Alberta now permits unvented Part 9 roofs, province-wide, without an engineering letter.

The variance was issued by the Provincial Building Administrator, Paul Chang, and is applicable throughout the Province of Alberta. STANDATA 23-BCV-017

One scope note that avoids a wasted conversation: Part 3 buildings never needed this variance. RDH states that construction under Part 5 already allows unvented spray foam roof assemblies, because all Part 5 buildings require design and engineering by registered professionals. 23-BCV-017 exists specifically to remove the per-project letter for Part 9. RDH 32139.000

For reference, the prescriptive venting requirements the variance displaces: unobstructed vent area not less than 1/300 of insulated ceiling area, or 1/150 where roof slope is less than 1 in 6 or the roof is built with roof joists; vents distributed uniformly on opposite sides with at least 25% at the top and 25% at the bottom; not less than 63 mm of clearance between the top of the insulation and the underside of the sheathing; and at least 25 mm of unobstructed air space at baffled eaves, with baffles extending at least 50 mm above the insulation. NBC(AE) · 9.19.1.2 and 9.19.1.3

1. Exterior-insulated. The entire required resistance as rigid or semi-rigid insulation on the exterior of the roof structure, with a continuous, effective air and vapour barrier at the interior of that insulation — on the sheathing.
2. Full-depth spray foam. The entire required effective resistance as spray foam directly against the underside of the roof sheathing. RDH’s wording: “MDSPF includes vapour control, and no additional vapour control is required, however if LDSPF [open-cell] is used, then a continuous effective vapour control in direct contact with the LDSPF, and sealed around penetrations, is required.”
3. Hybrid. Exterior rigid or semi-rigid insulation plus medium-density spray foam against the underside of the sheathing. With MDSPF no additional vapour control is required; with open-cell it depends on the R-value ratio and must be assessed case by case.

RDH 32139.000 — permitted assemblies

The required effective resistance for an unvented roof in Climate Zone 7A — which is Red Deer — is RSI 5.0 (R-28.5). RDH’s Table 1 draws this straight from NBC(AE) Tables 9.36.2.6.-A and -B. Zone 6 is RSI 4.7 (R-26.5); Zones 7B and 8 are also RSI 5.0 (R-28.5). RDH Table 1 · NBC(AE) 9.36.2.6

For hybrid assemblies that combine air-impermeable foam with air-permeable fibre, RDH reproduces Table 1202.3 of the 2024 International Building Code, “Insulation for Condensation Control,” expressed as the minimum share of total R-value that must be air- and vapour-impermeable.

The variance is conditional on following the report. The report attaches conditions, and they are as much a part of the permission as the R-value is:

• Interior wintertime relative humidity must not exceed 35%. This is the NBC’s own durability assumption in Note A-9.25.5.2; RDH recommends 25–30% in Zones 7B and 8. Elevated interior humidity puts every roof and wall assembly at risk, vented or not. RDH, citing NBC Note A-9.25.5.2
• No felt or building paper as underlay. “Traditional building paper or felt paper should not be used in unvented roof assemblies because it tends to break down more quickly.” Use a self-adhered membrane where ice damming is a risk, with a synthetic underlay elsewhere; some manufacturers recommend self-adhered beneath the whole roof. RDH 32139.000
• Framing must be encapsulated or continuously sealed. Built-up and stacked framing should be encapsulated in foam so the air barrier stays continuous. Where it cannot be — cathedral rafters that will receive gypsum, for instance — the wood-to-wood joint must be continuously sealed along its entire length with tape, flashing membrane or a movement-accommodating sealant. Gable ends and other vertical portions get the same treatment as the slopes. RDH 32139.000
• Installation must comply with the material and application standards. CAN/ULC-S705.1 or S712.1 for the material, and S705.2 or S712.2 for the application — “all criteria related to substrate moisture contents/temperatures, pass thicknesses, daily testing, daily logs, etc. must be followed.” In central Alberta the substrate-temperature criterion is the one that governs the calendar; it is set out on the winter application page. RDH 32139.000
• A certified installer. RDH: “The spray foam shall be installed by an installer that has successfully completed a training course and is certified as a Spray Polyurethane Foam Installer (Section 4.3.4, CAN/ULC-S705.2 or CAN/ULC-S712.2).” Note what this does not say: it names the standard, not a certification body. Four programmes are recognised by CCMC — CUFCA, Caliber, BPC and UFC — and each manufacturer designates one for its product. A specification demanding a particular body’s licence excludes most of the Canadian market. RDH, citing CAN/ULC-S705.2 §4.3.4

RDH sets out the advantages directly:

• Complex roof geometries where effective venting is difficult to achieve in the first place.
• No ingress of blowing snow, rain, or burning embers. RDH: “In many locations with high forest fire risk, ventilated roof systems are not allowed because of the history of fires due to embers entering the attic.” Alberta issued a separate variance on vented soffit protection (23-BCV-018, September 2025) in the same month, which tells you where the province’s attention is.
• HVAC equipment and ductwork brought inside the thermal enclosure.
• Usable conditioned bonus space.
• Fewer air-leakage penetrations at the roof deck than at a ceiling plane full of pot lights and speakers.

RDH 32139.000 — stated advantages

The report also explains why roofs are harder than walls, which is the part that changes how you detail one: night-sky radiative cooling depresses the roof surface below ambient temperature, so frost can form on a roof while the air is 2–3 °C above freezing. Roof sheathing runs colder, for longer, than wall sheathing at the same outdoor temperature.

Foam bonded to the underside of a roof deck removes the deck’s inward drying path — completely, with closed-cell. Building Science Corporation’s finding is the one to take seriously: “when failures occur in wood-frame roofs insulated with spray polyurethane foam at the deck, it is typically due to leakage of bulk water (precipitation) or vapour-diffusion condensation.” With closed-cell foam, leak water never shows on the ceiling. It sits on the foam and rots the deck out of sight. BSC BA-1312

The blanket claim that spray foam rots roofs is, nonetheless, overstated. BSC’s hygrothermal study of unvented wood roofs across all climate zones concluded that “so long as airtightness is provided and wintertime humidity is controlled, numerous unvented solutions using either or both spray foam (open and closed cell) and fibrous insulation can be successful.” The dominant risk variables are winter interior humidity and air leakage — not the foam as such. BSC BA-1001

Two consequences worth stating out loud, because they are the part that gets left off a sales page:

• Rainwater management becomes critical. The assembly’s tolerance for a roof leak is low, and the leak will not announce itself on the ceiling. Inspect and repair the roof covering before foaming, record moisture-meter readings pre-spray, and keep inspection access at ridges, valleys and penetrations.
• Treat closed-cell foam on a roof deck as permanent. It bonds tenaciously; published adhesion figures for one product run to 207 kPa on OSB and 275 kPa on wood. Removal is mechanical, a residue layer remains, and full remediation usually means new sheathing. That is an argument for getting the assembly and the installation right the first time, not an argument against the assembly.

One detail that follows from the physics: never add a low-permeance vapour retarder on the interior side of roof-deck foam. It closes the only remaining drying path.

RDH states the concealed-space reading explicitly, which is useful because it is the reading an inspector will already have seen: “in an attic or roof space that is considered ‘concealed’, the ceiling gypsum board below the attic or roof assembly will provide the required thermal protection.” NBC Note A-3.1.4.2.(1) defines a concealed space as one that is not visibly apparent and is provided with an opening for repair and periodic inspection. RDH 32139.000, citing NBC A-3.1.4.2.(1)

The acceptable finishes under Subsections 9.29.4–9.29.9 are plaster, gypsum board with taped joints, plywood, hardboard, fibreboard, particleboard and OSB or waferboard. Spray foam is combustible; if it is visible in a conditioned space, that is a code question, not a finish preference.

One documented Canadian friction point worth raising with a client before specifying a coating instead of board: in 2018 a Nova Scotia mutual insurer declined to accept a spray-applied intumescent coating as the thermal barrier over basement foam and required board covering, at a cost to the homeowner of roughly $3,700 — even though the coating complied with the code as an alternative solution. Confirm with the insurer, not only the inspector. CBC News, 2018