How do I seal the air barrier at the top of the exterior wall?


How do I seal the air barrier at the top of the exterior wall?

Nicolas Jolicoeur Jan. 3, 2022Last updated: Jan. 3, 2022

Hello, The air barrier was installed up to the top of the exterior wall. I wonder how to prevent the air from entering between the top plate of the wall and the OSB r4? The roof trusses are already laid on the top of the wall, so it is difficult to put red tape on the length of the wall.

I have the impression that the air barrier should have been installed over the top plate and the roof trusses installed on top afterward. How to air-seal it at the top of the wall opposite the plate? If I just put red tape on the wall at the top, the outside air could come in through the soffits, then enter between the OSB r4 and the top plate and end up inside the wall. Thank you

Responses (5)

Nicolas Jolicoeur
Nicolas Jolicoeur Jan. 5, 2022, 11:01 a.m.

Hello Emmanuel, Thank you for your suggestion! It does make sense, but unfortunately, in my case, it can't be done because the OSB covers the two plates. I thought I had linked to an image in my initial question. That would've helped you understand where the R4 OSB is!   Here's a link that shows how the construction is built. The difference is that I don't have the outdoor siding installed yet: My comments are in orange. The image originally comes from the Regie du Bâtiment du Québec, and the Tyvek isn't taped to the OSB nor the top plate. But I have a feeling that it should because air could leak between the OSB and the top plate. In my case, since it's R4 OSB, the air could go through the OSB and the R4 and between the R4 and the top plate. Once again, thank you very much for your help!

Emmanuel Cosgrove
Emmanuel Cosgrove Jan. 6, 2022, 9:54 a.m.

I can see now what you mean Nicolas. What that diagram shows is about what I thought you meant (shown below). You can see the interior poly membrane on the ceiling taped to the poly on the wall, so in this diagram they are relying on the interior 6-mil polyethylene as the air barrier, which is why there is no tape on the exterior. Which, is also an opportunity missed as you cannot have too many air barriers in a wall assembly. It also speaks a bit to the confusion often in buildings where construction crews aren’t always versed on what does what in a wall, whether it is the interior poly or the exterior weather barrier that is intended as the air barrier. Often you will weather barriers taped like crazy all over but then leave a crucial component - like not taping the top - rendering a lot of the previous work pointless. 

I would prefer to see that shown as a well-installed exterior membrane – preferably a peel and stick air barrier – that could be carried between the two top plates and then taped to the ceiling air barrier, then the poly on the wall taped to the ceiling as well. Then there would be a second barrier to further reduce leakage. But that’s all hindsight I guess, and in your position you may well be best at this point to get a few tubes of acoustic seal and make sure the exterior barrier is sealed to the top of the wall with no air allowed through the two top plates (which is where the acoustic seal comes in handy). how to seal an air barrier at the top plate of a wallImage via Regie du Batiment du Quebec

Nicolas Jolicoeur
Nicolas Jolicoeur Jan. 6, 2022, 10:45 p.m.

Ah..! Now I understand why the contractor did it that way. As you suggested, I'll go with the acoustic seal.  Thank you again for taking the time to write that detailed answer!

Emmanuel Cosgrove
Emmanuel Cosgrove Jan. 7, 2022, 1:51 p.m.

Happy to help Nicolas, hope it works out!