Air Sealing Plumbing Vents

Attics are cold in the winter and warm in the summer.  Since they are vented through the soffit they are also drafty spaces.    The attic could become an energy sink without lots of insulation and good air sealing details.  We are at a point in construction where we have to start penetrating out though the air barrier and into the attic.  I find this scary but its another reminder that we are progressing nicely.

With our blower door test measuring air tightness of the air barrier at 0.45 ACH50, I cringe every time somebody suggests that we have to drill a hole in it!  This being said, I knew it would be inevitable.  So we move forward with plumbing and wiring details.  Although we placed all the washrooms on one side of the house, it was pretty much impossible to tie everything into one main vent stack.   There service cavity on the ceiling is 2x4 strapping on face so there just isn't enough room to route each vent to a common place and terminate them together before penetrating into the attic.  Instead there are 3 individual ones which are drilled up through the top plates in the exterior service wall.

I knew there would be little distance between the edge of the pipe and the top plate of the exterior 2x8 wall.  I needed to have enough OSB to tape/seal my gasket.   My plumber, (Melvin Way, Perpetual Plumbing Inc) drilled through the top plate of the exterior wall with a hole saw slightly bigger than the pipe diameter.  He kept the pipe as close to the inside edge of the plate as possible so there would be enough OSB in the attic to provide a surface to air seal.  He stubbed up through the attic with a vent that was about 12" high so I could slip the gasket down over the pipe (Picture 1-1)

When I air sealed the ducts for my ERV and the air intake for the wood stove, I used Roflex gaskets obtained from 475 High Performance Building Supply.  They were pre-made and worked well.  Since then a friend of mine told me to make my own gaskets using an EPDM liner made for backyard ponds.  So I gave it a try.  I cut the gasket to about 6"x6".  I centered the gasket over the end of the pipe and used a utility knife to create a star shaped hole in the gasket by cutting successive lines while using the ID of the pipe as a guide (Picture 1-2).  This ensured that when I went to pull the gasket down, it would still be tight around the pipe.  Using my trusty Ryobi battery powered caulk gun I dispensed a good bead of acoustical sealant both around the pipe and at the edge of where the gasket would lay on the ceiling (Picture 1-3).  Not to get sidetracked but I should mention a couple of things here.  First, if you decide to build a passive house, purchase a battery powered caulk gun...unless you want to look like Popeye!  The caulk gun will pay for itself ten times over!   As a recommendation, I prefer to use use Mulco Acoustik ( for air sealing.  Its a low VOC formula acoustical sealant.  It is not quite as stringy as other brands and is a little thicker.  I also find that it is much easier to wipe from your hands....but you'll still get dirty....and it will go everywhere....its acoustical sealant!  What more can you expect!

Once the acoustical sealant is applied, I pull the gasket down over the pipe slowly until the gasket rests flush with the surface.  I apply rubbing pressure to the gasket to ensure it is seated in the acoustical sealant and then apply 3M 8067 tape around the edges.  I use a J-roller to apply pressure to the taped surface to ensure adhesion.  I then finish the detail by wrapping 3M tape around the penetration of the gasket and onto the pipe (Picture 1-4).

I think this is a good redundant air sealing system that is simple to implement on the build site and it doesn't require an fancy materials.   First, either the tape or acoustical sealant would have to fail in order for the air seal to be broken at the base of the gasket.  At the pipe penetration through the gasket, the EPDM is stretched and is compressing around the pipe because of the elasticity.  In this case, the three subassemblies would have to fail in order for the air seal to be broken:  the gasket, the acoustical sealant, and the tape.  Given that all subassemblies are going to be covered in 26" of cellulose indefinitely I expect that there will be little exposure to the elements and the details should remain intact for years to come.

As much as I don't like drilling holes in the air barrier, I'm pretty confident that these details will bring my air barrier back to passive house air tightness.

Picture 1-1.  Plumbing vent stubbed up in the attic.

Picture 1-2.  EPDM pond liner gasket, with a star line pattern cut the same diameter as the ID of the pipe.

Picture 1-3.  Use lots of acoustical sealant.  Its cheap!

Picture 1-4.  Final gasket detail.  Taped, sealed....and delivered!


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