GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

A pretty good garage

user-4885540 | Posted in General Questions on

In the process of putting up an advanced framed 36 by 36 foot detached garage that will have a “mother in law” apartment upstairs.

Climate zone 6 and planning on using rock wool batts on both the underside of a vented cathedral roof as well as the entire garage.

Trying to avoid using spray foam on both the garage ceiling as well as avoid foam on the roof deck. My question is regarding the air barrier for the apartment – is a practical approach to return the wall sheathing over the top plate of the garage part and then tape the seams of a sheet-goods (plywood, OsB, ZIP) on the garage ceiling?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Peter.

    Yes. That is not only a practical approach, but one that many high-performance builders are using regularly, and a good idea for a garage with living space above where an air-tight sealing is super important.

    Here is a photo of that flange in a double-stud wall. The bottom of the roof trusses will be sheathed with OSB, detailed as an air barrier. In this case, a mechanical chase will be framed below the OSB before the drywall is hung, but in a garage, you may not need to do that.

  2. walta100 | | #2

    Make sure you understand what your local fire code requires for living space over a garage some are very demanding. I think if I were sleeping over cars. Cars do have a tendency to catch fire I would want a 1 hour fire rated ceiling, staircase and door and a fire ladder even if no one is requiring it.

    You likely need at least a R38 ceiling to meet any modern code and pretty good implies you are going above the minimum requirements. My opinion building a vented R38 cathedral ceiling without foam inside a normal depth rafter bays sounds like mission impossible to my ear, if you plan please post a drawing.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |