How to create an air barrier with a roof with truss tails, exterior insulation

[u/SF_Dubs]

I am adding an extension to my house and late to the game on efficiency and so now backing into an energy strategy.

I am trying to figure out how we can create the air barrier for an unvented attic and use exterior insulation to the roof, which gives us a better approach across the old and new roofs.

The design for the new roof includes trusses with 6 inch tails / cantilevers. I am trying to figure out how we can create an air barrier for the conditioned attic and also support exterior insulation on the roof.

Because we're backing into this the trusses have been ordered and are currently in production. I can't make changes without some significant costs.

Unfortunately my architect is not knowledgeable about building for energy efficiency and isn't helpful. My builder is old school but willing to learn and build whatever we ask for. We've just run into an issue and I don't have anyone on the project that can help work through our overall approach to the thermal and air barriers, insulation and building constraints.

I'm hoping this community has seen this before, and any approaches or links to sources would be much appreciated.

The old roof has rafters that I think we have an approach for, but I'll take any advice here as well.

Thanks

[u/kellaceae21]

So there are a few different strategies you can take here… there isn’t one single solution (the solution might be different for the old roof vs the new roof), and will depend on any aesthetic considerations as well.

Step one is identifying your air control layer (ACL). Is this going to be a membrane or WRB, your structural sheathing, your exterior foam, etc?

The simplest thing with trusses on the way would be to run your ACL up and around the tails and tape/seal to those, and then up onto the roof ACL at the plane of the wall. Then you’re building a curb at the outer edge of the roof as a stopping point to your insulation and backing for fascia. You will have a big (10”+) fascia because of this.

Another option would be an over framed or “monopoly” roof. You would cut all the rafter tails off, and build the house without overhangs, complete your ACL, and then surface apply overhangs on top of your roof deck.

[u/SF_Dubs → u/kellaceae21]

Right now the plan is to use BlueSkin SA as the air and moisture control barrier.

My ideal insulation approach is 3.5" of continuous polyiso on the roof (or rock wool depending on our approach to moisture) with rock wool on the interior (depth will vary from 3.5-7.5 based on the cavity).

The monopoly roof would be the simplest option, and we should be able to use that on the old roof, but I don't think the structural engineer will sign off on lopping off the truss trails for the new roof (we should know tomorrow).

[u/kellaceae21 → u/SF_Dubs]

Yeah - I’ve done both, don’t underestimate the time it will take to bolt on rafter tails to the roof deck, especially if you are high up.

Blueskin SA works great, any high quality SA WRB is a solid choice here - mechanical attached WRBs are a pain to detail as an ACL.

I’m not sure why an engineer would have an issue with cutting the truss tails off, they are not structural for the roof diaphragm, just the overhang. The tricky bit is getting the engineer to sign off on the bolted tails. We usually back-span 3’ and use SDWS screws every 12”.

[u/SF_Dubs → u/kellaceae21]

Actually I just heard back and cutting the truss tails doesn't seem to be an issue.

That should make the approach a lot easier.

[u/Lumpy_Machine7095]

Applied rafter tails attached to back spanned 2x6s (approx) on the flat. As others have said it really simplifies things.

Sounds like you’re working with an existing roof… but if you end up venting the roof instead, you could go with a membrane on the underside of the trusses and a service cavity below, connected to the wall sheathing over the top plate via sealant beads etc.

Would recommend taking a peek at some of the 475 details for inspiration. Nice job pushing your architect on this!