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My home interior walls are thick redwood paneling in the bay area mountains. i'm about to start removing the wood in my second bedroom and want to install a moisture barrier on the 1 outside wall of the room.

I found this page: http://www.naturalhandyman.com/iip/infenergysavers/infthickenwalls.html describing the process of making a wall thicker by converting to a 2x6 stud allowing for more insulation. The last step before the drywall went on was covering the wall with a cellophane moisture barrier.

My question is, will adding this moisture barrier to the inside of my walls raise potential for damage inside the wall? There is currently insulation and T1-11 siding and no moisture barrier on either side of the wall.

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Increasing your wall thickness to facilitate 6in insulation is a fine idea, can't hurt. A vapor barrier of 4 mil plastic with taped seams should always be installed between the insulation and drywall. this will stop moisture from the outside from penetrating the back of the drywall and conversely, humidity from inside collecting in the insulation. T111 siding is notorious for not being a tight exterior sheathing, especially when installed without a backer such as OSB or plywood covered with a layer of Tyvec. The interior of the wall will vent itself and ambient moisture in the wall will not be a problem in most cases, as long as there is not any water infiltration into the cavity, you will be fine.

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great input, and thank you! unfortunately I didn't do this for our master bedroom as I had a slew of problems after demo. Sones room doesn't have any exterior doors though so it should be easier than the master BR. –  mattsn0w Apr 20 '11 at 5:07
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The paper backing on standard fiberglass insulation is a vapor barrier. When you install it, you want to be sure it extends one inch over the framing on all sides, which means separating some of the fiberglass from the paper on the top and bottom and cutting it several inches too tall so that you aren't just stapling it in via the sides. I believe the main goal is to prevent outside humidity from condensing on the back of the drywall surface, especially in areas that use air conditioning.

On the local Habitat projects that I work on, we always go with 2x6 framing of exterior walls when possible to improve energy efficiency from the thicker insulation, so I highly recommend it. It's a shame we don't do this more in residential construction.

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good to know about the paper backing. it's better than nothing for sure. I still feel the urge to rip off the T1-11 and put some tyvek over the studs. The other route (more time and $$$) is throw on tyvek over the T1-11 and put up some sort of vinyl or cement fiber siding. Any comment to either of these "solutions" ? –  mattsn0w Apr 20 '11 at 5:16
    
I've seen the cement fiber siding in areas near the ocean, and it seems to hold up better than anything else. Vinyl is easy to work with. We use it at Habitat due to the price. Just make sure your grill is away from the house when you light it. –  BMitch Apr 20 '11 at 11:29
    
good call! i'm not that close to the ocean but am in the middle of what feels like a rain forest in the winter. it will be sunny a couple miles away but the fog from the coast creeps over the mountain top that i'm below and keeps things moist. Thanks again! –  mattsn0w Apr 21 '11 at 18:04
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