BCAB #1741 - Unvented Roof Deck, Sentence 9.19.1.1.(1)

February 27, 2014

BCAB #1741

Re: Unvented Roof Deck, Sentence 9.19.1.1.(1)

Project Description

The project in question is a renovation to a single family dwelling which includes a covered roof deck over new living space. The building permit submission indicated the deck would be ventilated above the mineral fibre insulation but medium density spray foam insulation conforming to CAN/ULC-S705.1 has been installed without venting.

Reason for Appeal

Sentence 9.19.1.1.(1) requires roof spaces containing thermal insulation to be ventilated above the insulation unless “it can be shown to be unnecessary.”

Appellant's Position

The appellant contends that using the Code conforming 2.2 pound closed cell spray foam insulation provides an airtight coverage of insulation between all framing members and anything else within the assembly up to the underside of deck sheathing. This system greatly reduces the possibility of any moisture laden air entering the roof space and being able to condense. The product, when sprayed to a thickness of 2” or more meets code minimums for an air and vapour barrier. Any gaps in framing are caulked tight by the foam. In short, this creates a monolithic assembly with no air flow in the possible dew point areas, thereby rendering the need for ventilation above the insulation as unnecessary.

Building Official's Position

The building official maintains that it has to be “shown” to someone, presumably the AHJ, that venting is unnecessary so it is ultimately the authority having jurisdiction (AHJ) who determines if venting is in fact “unnecessary.” Further, the building official does not believe the phrase “it can be shown to be unnecessary” is an “acceptable solution.”   The acceptable solution is the descriptive ventilation requirements in Section 9.19. This makes an unvented roof space an alternate solution subject to the approval of the AHJ. The building official’s decision not to approve the unvented roof deck is based not only on the concept of moisture accumulation due to migration through the vapour barrier and air barrier from conditioned interior space below. When the roofing membrane fails at some point in the future it will likely not be evident to the owner for some time and this could lead to structural failure of the roof deck due to rot.

Appeal Board Decision #1741

The Board considers that showing the authority having jurisdiction that venting is unnecessary is an acceptable solution. It is the determination of the Board that the appellant has not provided sufficient scientific evidence such as independent research, testing and/or calculations to show venting is unnecessary. (See also Appeal #1719)

Lyle Kuhnert, Chair