After the publication of the article, “Making a Case for Sprayfoam in the Unvented Attic” by Peter Birkbeck, CTR, in the October 2017 issue of Construction Canada, we received a letter from one of our readers.
John Rutledge wrote:
The article is another case in the seemingly endless dialogue about ventilated vs. unventilated roof/attic spaces. Unfortunately, both sides seem to be winning, leaving designers no further ahead when deciding which one to choose.
I belong to the school of thought that believes having properly ventilated roof/attic spaces—and leaving the roof surface cold—minimizes and almost prevents problematic ice dams from forming along eave overhangs and roof valleys. All of my clients find this desirable.
As far as I know, insulation placed directly under the sloped roof sheathing—between rafters or top chords of trusses (i.e. leaving attic/roof spaces unventilated)—generates a ‘hot roof,’ where the exterior surface of the roof remains partially heated by the slow passage of heat through the insulation placed up against the underside of the roof surface. This article does not discuss how such ‘hot roofs’ can create problematic ice dams.
As an architect who designs many attic/roof spaces, and who has read many articles about ventilated vs. unventilated assemblies, I find few explore this ice damming issue. Any information pertaining to this concern would be appreciated.
For clarification, we reached out to the article’s author. Birkbeck offered this response:
Thank you for your comments on the article. I agree with you there is no shortage of papers, investigations, and other information on this topic. One of the main principles I was trying to get across is the intent of roof space ventilation is to remove unwanted moisture. The premise presented was the effectiveness of this inherently passive strategy is overrated.
Sprayed polyurethane foam (SPF) insulation was put forward as a multifaceted solution because, at typical code-prescribed insulation thicknesses, next to being a thermal insulation, it is an air barrier material. Regardless of where traditional insulation is located within the attic/roof space, it is generally agreed an aggravating factor in the cause of ice dams is an insufficiently sealed and insulated ceiling. Sprayfoam as a solution has the added benefit of making it easier to achieve sufficient sealing. The SPF insulation keeps moist air, which potentially has leaked from the conditioned interior space, from contacting the roof deck. This is why agencies such as Canada Mortgage and Housing Corporation (CMHC) have stated a well-sealed and insulated ceiling typically does not exhibit ice dams.
So, what is the connection of this principle with unvented roof spaces and SPF? The onset of an ice dam begins when, after large snowfalls, a mass collects on the roof edge. This problem is exacerbated by overhangs, which are cold if not insulated from above or below. The rest of the roof field will be warmer, and the melting snow runs down until the point at which it is blocked, at the eave or overhang. Traditionally vented roof/attic spaces make it difficult to properly insulate the roof overhang area and the space above the perimeter wall. The unvented attic insulated with SPF is insulated in its entire field as well as the eave area. Additionally, a high insulation value is easily achieved, and a substantial contribution to air sealing is provided. As a result, the entirety of the roof surface is kept at a uniform temperature and there is less potential for moist interior air to reach the roof deck.
Further dialogue on this topic is always encouraged. I believe we will see more definitive and possibly codified design guidance on this topic specific to Canadian climates in the future.