Cathedral Ceilings.
Since September 2025 Alberta has permitted unvented Part 9 roof assemblies province-wide, without a per-project engineering letter, provided the named RDH Building Science report is followed and submitted with the permit application.
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A cathedral ceiling is the assembly where the venting rules and the insulation depth fight each other. Alberta resolved that fight with a province-wide variance — and the resulting requirement is a specific, checkable number rather than an opinion.
The variance, the report it names and the worked example behind the seven-inch figure are set out in full in our unvented roofs guide including the conditions that come attached to it.
What the Variance Actually Permits.
STANDATA 23-BCV-017, issued September 2025 by the Provincial Building Administrator and applicable throughout Alberta, permits three unvented roof assemblies for Part 9 buildings. It supersedes the 2022 variance, which is obsolete for new construction.
- The requirement for an unvented roof in Zone 7A is assembly effective RSI 5.0 (R-28.5), drawn in the RDH report from 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.
- Assembly one — the full required resistance as rigid or semi-rigid insulation outboard of the roof structure, with a continuous, effective air and vapour barrier at the interior of that insulation.
- Assembly two — the full required effective resistance as spray foam applied directly against the underside of the roof sheathing. With medium-density closed-cell foam the report states no additional vapour control is required; with light-density open-cell foam a continuous vapour control layer in direct contact with the foam, sealed at penetrations, is required.
- Assembly three — a hybrid of exterior rigid insulation plus closed-cell foam against the sheathing.
- The worked example the report publishes: 178 mm (7 in.) of medium-density foam between 2x10 rafters at 406 mm on centre, at 13 percent framing and 87 percent cavity per NBC Table A-9.36.2.4.(1)-A, reaches assembly effective RSI 5.0. RDH is explicit that a real submittal must use the advertised RSI of the specific foam being installed.
- Interior wintertime relative humidity kept at or below 35 percent — the National Building Code's own durability assumption in Note A-9.25.5.2, and a condition of the variance rather than a suggestion.
- Roof underlay chosen accordingly: the report advises against traditional building paper or felt in unvented assemblies because it breaks down faster, and points to a self-adhered membrane where ice damming is a risk.
- Stacked and built-up framing encapsulated in foam where possible so the air barrier stays continuous, and continuously sealed along its full length where it cannot be — gable ends included.
Getting an Unvented Roof Approved.
- 01
Confirm the building is Part 9
The variance exists to remove the per-project engineering letter for Part 9 buildings. Part 5 construction already permitted unvented spray foam roofs, because those buildings are designed by registered professionals in any case.
- 02
Do the layer calculation with the real product
Air films, roofing, the parallel-path framing and insulation layer, any service cavity, the gypsum. The published example uses 0.04 RSI per millimetre for medium-density foam; the actual product's rated value is what goes in a submittal.
- 03
Submit the report with the permit
That is the condition of the variance. The RDH evaluation report is published free by the Spray Foam Coalition, so obtaining it is not a cost item — but leaving it out of the application is what turns an approved assembly into a query.
- 04
Build the conditions in, not around
Humidity control, underlay selection, framing encapsulation and continuous sealing at wood-to-wood joints are all part of the approval, not optional refinements.
Frequently Asked Questions About Cathedral Ceiling Insulation
Are unvented roofs legal in Alberta?
Yes, for Part 9 buildings, province-wide, since STANDATA 23-BCV-017 was issued in September 2025. It is a variance from the Provincial Building Administrator applicable throughout Alberta, and it permits unvented roof assemblies without a per-project engineering letter provided the design requirements of a named RDH Building Science evaluation report are used and that report is submitted with the permit application. It supersedes the earlier 19-BCV-022, which under Alberta's own obsolescence rule cannot be used for new construction once a new code edition is in force.How much closed-cell foam does a cathedral ceiling need?
Enough to reach assembly effective RSI 5.0, about R-28.5, which is the Zone 7A requirement for an unvented roof in the RDH report's own table. The report's worked example puts 178 mm — seven inches — of medium-density foam between 2x10 rafters at 406 mm on centre, and arrives at exactly RSI 5.0 for the assembly. That is a worked example, not a universal answer: RDH states directly that when the calculation is done for a specific project, the advertised RSI of the specific foam being installed must be used.Does a cathedral ceiling with spray foam need a separate vapour barrier?
With medium-density closed-cell foam, the RDH report states that the foam includes vapour control and no additional vapour control is required. With light-density open-cell foam it is required — a continuous, effective vapour control layer in direct contact with the foam and sealed around penetrations. That distinction is not a preference; the two materials are written to different standards, and open-cell permeance is deliberately kept high.What about the rafters themselves — do they leak vapour?
This is the standing objection to a full-depth foam roof, and there is a measured answer. Test data cited in the RDH report puts 2x4 framing at 36 ng/(Pa·s·m²) and 2x6 framing at 30 ng/(Pa·s·m²) on a dry cup basis, both already inside the code's 60 ng/(Pa·s·m²) vapour barrier limit. The wood is not the weak point. The joint beside it can be, which is why the report requires stacked framing to be encapsulated where possible and continuously sealed where it cannot be.Will an unvented roof rot the sheathing?
The blanket version of that claim is overstated, and there is Alberta-specific evidence. The RDH report reviews the literature directly: a 2010 hygrothermal study found no risk of moisture accumulation in the sheathing for a full-depth closed-cell foam roof in Climate Zones 6 and 7 with dark asphalt shingles, even at elevated interior humidity; a 2010 to 2012 field study in Ontario, run under deliberately conservative assumptions, found no increase in sheathing moisture content up to Zone 7A at an average winter interior relative humidity of 30 percent. The conditions attached matter — interior humidity at or below 35 percent, no felt underlay, and correct rainwater management, because the assembly's tolerance for a roof leak is low.Is there a minimum share of foam in a hybrid roof?
The guidance the Alberta-adopted report reproduces is a minimum percentage of total R-value that must be air- and vapour-impermeable: 60 percent for Zone 7A. It is worth being precise about where that table comes from — it is Table 1202.3 of the 2024 International Building Code, a US document, cited as building physics guidance inside the report Alberta has adopted by reference. It is not itself a Canadian code table, and we present it as what it is.