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Discussion Starter #1
To get more R value in my attic conversion project I am adding 1" foam board after putting R-21 into my 2X6 rafters. I am using unfaced batts and was planning on using foam board with foil backing. Question, most foam boards come with backing on both sides. Would this constitute a double moisture barrier? I didn't think so, but want to make sure before putting up. Sheetrock will be put up after foam board.

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No, do not add rigid insulation on top of fiberglass, it does add a second moisture barrier. You could install it next to the drywall between the studs, then add fiberglass on top.
 

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Mold!! Let's kill it!
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I hope that you are not insulating with R-21 fiberglass between 2x6 rafters. If you are insulating tight to the bottom of the roof sheathing you will have moisture issues. You need to have a 1-1/2" airspace between the top of the insulation and the bottom of the roof deck, with ventilation top and bottom. Rigid insulation wouldn't be my choice. It won't provide a solid surface to appy a finish over like drywall.
 

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I hope that you are not insulating with R-21 fiberglass between 2x6 rafters.
I missed that the first time I read the thread. I'm hoping he just used the wrong term and isn't really insulating his rafters tight to the roof sheeting.
 

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To get more R value in my attic conversion project I am adding 1" foam board after putting R-21 into my 2X6 rafters. I am using unfaced batts and was planning on using foam board with foil backing. Question, most foam boards come with backing on both sides. Would this constitute a double moisture barrier? I didn't think so, but want to make sure before putting up. Sheetrock will be put up after foam board.

Thanks,
I'm not seeing the foil faced board creating a double moisture retarder in that application. If that would be the case then where would you be able to use the stuff? Now, if the fiberglass had a VR and you installed the rigid, that would constitute a double VR.
 

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Discussion Starter #7 (Edited)
Thanks for the feedback.
I have already installed air channels on the bottom of the roof sheathing and then plan on using unfaced R-21 before putting on the foil faced foam board. The sheetrock will fasten to the 2X6 rafters with 3" screws so I don't understand why you think there will not be a solid surface.
I know I will have to compress the R-21 (5 1/2 in) just a bit not to affect the air channel.
 

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Mold!! Let's kill it!
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Foil has a vapor permiability of almost zero. It's the most efficient vapor retarder you can get. As long as it's on the warm side of the assembly, no moisture can get into the rest of the insulation. I've seen this done without any problems. I wouldn't do it on a wall, but a ceiling may be OK. The problem with using foam on the inside of the walls, is that if anyone leans against the wall, the foam will compress and the screw heads will pop through.
 
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