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Question on vapor retarder on interior dividing wall

Dublin1997 | Posted in General Questions on

First off, thanks to the many that respond to questions posted by us less experienced folks! I have learned a great deal from this site so thank you.

I have a workshop that I am currently finishing the interior. Currently everything is insulated and I’m turning my attention to the vapor retarder. First off, I’m located near ABQ NM which I believe is climate zone 5B. Very dry most of the year with a monsoon season that can get pretty wet. Anyway, my workshop is 32×40 and I have a dividing wall running down the middle so effectively two 32×20 spaces. Ceiling/roof heights are different for each with one at 10′ and the other at 12′. Flat roof. I am heating/cooling each space independently and for most times, will only be conditioning one side at a time. It will most likely be conditioned intermittently when working in one side or the other and given this is a hobby shop only, will probably only be used in the evenings and weekends by myself.

So my question is this: On the interior dividing wall, I plan to use Membrain up high where the lower roof trusses intersect into the higher ceiling area (2 ft). That seems clear enough to me. What I’m not sure of is the wall that is common to both work spaces. Should I be putting membrain on both sides of the wall, just one side, or not at all? What throws me is the fact that there are times I’d be heating/cooling one side and leaving the other alone whereas there are other times, I’d be doing the same but in opposite rooms. I should mention I will be insulating the interior wall (for insulation as well as sound reduction) with Roxul. I should also mention that the finish on the walls will be drywall/latex paint on one side and pine T&G on the other.

Thank you very much for your time and advice.

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  1. GBA Editor
    Martin Holladay | | #1

    All you need to worry about with this partition wall is airtightness. (It's always a good idea to reduce air leakage.) Vapor diffusion is irrelevant. You aren't going to have any diffusion issues in the partition. There is no need for MemBrain on either side.

    For more information, see Do I Need a Vapor Retarder?

    -- Martin Holladay

  2. Dublin1997 | | #2

    Thanks Martin. Can you explain why I shouldn't expect any diffusion going on? let's say its the dead of winter and I'm heating one side of the shop but keeping the other at "outdoor" temp. Isn't that the same as any other exterior wall (or the opposite scenario of cooling in summer)? I fully concur with regards to the air barrier that you point out and without getting too far into it, I was planning on installing the membrain in an airtight fashion but will now look at other methods/materials. Thanks much for the help.

  3. GBA Editor
    Martin Holladay | | #3

    1. The opposite side of your partition wall is not at outdoor conditions.

    2. As my article noted, most worries about vapor diffusion are based on misconceptions. Diffusion-related problems in walls are very rare.

    -- Martin Holladay

  4. Dublin1997 | | #4

    Thank you Martin!

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