David Joyce | September 12, 2016

Video // WRB & Flashing, Insulation & Air Sealing

SuperInsulated Sunroom (5): Outer Foam and Airsealing Tapes


Offset the joints and seal the gaps to keep air leaks out of the thick walls


Last time on Superinsulated Sunroom, we set the windows to be flush with the not-yet-installed outer layer of foam. The windows were sealed to the framing bucks along the sides. Above, a Z-flashing is formed to push any water out—if it ever gets behind the outer layer of foam, which is what we install in this episode.

Begin by ripping the panels to size and placing them on the wall. On the first layer, the full length panels were vertical, and they pieced in between with smaller pieces. On this layer the full length rips are installed horizontally—to offset the seams.

Because strapping will be screwed to the outside the foam, you only need two screws per panel to keep it from moving around. The screws are expensive, and wasting them where you do not need them can really add up. 

With all the panels in place, the crew turns to sealing the seams in the foam‚ along the window bucks on the sides and top, and at the gaps between the foam panels. 

Before adding insulation to the roof, they seal all the long gaps between the layers of the wall assembly. Zip System tape was added at the framing stage to seal the roof and wall sheathing. Calvin uses another flashing tape to seal the insulation to the Zip tape. 

The tape is tenacious, so they keep it up off the Zip tape, and make sure to align the edges tightly before smoothing it down into place. Then, Calvin overlaps the rake strip shingle style. The outer edge of the flashing tape is then folded down to seal to the foil facing of the polyisocyanurate foam panels.

Finally, Zip tape is used to make a tight connection between the Zip sheathing and the air sealing tape that just installed. On the wall, Dave covers the seam with Dow Weathermate tape, which he is a big fan of "because it sticks to everything."

After the video was shot, the roof of this sunroom got two layers of foam, which were staggered and sealed the same way as the walls, then covered with plywood and roofed. 

A super insulated sunroom may seem oxymoronic to energy nerds, but it is a tight addition to this energy independent home.


—Thanks to David Joyce, of Synergy Construction for opening his jobsite to our film crews. Thanks also to Calvin Cutts, PJ Burke, Damien Higgins, and John Albert for working with us and playing through the pain.



I am wondering why the joints between each panel of foam were not filled with a closed cell expandable foam, trimmed and then taped.  Don't polyiso panels have a history of shrinking over time?

Daniel Morrison's picture


I sent your question to David. Here is his response:

"When building this assembly, the air and moisture barriers are on the sheathing plane. When the house is sheathed, it is airtight and watertight. 

Then we apply bulk R value. Air and moisture can move through the foam plane, allowing hydronic redistribution. I also don't like to squirt foam between the panel edges because it can be messy, time consuming, and at the end of the day, is not inspectable. If I can't see that it was done, it was not done. 

We tape all seams or none at times, depending on the contract, client, details, material... I believe it is best to tape the outermost layer as you are flashing the wall. Tape helps prevent moisture from infiltrating between the foam. Tape also holds air into the assembly in the field of the wall. Spray foam in the seams would not allow any moisture that gets in to drain out the bottom. 

Air movement in the foam assembly is generally minimal. There are very small voids and very little force applied to the air. The primary force is temperature differential, but within the foam layers the temp changes very slowly.

Foam shrinkage issues.  From my experience, eyewitness accounts, and testing I have heard of or seen, polyiso doesn't shrink it is very stable and can get wet without a problem as long as it can dry. Long term exposure to hydronic pressure, however, will degrade it's R value over many years. 

XPS of the past had issues with shrinking. However as far as I know this has been solved years ago by the big names. 

Over the past decade or so of energy retrofits, I have only seen a few examples were there was shrinkage in my work, and it was an old off-brand product. 

As far as I know, EPS still shrinks. Type 9 may not because it is so tight. There are a couple redeveloped EPS foams that are new to the market promising better performance, but I can't vouch for them yet."

Hope that helps,​


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