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Community and Q&A

Vapor barrier/retarder with shiplap wall without drywall?

Rick49768 | Posted in Green Building Techniques on

I am building a small cabin in Michigan’s Upper Peninsula, using standard 2X4 walls, 16″ OC and the rafters are 2X10s, 24″ OC, defining a cathedral ceiling.  I’m using rain wall techniques on the exterior.  The cabin will be used mostly during the Spring – Fall months although we are planning winter visits as well.  Temperatures in this area of the world vary wildly, e.g., generally  from -15 to 90F.  A small camping trailer LP gas heater with a power vent will be installed.

My question relates to the interior walls and how they should be finished.  I want to use rock wool insulation so as not to adsorb moisture and also to at least deter mice, although I am thoroughly checking to be sure there is no access to the spaces between the framing members to allow them access.  I want to finish the interior walls with wood shiplap siding.  Would it be acceptable to install a vapor barrier/retarder onto the insulated walls and ceiling then applying the shiplap without drywall?  I have been told that drywall joints will likely crack due to large temperature changes.



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  1. Expert Member
    Zephyr7 | | #1

    I don't think you'd have any problems using a smart vapor retarder like MemBrain here detailed as an air barrier. I did that myself behind some drywall in my home office I renovated and re-insulated with mineral wool. I don't have enough exterior rigid foam to go without a vapor retarder on the interior, and I don't have the exterior redone for a few years yet.

    Note that if you put up sufficient exterior rigid foam for your climate zone you won't need an interior vapor retarder, and if you detail the exterior foam as your air barrier too, you won't need to do anything special behind your interior shiplap walls. Exterior rigid foam has advantages besides just adding insulation.


  2. Expert Member
    Akos | | #2

    I have a cottage with T&G ceiling with standard 6mil ploy as the air barrier under the T&G. If you are careful, tape all seams and use sealant around the perimeter, it works well enough. Putting a couple hundred nail holes through it didn't seem to cause any issues. Make sure to use air tight electrical boxes and seal the poly to around it.

    P.S. Critters like mineral wool, not much of a deterrent. You need to critter seal on the outside. Cover all exterior transitions with wire mesh. If you are building on posts, cover the bottom side of your floor joists with plywood+wire mesh.

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