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Flash and Batt Hybrid Walls in Ontario: 2 in. of Closed-Cell Foam Plus a Batt, the Condensation Arithmetic, When It Is Worth the $3.60 a Square Foot and When It Is Not
Insulation & Air Sealing
Flash and Batt: Half the Foam, Most of the Benefit, and One Thickness Rule You Cannot Cheat On
Full-depth spray foam in a 2×6 wall costs about $6.90 a square foot and gives you an R-24 wall, because the studs are still there. A batt and poly costs $2.00 and gives you R-17 and a wall that leaks air at every box and plate. Flash and batt is the compromise the trade settled on: 2 in. of closed-cell foam sprayed on the inside of the sheathing, sealing every crack and warming the wood, with a cheap R-14 batt filling the rest of the cavity. $2.50 plus $1.10 is $3.60 a square foot, plus masking, for about R-19 effective, an air-sealed wall and dry sheathing. The 2 in. is not a suggestion; it is the arithmetic below.
The three walls side by side
| Wall, 2×6 at 16 in. | Per sq ft | Effective R | Air seal | Vapour barrier |
|---|---|---|---|---|
| R-22 fibreglass batt plus 6 mil poly | $1.50 + $0.50 = $2.00 | about R-17 | Only as good as the poly sealing, which is usually not good | The poly, warm side |
| Flash and batt: 2 in. closed cell plus R-14 batt | $2.50 + $1.10 = $3.60 | about R-19 | The foam, continuous across the sheathing and every stud bay | The foam itself; no poly |
| Full-depth closed cell, 5.5 in. | 5.5 x $1.25 = $6.90 | about R-24 | The foam | The foam; no poly |
Add $150 to $500 of masking per job, default $300, to either foam wall. Closed cell runs $1.00 to $1.50 a board foot, $1.25 as the planning number; the low end is a big open wall, the high end is small bays and awkward access. The spray foam cost calculator does the board feet, the masking and the minimum charge.
What the 2 in. of foam is doing
Three things, and only one of them is R-value. First it seals: closed-cell foam on sheathing fills the crack at every stud, plate and sheathing joint, so the wall stops moving air. That alone is worth more than the extra R. Second it is a vapour retarder: 2 lb closed cell at 2 in. is around 1 perm, which is the OBC line for a vapour barrier in 9.25.4, so the wall needs no poly and should not have one. Third, and this is the thickness rule, it keeps the inside face of the foam warm enough that house air reaching it through the batt does not condense there.
The batt in front of it is just R. Fibreglass does not stop air any more than a sweater does, and in this wall it does not have to, because the foam already did. That is why the batt is the cheap R-14; the foam took 2 in. of the cavity and the batt has 3.5 in. left to fill. A compressed R-22 in a 3.5 in. space is an R-14 that cost more.
The condensation arithmetic, because 1 in. is a wet wall
Warm indoor air at 20 C and 35% relative humidity has a dew point of about 4 C. Any surface inside the wall colder than that, with that air touching it, gets wet. In a flash-and-batt wall the first cold surface the air meets is the inside face of the foam, so the question is how warm that face stays in January. The temperature at any plane in a wall splits the indoor-to-outdoor difference in proportion to the R on each side of it.
With 2 in. of foam: outboard of that face is R-12 of foam plus the sheathing, siding and air films, about R-14; the whole wall is about R-28.5. At minus 10 C outside, the face sits at minus 10 plus 30 x 14 / 28.5, about 4.7 C, right at the dew point. At minus 20 C it dips to about zero for the night. That is tolerable because the foam is the vapour retarder and the batt is open to the room, so what little frost forms dries back into the house by day. With 1 in. of foam the outboard share is about R-8 of R-22.5, the face runs at minus 10 plus 30 x 8 / 22.5, about 0.7 C at minus 10 outside and minus 6 C at minus 20. That is a wall that frosts for weeks, with a batt holding the water against it. One inch is a “flash” in name only, and in Ontario it is the mistake to watch for in a quote.
Two rules: at least 2 in. of closed cell in Zone 1 and 2.5 to 3 in. in the north and Muskoka, and never poly over the batt. Poly in front of a foam that is already a vapour retarder is the double vapour barrier, and the batt between them cannot dry in either direction. If the drywaller asks where the poly is, the answer is “the foam”. What each layer does.
A 2,400 sq ft wall worked through
Two-storey house, 2,400 sq ft of net exterior wall, open framing, new build or a gutted renovation. Batt and poly: 2,400 at $2.00 is $4,800. Flash and batt: 2,400 at $3.60 is $8,640, plus $300 of masking, $8,940, about twice the batt wall for R-19 instead of R-17, the air seal and the dry sheathing. Full-depth foam: 2,400 at $6.90 is $16,560 plus $300, $16,860, for R-24. Flash and batt to full foam is $7,920 for five more effective R-points; batt to flash and batt is $4,140 for two R-points and the air seal.
Now the fourth option the table on the effective R-value page shows: R-22 batt and poly inside, 1 in. of R-5 foam outside the sheathing, about R-21.5 effective, because the exterior foam covers the studs. At re-siding the exterior layer is $4.50 a square foot plus the jambs; on a new build the jambs are being built anyway. On a new house, batt-plus-exterior-foam usually beats flash and batt on R per dollar; flash and batt wins on a renovation where the siding is staying and the drywall is off.
Where it earns its money, and where it does not
- A gutted renovation with the siding staying. The sheathing is exposed from inside, the old poly is not worth keeping, and the foam gives the house an air barrier it never had. The best case.
- A room over a garage, a cantilever, a knee wall. Awkward cavities with cold on more than one side. Foam them, batt the rest.
- Not a new house where exterior foam is on the table. Same money, more R, and the studs stop being cold lines; see above.
- Not a 400 sq ft job on its own. Two men, a truck and the rig come with a $900 to $1,500 minimum. Bundle the rim joists and the garage ceiling into the same visit.
- Not with open-cell foam. Open cell is R-3.7 an inch and vapour-open; 2 in. against sheathing in Ontario is neither a vapour retarder nor warm enough. Closed vs open cell.
What to check before the drywall
Ask for the CUFCA card and the daily work sheet with lift thickness and density; 2 in. is the spec and the sheet is where it is written down. Then check with a nail: push it through the foam to the sheathing at a dozen bays, corners and headers included, and read the depth. Foam that is 2 in. in the middle of the bay and 3/4 in. at the stud edges is the common short, and the stud edge is where the condensation starts. Foam that has pulled away from a stud leaving a crack was sprayed on a cold or dusty surface, and it is not an air barrier through the crack; the spray foam page covers the product and what goes wrong with it.
The batt goes in after the foam has cured and been inspected, not the same afternoon, fitted to the 3.5 in. that is left and cut around boxes, not stuffed behind them. Foam exposed in a living space needs the 1/2 in. drywall thermal barrier of OBC 9.10.17; in a finished wall it is covered, but where the wall stays open in a garage or a mechanical room that line applies. The materials section covers the fire rules.
Questions people ask
How much does flash and batt cost in Ontario?
About $3.60 a square foot of wall: 2 in. of closed-cell foam at $2.50 plus an R-14 batt at $1.10, plus $150 to $500 of masking per job. A batt-and-poly wall is $2.00; full-depth foam is about $6.90. On 2,400 sq ft of wall, about $8,940 before HST.
How thick does the foam need to be in a flash-and-batt wall?
At least 2 in. of closed cell in southern Ontario, 2.5 to 3 in. north of Barrie and in Muskoka. With 1 in. the inside face of the foam runs below freezing for weeks in January and the batt holds the frost against it.
Do I need a vapour barrier with flash and batt?
No, and you must not add one. 2 in. of closed-cell foam is about 1 perm, which meets the OBC vapour barrier requirement, and poly over the batt would trap the batt between two vapour barriers. Drywall and paint go straight over the batt.
What is the R-value of flash and batt?
R-26 nominal in a 2×6 wall, R-12 of foam plus R-14 of batt, and about R-19 effective once the studs are counted. Full-depth foam is R-31 nominal and about R-24 effective; the batt wall it replaces is R-22 nominal and about R-17.
Is flash and batt better than full spray foam?
For the money, usually. It gets the air seal and the dry sheathing for half the foam cost; the extra 3.5 in. buys five effective R-points for $3.80 a square foot more. On a new house, exterior foam over a batt wall beats both on R per dollar.
Price it against the other three walls
The wall assembly comparison calculator prices flash and batt beside a batt wall, a batt-plus-exterior-foam wall and an ICF wall for your square footage, with the effective R and the air seal on the same line. If you already have a foam quote, the $99 review will read the thickness, the product and the masking against what this page says they should be.
Sources and further reading
- 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.
- 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.
- 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.
