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Old 03-31-2013, 07:27 PM   #1
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Install Vapour Barrier from Outside?


Haven't seen this question:

The south wall on my (old) two-storey frame house has no vapour barrier. I don't wish to remove the drywall, but I will soon have the back of the house off to install new siding. I will at the same time replace the old pink insulation, add styrofoam insulation, re-sheath with plywood and add Typar before siding.

My question: With everything out, and the back of the drywall exposed, can I install a vapour barrier from the outside? It would mean wrapping the poly around the outside of the 2x6 studs, leaving them on the warm/humid side. Will this be a problem? Is it just better to leave the wall without a vapour barrier?

Thanks for any help!

Doug Earl
Toronto

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Old 03-31-2013, 07:47 PM   #2
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Install Vapour Barrier from Outside?


Much of the vapor and condensation potential will be eliminated with proper insulation (rigid foam to the exterior) and air barrier details (i.e. airtight drywall approach or ADA).

You could apply a vapor retarder paint if you so chose to the backside of the drywall but that would limit the walls ability to dry to the interior as would poly.

I am not a fan of vapor barriers, personally, and think that if the wall is air tight and thermally broken to the exterior, the potential is minimal at best.

Gary has the best links in the biz and will be along shortly to clarify and post up all the stats.

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Old 04-02-2013, 02:00 PM   #3
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Install Vapour Barrier from Outside?


I was taught in school that the vapour barrier should always be on the warmside of the insulation. By putting it on the outside you will be trapping moisture in the insulation and if you have the pink stuff will be rendering it useless and be creating a mold hazzard.

Last edited by Burnt; 04-02-2013 at 02:02 PM.
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Old 04-02-2013, 05:28 PM   #4
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Install Vapour Barrier from Outside?


Quote:
Originally Posted by Burnt View Post
I was taught in school that the vapour barrier should always be on the warmside of the insulation. By putting it on the outside you will be trapping moisture in the insulation and if you have the pink stuff will be rendering it useless and be creating a mold hazzard.
The OP was still inquiring about placing the vapor control layer on the warmside (i.e. the backside of the drywall).
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Old 04-02-2013, 07:54 PM   #5
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Install Vapour Barrier from Outside?


The potential problem I see is if warm moist air makes its way to area of the stud facing the outside. That area will be cold and you might get condensation on the outside of the 2x6. To be honest, I am really not sure how high the likelyhood is of this happening.
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