This is the fifth entry in a series of columns about the design and construction of my newly certified passive house. Passive House Institute US (PHIUS) offers a certification program for highly energy-efficient homes and separate certification programs for home design and building professionals seeking to build them (www.phaus.org for more information).
In this column, I will describe the roof insulation in detail.
For energy-efficient homes, one of the key plans is a “section” view; imagine slicing a house in half to provide a perfect view of how the walls and roof are put together. To see a section view of my house, go to my blog at http://www.bpcgreenbuilders.com/blog, and look for passive house (Part 2). When you find the section, click on it to get a full screen view.
If you’ve read the previous columns on my house, you know that the three fundamental criteria for a passive house are high levels of insulation; a continuous layer of insulation not interrupted by framing; and a completely air-tight perimeter boundary for the conditioned spaces.
To meet the above criteria, I chose to frame the roof with an engineered wood truss, which forms both an attic and vaulted ceiling. I chose a truss, rather than a traditional roof rafter (e.g. 2×12). Over the vaulted ceiling area, I wanted the truss to accommodate 18” of cellulose insulation — about R-65. Because the truss has separate elements — a 2×6 top chord and 2×4 bottom chord and supporting elements — insulation can fill the spaces in-between, which prevents thermal bridging, or “heat bleed” through the wood, which is a poor insulator.
The attic area is too small for storage space so we simply blew in cellulose to a depth of 24” — about R-86. On one side of the attic, the cellulose is continuous with the cathedral ceiling. On the other side, it terminates against the front wall where I have 12” between the attic floor and the roof sheathing. Twelve inches of cellulose works out to R-43, sufficient to keep the roof cold, thereby avoiding ice dams in winter.
To satisfy the air-tight requirement, we attached 1/2” plywood to the interior underside of the roof trusses and then taped all the joints with a European flashing tape, which will adhere tenaciously to the plywood for as long as the home stands. Where this interior plywood meets the exterior plywood sheathing on the walls, we used a wider flashing tape to seal this critical joint.
We used flashing tape at all other intersections of floor, wall and roof planes, as well as to install the windows and doors. The end result is a very, very air-tight house, as demonstrated by a third-party blower door test result of 0.46, which easily meets the PH certification standard of 0.6 ACH50 (air changes per hour at a 50 pascal air pressure difference between inside and outside the house). The current code requirement in Connecticut is 7.0ACH50, nearly 12 times the PH standard. This is yet another instance where the code standard for this critical efficiency element is well behind the times.
In my next post, I’ll provide details about the European high performance windows.