Air barrier on cape w loft
Hey all,
I’m having trouble deciding on how to best go about installing the air barrier on our project that is currently in the design stage. Zone 5, cape style, single story with loft. Simple roof without dormers.
The wall will be double stud with blown in cellulose, with the plywood acting as the air barrier. Currently I am thinking that the lower 2/3rds of the rafters will be the loft space, and the upper 1/3rd will be a vented attic with blown in cellulose on the deck.
The lower 2/3 of the rafters will likely also have to be double stud to achieve sufficient R value, and to accommodate the space for the vent channels, which will start at the soffit.
My question is, has anyone here run into a similar situation, and what did you come up with?
Lately I have been considering taping the corner from the external first floor sheathing to the plywood on top of the lofts floor joists. From the plywood on the inside of the rafters, I was thinking of taping something like intello and running the membrane up and under the collar ties / attic floor joists. Inside of the intello, perhaps a service cavity for simplifying the detailing when installing whatever lighting is decided upon.
Thanks!
Ben
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Replies
Why would you not insulate the roof with polyiso on the top of the roof deck. You make the assembly and air barrier install so much simpler.
If you want to use cellulose, to insulate the roof in a similar fashion to create a warm roof, look at the BS&Beer show
https://www.greenbuildingadvisor.com/article/the-bs-beer-show-unvented-roofs-and-fluffy-insulation
Creating a cold space behind a double stud wall at the lower part of the roof cause all kinds of challenges and you have already hit one big one.
Also, with a warm roof (insulation on top of the roof), you just gained an incredible amount of conditioned storage space. It’s a win in so many ways.
My opinion the half story choice is a poor one. Start over and keep it all on one floor or shrink the foot print and add a full second floor. Getting the air sealing details correct designed and built correctly for the half story is no small feat and seems likely to cost the same as a full two story.
Avoid the other common popular design flaws sloped ceilings, heated floors, skylights, large % of windows to wall, HVAC in attic.
Walta
Actually, I agree. That is a better response than mine.
Thank you both. The BS and Beer was great to watch, great presentations and discussion. Interesting to see Chris Corson’s (sp?) roof with what could be wood fiberboard over the roof sheathing, utilizing battens for airflow. Though I say that now, but I’m unsure how the effort of building his design in the sun would compare to what I was considering above.
Also, I in no way disagree about the similar effort of building a loft vs second floor. As it happens the loft is something my GF has wanted since before we started planning the build. Both of us have compromised on a number of things that are important to each of us, but the loft remains. For months now I have been thinking of how I can avoid putting in unnecessary work when construction begins next spring.
Again, I appreciate the honest advice
Ben
You might take a look at this video and article by Josh Salinger for details on transitioning your plywood air barrier on your wall to a membrane air barrier on the inside of your roof.
Video: https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Article: https://www.finehomebuilding.com/2021/04/28/a-new-take-on-insulating-a-roof
I think the detail is similar to what you are describing, and it's nice to see some video and pictures!
Josh Salinger's detail uses 2x4 strapping to make a small service cavity to run wiring between the air barrier membrane and the ceiling finish.
That article also includes a novel approach to the vent channel at the outside of the roof, but that part of the detail can be ignored if it isn't useful to you.
Look at post #26 here:
https://www.greenbuildingadvisor.com/article/maine-authors-publish-pretty-good-house
This seems like the simplest and cleanest way to build a 1 1/2 story. This avoids having to build knee walls and your floor/roof joists don't poke through your air barrier.
As for the dormers, the simplest is to make the dormers the full width of the house bellow and have bolt on roof overhangs on the side. This avoid the very messy roof to wall to roof transition that there is no good way to get right without sprayfoam. Besides being easier to air seal it is also way simpler to build and looks about the same.
That’s some great info right there.
Josh’s roof assembly is clearly a much more refined and well thought out version of what I described in my original question.
I’m going to have to give serious thought to the 1.5 story house on the book (likely should just buy the book). It’s a design I have always been drawn to. Using the space that would otherwise be lost to knee walls is a huge win for sure.
Thanks again!
Ben
You are basically the target audience of the book (owners wanting to build a pretty good house!). I think it's well worth purchasing! My copy is currently on loan to a client, and they seemed pretty excited to read it.
Purchased!