Exterior Assembly Details for a Pacific Northwest Passive House

April 6, 2017

 

Dan Whitmore of Hammer & Hand explains that the most effective (and easiest) time to detail the air barrier is during the framing stage. 

 

What he doesn't say is that if it doesn't happen during design, ain't no way the framers are gonna get it right.

The house has a pretty complex form, and that makes detailing the air barrier more difficult and important. Case in point: a big structural steel beam that is carrying the whole wall system above and part of the roof load. One side of the beam represents the thermal envelope of the house, the other side is interior space.

"This means a fairly complex thermal envelope and air barrier coming down and then going back up. So what we wanted to do is build the shell, and then add that wool sweater on the outside."

So we were able to put our air barrier in a protected location buried within the thermal envelope, detail it appropriately and then head vertically above it.

The back deck is another tricky spot because the air barrier transitions from the foundation to the rim joist, to the deck floor, to the step, to the door. Then up above, it comes back out and goes vertically again. 

"So we have to make sure the ‘innie’ door is detailed for air tightness, thermal efficiency, long-term durability, and for aesthetics."

"We need to make sure we are dialing it in as we build the building."

(HINT: It is easier to make an air barrier continuous if you give it The Pen Test)

Another roof/wall interface illustrates another spot that must be free of thermal bridges, be air tight, structurally sound, and water-tight. So air barrier/WRB goes down behind the insulation (Zip system sheathing) where it will hit another deck.

"We’ll need to air seal at that deck below it. On top of the deck is going to get additional insulation, then we’re going to integrate this WRB into the top of that deck with a continuous TPO membrane."

The critical point is to make sure that you hit all of those spots during framing—

"You can always remediate it down the road, but wow, what a pain. So if we can hit it right now, identify where the critical spots are, that’s when we want to hit it, during framing."

Hammer and Hand is a general contractor that specializes in high performance building with offices in Portland and Seattle

 


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