©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
I live in Raleigh NC Zone 4. I am finishing off my attic and have a few questions about insulating between the rafters. I have continuous soffit vent and ridge vents at the top. The rafters are 8" and I am planning on attaching 2x2 furring strips to the end of the rafters to get 8 3/4" of depth to work with. I unfortunately purchased my insulation products before I found out that insulating the roof line wrong will cause much trouble. I have purchased xps rigid foam and R-19 fiberglass batts. I only need to achieve a R30 at the roof since this is finishing off an existing space. I was planning on attaching two 1" spacer strips to the underside of the roof sheathing followed by a radiant foil barrier. Next I was going to put 2 layers of 1" xps foam on sealing all seams and against the rafters with cans of foam. This will give me 1" of space off the sheathing for airflow. After the xps I was planning on putting the R-19 batts in and stapling them to the outside of the 2x2 furring strips paper towards the interior space. Finally I was going to put in a plastic vapor barrier on top of the insulation and finish it with 5/8" drywall. Does anybody see a problem with this configuration? I have read many articles and have not seen anything configured like this. I was told by one person that the batts should go on before the xps so it could dry out if I had condensation problems. My problem with this is I would not have an air baffle under the sheathing and do no think I would get good airflow. Would adding a final layer of xps on top of the furring strips to reduce the thermal bridging help? If so would that also pose a condensation problem since that will sandwich the batts between the xps?
I am sorry this is so long but I also have one other challenge. I have two valley rafters in a small section of the roof line. I will not be able to ventilate this part of the roof at all so I am planning on only using layers of xps in there and making it as air tight as possible. Is it reasonable to think that I can use xps cut and cobble as they call it and get it air tight? If I do this should the xps be flush against the sheathing in this section or would it be smart to still leave a 1" air gap between the sheathing and the first piece of xps?
I am open to any suggestions except hiring out a foam truck to do the job for me. Thank you