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Moisture issues installing SM, BATT, and Vapor Barrier?


chrissiemens4's Avatar
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08-29-12, 02:44 PM   #1  
Moisture issues installing SM, BATT, and Vapor Barrier?

Im re-insulating some of the exterior walls on the main floor. They are 2x4 walls, so i wanted to get the most insulating R value possible. I went with 1/2" SM with an R-value of 5, and R-13 Batt insulation. I have yet to put it in, and a friend told me that doing the SM, and then Batt, plus vapor barrier can cause moisture issues as there will be no airflow and condensation can occur. I have tried to research it, and it seems to be ok to do. Just wondering if he is right, or am i ok to proceed?

 
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chandler's Avatar
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08-29-12, 03:03 PM   #2  
The XPS or foam board is a vapor barrier and you don't want two inside a wall. In addition, a 2x4 wall will only hold R13. You cram it in the void and crush it due to the XPS in there already, you lose efficiency and R factor. Are your batts kraft faced or plain?

 
chrissiemens4's Avatar
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08-29-12, 03:11 PM   #3  
they are plain. the reason i chose to add them was cause the .5 inch that i will compress the batt, will be less of an R-value loss than what i am adding with the R5 rigid. but if it is bad to have it on the outside then i may not use it. im reading up about putting it on the inside of the wall. is that a better idea?

 
Bud9051's Avatar
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08-29-12, 05:19 PM   #4  
Very common to insulate and then add a layer of rigid over the entire inside. You reduce the thermal conduction through the studs while adding some extra r-value. They do make a high density 3.5" fiberglass batt at r-15, but I would also look at mineral wool. Being in Canada you should have easy access. Google Roxul. While the cavity is open, seal anything you can to reduce air leakage. Lots of detail work is good around electrical boxes.

Bud

 
GBR in WA's Avatar
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08-30-12, 08:32 PM   #5  
"but if it is bad to have it on the outside then i may not use it."---- better to have it on the outside keeping the wall cavity/sheathing warmer than the dew point, depends on the wall make-up.
You haven't described the rest of the wall, siding, sheathing, building paper?
OSB or plywood/solid boards?

Gary

 
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