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Insulation and vapor barrier for external walls in Dallas area?


SwingingHammer's Avatar
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08-16-13, 09:56 PM   #1  
Insulation and vapor barrier for external walls in Dallas area?

Hi -
Just purchased a home in the DFW area, and found moisture in some exterior walls. As I repair them, I'm struggling with what type of insulation to use. Pulling out the old drywall and insulation I was surprised to find plastic over faced insulation. Working my way from inside to outside, I have drywall --> plastic sheeting --> faced insulation, with facing toward interior --> exterior sheathing.

First question - should I use faced insulation again? And if I do, should the kraft paper go to the inside or outside (of the wall)? While faced insulation (facing indoors) makes sense in colder areas, it seems logical (to me) to have the paper facing out (i.e. to better keep the moisture/humidity out of the wall altogther.

Second question - do I really need the plastic under the drywall? My fear is that I'm trapping moisture in the wall with it...especially since it does get hot here in the summer.

Knowing the materials and order of them in the current scenario, I'm looking for info about how to put these walls back together - thanks for any help!

 
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08-17-13, 02:49 AM   #2  
Welcome to the forums! What is happening is you have two vapor barriers. You only need the kraft facing OR the plastic, not both. I would opt for kraft facing alone, facing the warm side (inside).

 
SwingingHammer's Avatar
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08-17-13, 07:09 AM   #3  
Thank you! I thought the plastic seemed unnecessary...I'll go with the faced insulation on the interior. Wonder if the dual VB's were creating another issue with moisture?

 
Gary in WA's Avatar
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08-17-13, 06:02 PM   #4  
The plastic was stopping the warm exterior air, condensing on the cavity insulation. NO PLASTIC anywhere, unfaced batts are suggested for your location;Info-310: Vapor Control Layer Recommendations — Building Science Information No faced-insulation is required per code, either; Chapter 14 - Exterior Walls

Gary

 
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08-20-13, 10:32 PM   #5  
Great article - thank you! I've been considering using closed cell foam insulation as a base on the exterior, then topping with insulation. My reasoning = stop as much air flow/moisture from entering the walls altogether. Given this info, I'd put unfaced insulation on the inside. Working my way from inside to outside, I'd have drywall --> unfaced insulation --> ~1" closed cell foam insulation --> exterior sheathing. Thoughts?

 
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08-22-13, 12:15 PM   #6  
The sheathing may get wet because the foam only allows drying to the exterior. Better if f.b. is exterior to sheathing, stopping moisture RH there. Sheathing could`still dry to inside (if ever wet), if needed; http://www.buildingscience.com/docum...nd-wall-design

http://www.buildingscience.com/docum...ting-sheathing

Gary

 
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08-22-13, 11:03 PM   #7  
I was thinking spraying a closed cell foam vs using a foamboard. Wouldn't that seal the cavity (i.e. sheathing and studs)? Thinking a sealed cavity would decrease the chance of moisture ever entering. And perhaps let the wall dry from the inside if any did get in (from the inside).

 
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