Air sealing
Building in Zone 2 and am considering changing from my original plan to use spray foam.
Would like to use Roxul ComfortBatt in a 2×6 stud bay and Roxul Comfortboards outside of sheathing instead. Plan on using a self adhering WRB and the baseplate will be sitting on a EPDM gasket. Would like to gather some more detail on water and air sealing with this approach, specifically:
1. important areas of the framework and sheetrock to seal and the best sealant to use. This is a single story house sitting on a concrete slab with a single pitch roof.
2. transition between roof WRB and wall WRB at the overhang. I have seen some diagrams where the WRB was continuous between the two and some where the WRB stops at the soffit. It seems the continuous approach is best but how can that be done correctly when the overhang is 2 feet long.
Thank you.
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Replies
Hi Deerefan,
The WRB on the wall and the roofing underlayment do not need to be continuous. This is not possible when you have roof overhangs, and the overhangs themselves protect the top of the wall. What needs to be continuous around all six sides of a house is the air barrier.
Sometimes the WRB and the air barrier are the same material. For example, in your case a fully-adhered WRB can be the air barrier on the wall sheathing, but you need a plan for the air barrier on the lid of the house. In other words you need to decide if you are barrier will be at the upper floor ceiling (if the attic floor is insulated or along the roof line (if the roof is insulated) and how you are going to make a continuous transition from the air sealed wall sheathing to the ceiling or roof air barrier. You may find this article helpful to understanding the difference between your water and air control layers: The Four Control Layers of a Wall
Attached are a couple of photos from one of our contributor's projects. They use an OSB flange (a rip of ZIP sheathing in this photo) that is run over the top plate and taped to both the WRB on the outside and the ceiling air barrier for continuity. Some builders just air seal the top plate with caulks, canned spray foam, etc. and install the ceiling drywall in an airtight manner.
Thank you.
Can I use an EPDM gasket between bottom, top and perimeter sheathing and frame?
What sealant is best on the inside of the frame? Tremco (or similar) Acoustic Sealant or 2 part closed cell spray foam or other?
The overhangs are integrated into the trusses in our structural design so I cannot use the air sealing detail you demonstrated. How else can I accomplish this effectively without having to spray a bunch of foam?
Thank you again.
Before using Tremco (or any other) acoustical sealant, I recommend reading the manufacturer's instructions. In most cases, this will lead to not using it for air sealing.
Jon,
Thank you for the reply. Could you explain as to why, it seems that I have seen it described as one the best, and extensively used, products for that application on this board.
I’ve been using a urethane caulk from loctite for sealing recently. I haven’t been using it long enough to really know how well it holds up over time, but I expect it to do pretty well. It sticks to everything and stays a little flexible, and it’s a lot better at bonding than silicone is (less tendency to pull out like a piece of rope from a crack).
Bill