foam sprayed to the underside of the sheathing. A full-depth batt with no channel and no foam is neither, and it is the assembly that rots sheathing and grows ice dams.
How much does it cost to insulate a cathedral ceiling?
Per square foot of roof: about $2.50 to $3.00 for a vented batt assembly with baffles and poly in new framing, $4.25 for dense-pack cellulose into an existing bay, $4.00 for 2 in. of closed cell plus a batt, and $6.90 for 5.5 in. of closed cell to the sheathing, plus $300 masking and the minimum charge on a small job, before HST.
Can you spray foam a cathedral ceiling from inside?
Yes, with the drywall removed, closed cell sprayed to the sheathing in lifts of about 2 in., then 1/2 in. drywall or an intumescent coating over the foam before the room is used. Open cell against the sheathing needs a vapour retarder in Ontario and is the wrong product for the job on its own.
Why do I get ice dams over my cathedral ceiling and not over the attic?
Less insulation (R-31 against R-60), air leaks at every pot light going straight to the sheathing, and a vent channel that is usually blocked or missing. Warm sheathing melts the snow, the water refreezes at the cold eave. The fix is the channel or the foam and the sealing, not a heat cable.
Price the roof you actually have
The attic insulation cost calculator takes the cathedral portion of the roof as its own area with the dense-pack and foam rates, so a house that is half attic and half great room gets priced as both. If you have quotes for the cathedral that do not say vented or unvented, how many inches of foam, or who is putting up the thermal barrier, the $99 review will ask those questions for you.
Sources and further reading
- Ontario’s Building Code, Supplementary Standard SB-12: the energy-efficiency packages that set the R-values for attics, walls, basements and slabs in a new or renovated Ontario house.
- Building Science Corporation: the cold-climate basement, wall and roof assembly guidance that explains where the condensation goes and why foam goes against the concrete.
- 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.
