Should I vent my attic? What happens if I do?

Should I vent my attic? What happens if I do?

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.

Julio Reynel

Julio Reynel-Gracia is the president of JRG Building Engineering, Inc. He is a Building Envelope Professional Engineer specializing in the building envelope and energy efficiency for Part 9 buildings and small structures. Julio works closely with residential home builders on energy efficiency as it pertains to the Building Code and also on building envelope design and building assemblies.

This Post Has One Comment

  1. Urs Kissling

    Hi Julio: May I ask for your expertise, please? We are building our own cabin up north (Kamloops area). We have a cathedral ceiling. All exterior wall are getting a 2.5″ spray foam insulation. Roof is split like a big barn. Engineered roof rafters are 22″ deep, with a c/c of 24″. Upper / gable section is intent to be unvented, with a 2.5″ spray foam insulation and balance traditional fiber-insulation. Is this a good way of sealing a cathedral ceiling to prevent any wind drafts & retail as much heat as possible. I appreciate your feedback. Thank you taking out a few minutes of your time to support our “love” project to build a highly efficient home. Sincerely Urs Kissling, 604-532-3881

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