Air Barrier?

OK these last 2 pictures were taken literally 2 minutes ago. These guys are flying!!! Notice the Tyvek below the insulation? What is that all about? No no, we don't have it backwards. It is there for a reason!

That is our primary air barrier silly!

We want this enclosure to be as air tight as possible. When an Architect is designing their building enclosure in section, they should be able to draw a line from the roof all the way down to the basement slab without picking up their pen to define where the air barrier is. It has to be a constant line because a break in the line would be what we call a hole. Holes let air in. Holes are bad.
Look...ill do it. Even do the basement. (there is lots of cracks, pipe penetrations, conduits and stuff you and I don't even know about below grade that connect with the outdoors) Doing this exercise makes you critically think about how the air barrier will be maintained through the sometimes tricky details. ...like a roof edge detail.


Marker down.

Keep going.

I think im done....

No. Wait. almost forgot... the basement... Done. Bam.

Why did we draw the line there and not somewhere else? Well...

This is a 100yr renovation. What does that mean? It means that we are trying to maintain a high level of quality and durability in all aspects of the project so that the building will not need any major work for at least the next 100 years. That is 2110. That is far.

When we were thinking about where the air barrier should be, we first though well what about the the Tyvek and roof underlayment on the outside? We are already putting it up...lets just do an extra good job taping it up and call it a day. Ideally, yes that would work but keep in mind we are talking 100 years here. If in 50years Joe contractor is hired to replace the siding, rips opens up the wall and slices up the tyvek...that would be an air barrier compromise. That is bad. So keeping in mind that we don't know what is going to happen in the next 100 years regarding building maintaince, we wanted to bury it deep where nobody could mess with it, keep it dry and warm and hopefully SAFE because it is so important.

What about the foam? What if we did a good job taping and foaming the foam edges? Ehh same thing. Buildings move, those sheets of foam are going to be bouncing around and shifting as the buildings timbers expand and contrast. The tapes and foams that seal up the edges of the foam are relatively new. Will they last 100+ years? I don't know. We don't know. Nobody knows. What happens to the low expand foam when XPS sheets want to pull away from eachother? Does it stretch or does it fail? I don't know. It is a risk we don't want to take. We are going to tape and foam all the edges anyways but we are going to expect it to well...fail over time in some places.

What about some type of spray applied air barrier? Hah. Our existing rough cut sheeting has gaps in it as large as 1". I don't think that a spray applied membrane can bridge that gap.

What if you taped the exterior sheathing? Well we are going to do that anyways but there is a joke about peel and stick tapes & membranes....they call them stick & peel membranes. haha It is a small durability risk. The big issue with this method is how the air barrier will be connected and worked through the roof overhang. It is too complicated. Too many beads of caulk to fail. Lets Keep It Simple Stupid

Ok so..

Taking everything into consideration, we decided on rolling out a layer of Tyvek ontop of the existing wall & roof sheathing and then build everything new on top of that. It is constant, easy to inspect and is not labor or material intensive.

Here are some step by step sketches that attempt to break down what is going on up there.


Strip the building down naked to the existing sheathing.

Roll out some Tyvek right over the edge of the roof.

Put foam & nailer for roof edge ontop Tyvek.

Build the roof edge and not have to think about the air barrier. Nice.

I will get into WHY we want air tighness another day!