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Insulating an attic ceiling

David Jastrow | Posted in General Questions on

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 2×2 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 2×2 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

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "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?"

    A. Yes, there is a problem. In your climate zone (Zone 4 -- Raleigh, North Carolina), you definitely don't want to have interior polyethylene. To learn why, read this article: Vapor Retarders and Vapor Barriers.

    There is nothing wrong with using rigid foam to define the ventilation gap that you want to create between the insulation and the roof sheathing. Here's some feedback, though: A deeper gap -- 1.5 inch or 2 inches -- would be better than 1 inch.

    Moreover, if your roof has valleys, it's not a great candidate for a ventilated roof. You can do it, but it won't ventilate very well, and it would be better to design an unventilated roof assembly for your type of roof.

    For more information on all of these details, see How to Build an Insulated Cathedral Ceiling.

  2. David Jastrow | | #2

    Thank you for your responce Martin. I am glad I asked rather than installing the vapor barrier on the interior wall. So would the rigid foam installed across the interior side of the rafters also cause vapor issues as well since I will have fiberglass sandwiched inbetween the top layer of foam and the bottom layer? I was hoping to use foam to act as a thermal barrier between the rafters and the drywall. If I can use foam across the rafters is there a prefered type of rigid foam that would be best or is xps the best choice?

  3. GBA Editor
    Martin Holladay | | #3

    I don't think that using rigid foam on both sides of your batts will create any moisture problems, as long as the rafters are dry when the work is performed. Pay attention to air sealing when installing the lower layer of rigid foam.

    Most builders consider polyisocyanurate to be the most environmentally benign type of rigid foam, although XPS will also work. For more information on the environmental characteristics of different types of rigid foam, see How to Install Rigid Foam Sheathing.

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