March 28, 2016

JLC Reports on Success of Foam-Free Unvented Flat Roof Assembly

475 enables more robust assemblies that are less toxic. Such was the case with the Whitchurch home in northern Vermont – working with the owners and their dedicated Passive House team at Montpelier Construction. The walls were a variation on the I-Joist outrigger configuration with dense pack cellulose between DB+ membrane inboard and SOLITEX Mento Plus outboard. The tricky part was the roof. Looking to avoid foam insulation at the proposed flat roof, 475 worked closely with the project team to define the assembly and produce a safe and robust unvented flat roof using INTELLO. You can see our initial blog post on the project here.

Cover of 2015 JLC Magazine with article by Indigo Ruth-Davis (click image)

Cover of 2015 JLC Magazine with article by Indigo Ruth-Davis (click image)

The principles involved in making a safe unvented flat roof are well understood and we’ve written about it in the blog posts The Ten Golden Rules for Foam-Free Flat Roofs as well as in Unvented Flat Roofs: A Technical Discussion. Recently, we’ve had validation of this approach from the German Federal Agency Deutsches Institut für Bautechnik (DIBt), as noted in our post INTELLO & DB+ Approved by DIBt for Use in Unvented Hot Roof Assemblies.

To add to this growing list of serious analysis we add Unvented Flat Roofs: Theory Meets Practice in Rural Vermont by Ted Cushman in the March 2016 issue of Journal of Light Construction. The article is an insightful look at the issues and results at the Whitchurch roof thus far. Cushman sets it up, noting:

One of the addition’s unusual elements was an unvented low-slope roof system insulated with 22 inches of dense-blown cellulose and topped with black EPDM membrane roofing. With no way for moisture to escape upward, the roof’s moisture-management concept relied on a “smart membrane” on its underside, just above the finish ceiling facing the conditioned space.

The article goes on to explain the process, working with engineer Bill Root and certifier, PHIUS – and is careful to note the limitation of the moisture simulation modeling tool WUFI. Moisture sensors were installed during construction and the owner has been taking regular readings.

But the roof hasn’t failed yet. In fact, so far, Whitchurch’s granny-annex roof is an exemplary success: Any wintertime increase in lumber moisture content is more than balanced out by summertime drying, and the overall trend after almost two years of data collection is toward the dry direction. Like a well-built conventional attic, this roof is drying out over time.

The assembly is drying out and building drying reserves – behavior we want to see in a high-performance enclosure. The owner continues to collect data and we look forward to publishing more extensive results going forward.

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