b'UNVENTED WOOD ROOFCAUSE FOR CONCERN? Adoption of the unvented wood roof in AlbertaBy Fred Edwards, B.Sc., P. Eng. LEED APU nvented wood-framedroof assemblies is only necessary unlessleads to stratification and stagnant air roofing is gainingit can be shown to be unnecessary.flow in the vented cavity. momentum in Canada.Enterprising spray foam manufacturersThat is a big problem with vented flat It is a substantial shift inand distributors take note: the door isroofing, which Part 9 has never taken the residential construc- open. It has been for years, only nowsteps to address. In many cases, the tion landscape in thisthe Alberta government has taken thealternate solution has been to abandon country; nearly all wood-framed single- next step via the STANDATA refer- the concept of ventilation in favour of family homes have historically beenenced above. increasing air-tightness and lowering constructed with a vented roofthatUnvented wood roofs are almostvapour permeability (unvented solu-cold (or hot) attic space above yourexclusively constructed using spraytion). It is a wholly different strategy. upstairs bedroom.foam as the air and thermal barriers.Rather than accommodate some air Alberta is leading the charge. OurThey are quite common. Balconies overleaks and moisture originating from provincial government has takenoccupied living space were, up untilinside the building via venting, stop the the significant step in permitting annow, the most prevalent occurrences ofsource of moisture into the roof and alternate solution to vented roofingunvented roofing. Flat roofs like this aretherefore eliminate the needprovincewide. While other jurisdictionsnotoriously difficult to vent effectively.to accommodate. in Canada (like B.C.) have allowedAlthough flat roofing is addressed inThe problem is and has always been that such variances on a case-by-case basis,Part 9, its venting requirements wereachieving perfection in an air barrier is Alberta seems to be the first to approvenot conceived with flat wood-frameddifficult at best. Spray foam application a blanket variance based on a report notroofing in mind. Clause 9.19.1.1, whichhas come a long way in the last 20 years, linked to a specific property. The days ofhas been included in the NBC in someand it has never been more possible to the vented roof as the only prescriptiveform since 1960, was conceived toachieve a near-perfect air barrier than solution (i.e., not requiring profes- accommodate vented attic spaces imme- it is now, but perfection is a unicorn. sional involvement) in wood homesdiately below steep-slope roofing. In theWe want to believe that perfection is appear to be behind us. I am refer- traditional residential home, convectionpossible, but cold, hard reality tends to ring to the Alberta governments 2022within the attic space drives the ventila-STANDATA variance number 19-BCV- tion with little need for mechanicalget in the way. One fundamental reality 22, which establishes unvented roofingintervention. Flat roofing is different.is that all roofs eventually leak.as a provincewide acceptable alternateThere is less of a temperature gradientHowever, once you establish that a solution to the requirements of thebetween the intake and exhaust, and theperfect air barrier is possible and assume National Building Code2019 buoyancy of heated air has little or noas much in your calculation of moisture Alberta Edition. slope to follow to the exhaust vent. Thismovement through roofing, the pieces Notably, the National Research Council, who are ultimately respon-sible for updating Canadas National Building Code, did not take the lead on the matter. The latest edition of the National Building Code, NBC 2020 THE PROBLEM IS AND HAS ALWAYS BEEN now in force in Albertareads much like the previous versions of the code when you get to the part about roofTHAT ACHIEVING PERFECTION IN AN AIR ventilation, Article 9.19.1.1. Roofing used in NBC 2020 Part 9 compliantBARRIER IS DIFFICULT AT BEST. buildings should be constructed as vented assemblies with prescribed areas for intake and exhaust vents. The NBC, however, provides an out. Venting of SPRING/SUMMER 2025 13'