Middlesex VT: Foam-free, WUFI-verified, and OSB-free high performance
The town of Middlesex, Vermont, located just north of Montpelier, has put itself on the high performance building map. Leading the way is the new "Whitchurch Passive House Cottage" - not only in terms of energy efficiency (Passive House certification is in the works), but also as a unique and adventurous example of "form follows function". The project was executed with thorough commitment by the owner, Greg Whitchurch, along with CPHC Indigo Ruth-Davis and Passive House Certified Builder Chris Miksic of Montpelier Construction. Though simple in geometry, the house challenges conventional construction practices on a number of levels. Some of the most interesting innovations are hidden behind the cedar cladding, where advanced Pro Clima membranes and tapes provide state-of-the-art airtightness and moisture protection. Let's take a closer look. (Photos by Greg Whitchurch)Whitchurch Passive House Cottage - with storefront windows showing the post & beam structure behind
The house employs some of 475 High Performance Building Supply's best practices, including foam-free super-insulated TJI assemblies with interior and exterior airtight membranes to maximize moisture protection and insulation performance. (Visit DWG Construction Details for free access to our recommended high performance assemblies). The owner also wanted a flat roof that kept snow from sliding off, but he wanted to avoid using foam. What he got, according to a professional engineer familiar with the project, was a "non-vented cellulose dense-packed insulated flat roof assembly". In the past, unvented flat roofs have routinely failed due to poor detailing and inadequate moisture control. We discuss flat roof issues in a couple of blog posts - see here and here. The game changer on this project was one product: INTELLO Plus "smart" vapor control membrane. Serving simultaneously as the airtight and variable vapor control layer, INTELLO made it possible to keep moisture transmission through the roof assembly within safe levels.
WUFI further showed that in Vermont's climate it is beneficial to have a dark roof. The INTELLO Plus on the warm side of the assembly keeps wintertime moisture from the interior from diffusing out of the assembly, while allowing drying to the inside in summer. This process is aided by the highly absorptive dark roof. The graph below shows that the dark membrane plus INTELLO keeps the pine board sheathing below 20 M% in the first winter, though for a few weeks it exceeds 18%, the conservatively safe level for solid wood. After the 2nd winter, the moisture content goes down to very safe levels. The light-colored roof peaks higher and longer in the first winter (21 M%) and only in year 5 does it drop below 18 M%. Hence the black roof membrane leads to a roof with a high drying reserve and hence a very safe assembly. 475 worked closely with the team and a structural engineer to accept this assembly as a cold climate alternate solution.WUFI moisture % in pine boards above cellulose and INTELLO (Grey line is with white roof. Dark line is black roof.)
Ultimately, a successful roof outcome for this project depended on careful airtight installation of the roof layer. Typically, INTELLO Plus is stapled to the bottom of rafters, then horizontal battens are fastened over the membrane to help distribute the load from blown-in insulation such as cellulose. In the Middlesex project, the interior of the roof assembly was built in reverse. Interior 2x8 pine tongue and groove boards were attached to the outboard side of the structure, over the traditional post and beam structure. This layer served as a sturdy backing for the densepack cellulose that followed, and the vapor open properties of the wood didn't compromise the vapor intelligence of the INTELLO. It was then a breeze to achieve a perfect airtight layer with INTELLO across the top of the solid wood flat roof. Seams were made airtight with TESCON Vana solid acrylic tape.