I am finishing off my attic and have some questions about insulation options. My rafters are 2x8. I have a complex roof with valley rafters. I have read many articles about different methods for the insulation but want to try and use the materials I have already purchased. I have bought enough 1" xps rigid foam to put a couple of layers into the rafters as well as R-19 batts 1.5" x 1.5" wood furring strips and a radiant barrier material. For the parts of my roof line where I have a clear shot from my soffit vents to ridge vent I was planning on using 1" foam spacer under the roof sheathing followed by the radiant barrier for an insulation air baffle. Below the radiant barrier I would install 1" rigid foam and caulk it in. The next layer would be R-19 batt insulation with the paper towards the interior. I will then install the final 1" of rigid foam on top of the 1.5" x 1.5" furring strips. All seems will be taped on the rigid foam. My first question is will there be any issues with sandwiching batt insulation between the two vapor barriers (rigid foam)? Will having a vapor barrier on the outside and inside of the insulation structure cause condensation in the batts that will cause mold issues? My next question is what should my insulation structure in the rafter valleys look like? I have seen articles about drilling holes in the jack rafters to allow air circulation and would rather not do that. I have seen other articles about insulating without ventilation by spraying closed cell insulation. I would like to install the same insulation structure that I have on my other roof line minus the air space between the sheathing and rigid foam. Would this insulation method work? Will this cause issues with condensation and excessive heat to the shingles? Are there any other suggestions?
I don't think you will have too many issues with the two 'vapor barriers" as the paper facing of insulation is not a perfect seal.
Starting from the outside, I would install the radiant barrier first. Then a couple of tracks of rigid foam alongside each rafter to create an air space. Then several layers of rigid foam. Finally the R-19 faced. You will probably only need an inch or two of rigid foam - you have 7-1/4" of rafter space, take out 1" for ventilation and you have 6-1/4" left. R-19 is designed for 5' to 5-1/2" space, try to compress it into a smaller area and you just loose R-value. At the same time, the rigid foam has a much higher R-value per inch than the fiberglass so a little loss by compressing the foam could be worth it by the additional insulation of an extra layer of foam. Just don't compress the fiberglass much past the 5" mark.
Other options are to just insulate with the rigid foam layers or add some framing onto each rafter to give you more room for insulation. 1" of rigid foam will give you R-4 to R-5 (depending on foam cell size and density). 6" of rigid foam will give you about R-30 (the minimum needed for an attic space) so you may want to return the fiberglass and invest in some more foam.
I thought the radiant barrier had to be separated from the shearing to allow it the air space to reflect back the radiant heat. Other than that I think I might take your advice on the straight foam and not have to attach the 2x2 to the rafters. My only concern now is what to do for my closed rafter bays in the valleys where there no opening for airflow. Thank you for the advice.
I would drill upper and lower vent holes into those closed rafter bays. It doesn't take much, two or three 1" holes at the top and bottom will allow for some airflow.
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