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

Exterior vapor barrier and an interior barrier between garage and apartment above

Irishjake | Posted in General Questions on

Hello all,

So I’m in the middle of building a super-insulated/zero energy home using with a detached barn/garage that has an in-law apartment above. The barn is being super insulated like the house (REMOTE/PERSIST style), because it is a workshop, a garage and apartment above. It is easier to just detail the whole wall in one plane than varied planes. We have already applied all the peel & stick ice & water shield on the roof, and yesterday we started applying the liquid applied WRB (Prosoco R-Guard products/system) on the exterior sheathing/walls/windows.

We thought we had nailed every detail, but now I’m back pedaling a bit, because I’m concerned with how to detail the vapor/air barrier that needs to go between the garage area and the apartment above. (I probably should have placed some strips of Stego between the top plate and ceiling joists to totally isolate the apartment). Normally the interior vapor barrier would just wrap up and over the walls/ceilings.

The garage has to have the apartment above.
It will have all required fireproofing, etc.
It will be insulated within the joists bays.
The apartment will have TwinFresh ERVs.
The garage area will have a ventilation fan whenever cars pull in or out.

Any ideas on how to isolate the apartment?

Thanks, Brad

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
    Martin Holladay | | #1

    Usually, plywood subflooring is installed with construction adhesive. That makes for a fairly effective air barrier. Presumably, there will also be an air barrier at the drywall ceiling that you will install on the underside of the floor joists.

    If you are worried about leaks at penetrations, cracks, and transitions, you can use a theatrical fog machine and a fan to track down the last few leaks.

  2. Irishjake | | #2


    Yes Advantech T&G subfloor, with construction adhesive was used. Yes I have planned to put a vapor/air barrier (10 mil poly or Stego) at the ceiling on the underside of the joists, but normally that barrier would just run right down the walls and be taped at the ceiling/wall intersection to encapsulate the garage.

    So the question is how to continue the air/vapor barrier from the ceiling and out to the edge of the walls (underneath the joists and to the outside of the structure)?

  3. GBA Editor
    Martin Holladay | | #3

    The title of your post, and your most recent question, shows a concern with vapor barriers. Why?

    It sounds like the insulation is on the outside of the wall, and that both the garage and the apartment are inside the conditioned envelope of the building. (By the way, will the garage be heated?)

    So why do you anticipate vapor drive through this floor assembly?

  4. Irishjake | | #4

    You are correct...
    50% of the insulation is on the the exterior side of the walls, and on the roof it is all to the outside.

    The garage, apartment and workshop are all heated and on the conditioned side of the air/water barrier. Ice&Water being nearly impermeable, and the Prosoco being vapor permeable. There are varied levels of permeance. The varying levels of EPS, along with Advantech, also add to the varying permeance.

    My concern of vapor barriers, is that gases are a vapor. Air sealing two spaces doesn't prevent gases from passing between them necessarily (right?). Last I checked most of the "vapor" we talk about, is just a gaseous form of water. So my concern is that moisture (water), benzene, CO, and other products emitted from combustion engines, along with various chemicals used within a typical multi-use shop space (degreasers, spray paint, glue, cleaners, etc.) can push their way through an "air sealed wall" in the form of a vapor (just like water). I'm trying to reduce the prevalence of this by negatively pressurizing the garage. Hopefully ventilating the apartment via ERV will help, and the bath and kitchen exhaust won't reverse the pressurization of the garage/apartment.

  5. GBA Editor
    Martin Holladay | | #5

    If you install a decent air barrier, you don't have to worry that benzene molecules will diffuse through the painted drywall ceiling and your plywood subfloor. Even water vapor molecules will have a hard time diffusing through those materials, especially once the drywall is painted and the finish flooring is installed on top of the subfloor.

    And unless I'm mistaken, water vapor molecules are very small -- smaller than benzene molecules -- so that all you need is an air barrier, not a vapor barrier, to keep your apartment separated from the VOCs in your garage.

  6. Expert Member
    Dana Dorsett | | #6

    A benzene molecule is pretty small, at about 6 Angstroms.

    But a water molecule is smaller still, at about 2.75 Angstoms.

    CO is comparable in size to water, at about 3 Angstroms.

    The health consequences of small amounts of water or CO diffusing through the subfloor are considerably smaller than those of benzene, but the high dilution factor of the ERV renders all the vapor diffusion aspects down into the noise (you might be bringing in more benzene from your neighbor's parked car with the ERV than is getting into the room via vapor diffusion from your garage/shop.)

    Air tightness is at least an order of magnitude more important than vapor tightness to small molecules, but even minor air leakage is dealt with by the ERV dilution factors.

  7. Irishjake | | #7

    Martin & Dana,

    Thanks for the feedback. It will definitely be air sealed well. Thanks for addressing my concerns so quickly, and putting them to rest too!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |