BCAB #1719 - Roof Venting Above Spray Foam Insulation, Article 126.96.36.199.
October 17, 2012
Re: Roof Venting Above Spray Foam Insulation, Article 188.8.131.52.
The project involved is a new triplex residential building. It is proposed to insulate the roof using half pound spray foam insulation between the roof joists applied directly to the underside of the roof sheathing. That is, no air space or venting is proposed between the insulation and the underside of the roof sheathing.
Reason for Appeal
Article 184.108.40.206. requires a vented space between the top of the insulation and the underside of the roof sheathing for the transfer of moisture to the exterior “except where it can be shown to be unnecessary.”
No vented air space is proposed since spray foam insulation will be applied directly to the underside of the roof sheathing.
The appellant contends half pound spray foam insulation applied directly to the underside of the roof structure provides an airtight barrier to the transfer of moisture (water vapour) from the conditioned space to the underside of the sheathing. If water vapour can’t reach the cold underside of the sheathing and condense there is no need to provide the vented air space. The appellant has provided documentation of the building science principles to support the contention that venting is not necessary with this type of roof assembly.
Building Official's Position
The building official maintains that insufficient evidence has been provided to prove the roof assembly will be airtight enough to prevent air leakage and subsequent condensation or water vapour on the underside of the sheathing. The roof assembly would need to be tested by an approved agency to demonstrate its air permeance would not be greater than 60 ng/(PaŸsŸm2) as specified in Sentence 220.127.116.11.(1). The building official also considers the elimination of venting should be treated as an alternative solution as permitted by Division A of the Code.
Appeal Board Decision #1719
In this appeal the Board is ruling only on the concept of unvented roof assemblies and acceptance of a particular design falls under the jurisdiction of the building official.
It is the determination of the Board that Article 18.104.22.168. permits an unvented roof assembly where venting can be shown not to be necessary. The Board considers that sufficient information is available from various sources to demonstrate to a building official that venting is not necessary with this type of roof assembly due largely to its inherent air-tightness. It needs to be clarified that 60 ng/(PaŸsŸm2) refers to vapour permeance and is not a measure of air tightness.
George Humphrey, Chair