5/18/2023 0 Comments Tight building envelope![]() With the airtight shell complete, a preliminary blower door test was run to determine airtightness and to locate air leakage points that could be sealed. Finally, the windows and doors were installed, using the same tape to seal them to the framing. The intersection of the subfloor and wall sheathing could not be taped, so this intersection was sealed with EPDM gaskets. The same tape was used to seal all joints and intersections of the plywood subfloor, exterior wall sheathing, and underside of the roof trusses (where plywood was installed to help guarantee the continuity of the airtight thermal envelope). These joints were taped with a high-quality construction tape to provide a durable and airtight connection. The plastic was cut to lap the plywood wall sheathing and plywood subfloor (by the old foundation) by 3 to 4 inches. I began by wrapping the concrete slab foundation with a reinforced plastic sheet, which was installed with great care to eliminate possible holes. Maintaining air barrier continuity at transition points in the thermal envelope is critical. Building the house with an airtight thermal envelope is one of the most difficult Passivhaus requirements to achieve. This is yet another instance where the code standard for this critical efficiency element is well behind the times.įloor, wall, and roof areas that adjoin unheated spaces make up the thermal envelope of a house. The current code requirement in Connecticut is a maximum air leakage rate of 7.0 ach50, nearly 12 times the Passivhaus standard. The end result is a very, very airtight house, as demonstrated by a third-party blower door test result of 0.46 air changes per hour at a pressure difference of 50 pascals (ach50), which easily meets the Passivhaus certification standard of 0.6 ach50. We used flashing tape at all other intersections of floor, wall, and roof planes, and to install the windows and doors. Where this interior plywood meets the exterior plywood sheathing on the walls, we used a wider flashing tape to seal this critical joint. ![]() To satisfy the airtightness requirement, we attached 1/2-inch plywood to the interior underside of the roof trusses and then taped all of the joints with a European flashing tape which will adhere tenaciously to the plywood for as long as the home stands. Kicking the Tires on a Passivhaus Projectīuilding a Small House in the White Mountains Twelve inches of cellulose works out to R-43, sufficient to keep the roof cold, thereby avoiding ice dams in winter. On the other side, it terminates against the front wall where I have 12 inches between the attic floor and the roof sheathing. On one side of the attic, the cellulose is continuous with the cathedral ceiling. The attic area is too small for storage space so we simply blew in cellulose to a depth of 24 inches - about R-86. 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. Over the vaulted ceiling area, I wanted the truss to accommodate 18 inches of cellulose insulation - about R-65. I chose a truss, rather than traditional 2x roof rafters, to make it easier to achieve my efficiency goals. I chose to frame the roof with engineered wood trusses, which form both an attic and vaulted ceiling. The first part was published as “Building My Own Passive House.” Editor’s note: This is the third installment in a series of blogs by Michael Trolle about the construction of his Passivhaus home in Danbury, Connecticut.
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