The Sealing details that make a Passive House
Comments of Al Cobb
The Panelwrights crew has completed all panel work and the structure is covered with roofing felt allowing the trades to begin. While we prepare to install the “outsulation” along with all windows and doors, our crew is busy ensuring the fine detailing of air sealing. It is this critical element that is one of the three performance metrics of a “Passive House”. I like to think of it as the “trifecta of performance”
You’ll notice that the crew installs an interior tape at the corners before installing drywall “backup”(this is the necessary framing required to properly install the gypsum board. It is this tape that adds a second level of protection from air entering the wall assembly. Some would wonder why we don’t tape the exterior of the joint where interference is much less. However, in this climate, our vapor retarder must go on the inside. We do this to prevent any air with entrained vapor from coming in contact with a cold surface (Exterior sheathing). It’s the prevention of uncontrolled air movement that both ensures durability and improved energy performance.
So why didn’t we tape the whole joint you ask? Because our first blower door test will be before final taping. We have planned two tests to verify how effectively the SIPs seal by themselves, and then a second test to determine the % of added benefit provided by the interior joint taping.
In addition to using a non-permeable SIP tape, the crew uses an expanding single component foam that is injected into any open crack. This foam will be rasped smooth and then later covered with SIP tape as well. This shot is looking out the front hip dormer.
In addition to sealing our work, a big part of successfully controlling air leakage is working with other trades to ensure they’re envelope penetrations are sealed. O’Neal Development coordinated a pre-construction meeting to gather any trade that would be putting a whole in our newly installed envelope. This gave us an opportunity to coordinate the details that ensured our collective success. This picture shows how the plumber ran a vent stack through the roof SIP. Per our plan, the hole was bored a full inch oversized. This allowed us to use a two component expanding foam to fill the void completely around the pipe and ensure no air leakage. You’ll notice we used a simple scrap of cardboard to act as a damn while filling the void.
For the sake of comparison, I’ve included a picture that shows a similar vent stack that was installed by a plumber that didn’t consider air-sealing important. In addition, the general contractor hadn’t checked the work to verify the envelope wasn’t compromised.
The last shot of the plumbers work shows a typical use of a “wet wall” placed inside the SIP wall. We don’t like to run pipes in exterior walls for a whole bunch of reasons. The simple solution is clearly shown here.
You should also notice the use of an interior partition as a means to run the vent pipe.
Check back next week for more sealing details and the start of exterior insulation wrap.