Stringent control of airflow into and out of the house is required for the PassivHaus design to function as intended. As a result, one of the primary features of the house is a continuous air barrier. The continuous air barrier stops uncontrolled air from flowing from inside to outside or from outside to inside in order to keep fat lazy pets reclining on memory foam beds in draft free climatic nirvana despite what owners are experiencing outside.
Executing an air barrier properly amounts to: 1. choosing materials that are air impermeable, 2. assembling air impermeable materials into air impermeable components, 3. assembling air impermeable components into an air impermeable building. And therein lies the problem in one simple word--assembly.
One assembly problem that's haunted me for months is how to join the ultra durable and ultra slick (as in things don't stick to it) 15mil Stego Wrap which forms the bottom of the air barrier to the air sealed plywood sheathing. The first failed attempt to join Stego Wrap (yellow) using Grace Vycor Plus tape (black) and a bit of Prosoco R-Guard Fast Flash (red) appears below.
The system did not fail for lack of adhesion. I was worried about the tape and Fast Flash sticking to the polyethylene Stego Wrap. Neither had a problem. In fact it took a massive amount of mechanical force to pull the materials apart when the system was disassembled. Reason for failure: I could not get all three materials joined together in the corners in such a way as to make me confident in the air tightness of the assembly. Mind you that's on an outside corner. I cannot imagine the mess I would have made on an inside corner.
The joint is so complex and took so much time and effort to install (imagine using a heat gun and rolling that Vycor tape by 3" increments to make it stick in near zero temperatures) it just had to work. Wrong. This assembly was put to the test shortly after installation by a big rain/slush event followed by a hard freeze. Ever heard of Ice Spikes? Me neither. And although Ice Spikes are particularly difficult to produce in a lab environment, they were like rabbits in spring at our build site. My theory is the Ice Spikes were pulling water through the corners and into the joint between the sheathing and the mud sill where it then defied gravity and made all sorts of ominous vertical formations on the bare wood framing.
Wall and Floor adhesive is used because it has relatively low expansion compared to the other available foams (and it's what I had on hand). Once the foam skins over I mash it into the joint with a finger. The results of the finger mashing step are included below.
While it can't be seen in the picture above, foam wall and floor adhesive was used to bridge a 1/2" gap between the concrete coated in red Fast Flash and the bottom edge of the wall sheathing coated in pink Joint and Seam Filler. A bit more Fast Flash on the bare concrete spots and the Stego Wrap is joined to the sheathing with a tough monolithic elastomeric shell. The air sealing turd has been sufficiently polished.
Executing an air barrier properly amounts to: 1. choosing materials that are air impermeable, 2. assembling air impermeable materials into air impermeable components, 3. assembling air impermeable components into an air impermeable building. And therein lies the problem in one simple word--assembly.
One assembly problem that's haunted me for months is how to join the ultra durable and ultra slick (as in things don't stick to it) 15mil Stego Wrap which forms the bottom of the air barrier to the air sealed plywood sheathing. The first failed attempt to join Stego Wrap (yellow) using Grace Vycor Plus tape (black) and a bit of Prosoco R-Guard Fast Flash (red) appears below.
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| Stego Wrap (yellow) + Vycor Plus (black) + Fast Flash (red) = Fail |
OK. So we now know that Vycor tape sticks like mad to CDX plywood and polyethylene Stego Wrap. Fast Flash is nearly impossible to remove from Stego Wrap and concrete. The problem is making the transition in the corner. Shower systems like those designed by Schluter use preformed waterproof corners. If they're waterproof they must be air proof as well. The image below shows the benefit of adding complexity to the simple task of sealing out air.
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| Stego Wrap (yellow) + Henry 208R Roofing Cement (black) + Fast Flash (red) + Vycor Plus (black) + Joint & Seam Filler (Pink) + Preformed Corner (pale yellow) = Fail |
At this point Grace would be my number one fan except for the fact that I keep ripping up their products because I can't manage to make an air tight (or water tight) assembly using them; a huge buzz kill for the marketing department--"Use Vycor Plus for air sealing. It's disastrous!". Fortunately the 208R roofing cement stuck hard to the Stego Wrap. And with gravity jamming a substantially non-curing material into the joint with the concrete the air seal should improve as time goes on. No need to tear the 208R out. The Fast Flash and Joint & Seam Filler stuck to the concrete with gusto so they were left in place.
While the wounds of the Ice Spike fiasco healed the joint between the first and second floor sheathing required air sealing attention. If I were a genius carpenter I would have had a nice uniform 1/8" gap to seal with a small bead of Joint & Seam Filler. But I'm not and that's what provided the inspiration necessary to wrap up the air sealing saga. The picture below shows a bead of Great Stuff Pro Wall and Floor Adhesive that is used as a shapable caulk backer rod to stem the tide of money pouring out of the caulk gun.
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| Great Stuff Pro Wall and Floor Adhesive as Caulk Backer Rod |
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| Great Stuff Pro Wall and Floor Adhesive Mashed Into Sheathing Joint
With a firm substrate in place the Joint and Seam Filler works its magic. It seals up all the small voids in the foam. And because it's elastomeric (like silly putty rubber), Joint and Seam will remain flexible over time. Believe it or not, it'll stretch with the foam even if the foam isn't fully cured when the Joint and Seam is applied. Wild. An image of the state of the art Stego Wrap to building sheathing connection is included below.
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| Stego Wrap (yellow) + Henry 208R (black) + Fast Flash (red) +Wall and Floor Adhesive (hidden) + Joint and Seam Filler (pink) = Success |
In closing, at this point I'm a firm believer in the use of liquid flashings (Fast Flash and Joint & Seam Filler) for making the connection between sub-slab polyethylene and plywood sheathing to form a continuous air barrier. Liquid flashing allows for an efficient, durable, and verifiable seal at complex material intersections. The system is highly adaptive when coupled with spray foam to form a solid substrate for subsequent liquid flashing application. Rest easy pets. Air sealing crisis averted.
Note:
1. Getting hold of the Prosoco R-Guard liquid flashing products is not an easy task for the intrepid individual.
2. One source of Prosoco R-Guard products is Small Planet Workshop.
3. A second source of Prosoco R-Guard products is Allied Building Products. Prices are better but required quantities may be larger.
