Superinsulation Can't Work if the Windows Leak
By ROB WOTZAK
A GOOD AIR BARRIER CONNECTS THE LAYERS
Some of the hardest parts of a house to seal are the windows, which are really just big holes in a wall filled with glass, wood, and plastic. The reason it's so difficult to seal around windows is that the hole spans many layers of different materials—drywall, framing, exterior sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , housewrap, and siding. In a superinsulated house, there are even more—the house featured in this video has an extra two layers of rigid foam on the outside. All of those layers need to connect to the window frame, which is usually made from a thin piece of wood, plastic, or metal. Because stopping air leaks is so important to energy efficiency and the durability of a house, it's a good idea to double and triple check the air-sealing details at every seam of every layer in every wall.
Thick walls complicate window connections
Building Science Corporation designed the home in this video so that the drywall creates a continuous interior air barrier. The home's 4-in.-thick layer of rigid foam sheathing puts the outboard-mounted windows several inches away from the interior edge of the wall. This deep gap around every window has to be bridged to make the airtight drywall work. BSC's solution is a plywood box extending from the inside of the framing to the outer edge of the rigid foam, within each rough opening; everything from the foam to the framing gets caulked or taped to this plywood, making air sealing consistent and reliable. BSC principal Joe Lstiburek, Ph.D, P.Eng., ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. Fellow, uses his eyes and experience to make a thorough inspection of the various sealants and tapes to be sure this detail works as planned.
Understand air barrier principles to solve real-world problems
Dr. Joe points out that in this house, just as in any other construction project, things can change at the last minute. An increase in window sizes forced the carpenters to remove some pieces of plywood meant to bridge the gap between the airtight drywall and the back of the window frames. This just meant that each of the layers of the wall needed to be sealed separately — an inconvenience, but not a major problem since everyone working on the job site understood how to connect the different materials.
Belt-and-suspenders approach to air sealing
David Joyce and Gary Bergeron from Synergy Construction have built enough extremely airtight houses like this one to know it's worth following Joe's recommendations. They don't question Joe's request to add beads of silicone to already well-adhered strips of peel-and-stick membrane. That doesn't stop Dr. Joe from reminding them that it's a bigger deal than many people might think. "We've got 35 windows in this house and it adds up, " Joe tells them. "And we're going to be absolutely perfect."
Feb. 16, 2010 - UPDATE:
David Joyce called me this weekend to tell me about the blower door reading they got: 1.37 air changes per hour at 50 pascals. And this is before the drywall is even up. Their target was 1.5 after air-tight drywall. So far, so good.
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