Ever wonder why we have so much space in our attics? Or why we have soffit vents and roof vents? Sometimes these questions go unanswered and installing vents in our roofs happens just because the building code says to do it.
So what happens if we don’t vent our roofs? How can this happen and what is the result of not venting?
Roof spaces are to be vented unless it can be shown not to be necessary. I’m paraphrasing the building code for simplicity, but this is very much an option. Certain building envelope assemblies are built in a way that allows the free flow of air in the roof spaces and some, well, not so much. Venting roof spaces can sometimes be a challenge when designing modern type flat roofs, cathedral ceilings and other roof type assemblies. Adequate clearance for insulation or cross purlins –sometimes referred to as sleepers – is necessary to meet building code compliance; however, should the clearance not be available, an alternative method of compliance can be made available: An unvented roof assembly.
Unvented roofs are more common in today’s modern home designs. These roofs are code conforming, except they do not display the venting requirements expressed in the building code. The most common way to eliminate venting from a roof assembly is to use a spray foam (SPF) insulation. By installing SPF insulation (spray in place insulation) in lieu of mineral fiber insulation in the joist cavity of a flat roof, or between rafters of a cathedral ceiling, the assembly may perform the same as, or better than, that of a typical one and venting can be shown to be unnecessary.
Let’s take the example of a cathedral ceiling renovation project on older home in BC. The assembly may simply be a 2×6 rafter joist with a standard R20 (5.5”) mineral fiber batt insulation. Current codes require a minimum of R28 (7.5”) with a 1.5” air space between the top of the insulation and the underside of the roof sheathing. Also, cross purlins are needed for the venting requirements unless each rafter cavity is individually vented. Due to inadequate depth for the R28 batt insulation, and the absence of cross purlins, medium density SPF may be installed directly to the underside of the roof sheathing to eliminate the need to vent and to comply to the thermal requirements of the building code. Here, venting was eliminated and shown to be unnecessary.
In BC, a registered professional, such as a Building Envelope Engineer, may submit to the authority having jurisdiction, on behalf of the designer, builder or architect, an alternative solution showing that using spray foam insulation in lieu of venting is acceptable. The City of Vancouver mentions the requirements for such and alternative solution in the “BULLETIN 2014-008-BU” which is available as a free download here.