Replies

1. 
   schrickker | Feb 17, 2015 07:57am | #1

   Low slope insulation

   I can't advise you as to the best combination of materials but we did something similar in the vaulted ceilings of our house. In our case there is no attic and the roof is framed with 2 x 8s. We took out all the ceilings and glued 4 inches of Celfort XPS to the underside of the roof in each rafter bay, then used many tanks of two part closed cell spray foam to fill the gaps between the XPS and the rafters. I also sprayed about two inches of closed cell foam over the XPS and between the rafter bays. There is no ventilation or poly vapor barrier.

   That was done about five years ago. In early November 2014 we took out a 6' x 35' section of roof and insulation to tie rafters for a kitchen addition to the existing roof and found no evidence of moisture or dark streaks from air movement. One of the objectives with a hot roof is to stop inside or outside air and moisture from moving into the area, and we seemed to have been successful. I also contacted a well-known building technology consulting firm about building envelope details for the kitchen addition, which included discussing insulation and vapor barriers in the existing house. They suggested that a thin layer of two part closed-cell foam on the underside of the roof, then a layer of XPS, filling the remainder of the cavity with rock wool, and using a smart vapor barrier would provide good insulation value, and the rock wool would reduce external noise better than just XPS. In our case, using the XPS is a way of achieving the required R value in the 7 - 8 inch depth of the rafter bays. If you have a greater space you may be able to use rock wool only as long as all the air leakage from outside is sealed off.

   A few things I learned: I've used both a table saw and a hot-knife for cutting XPS. A table saw gives a cleaner, more even cut but produces a lot of foam particles. If you are doing the cutting in an enclosed area be sure to hook a vacuum with a container bag to the table saw, otherwise the vacuum filter will plug up quickly. Closed cell foam is denser than canned foam and seems to do a better job of sealing and insulating (purely subjective opinion), but it comes out a greater pressure than canned foam and takes a while to learn the best technique for dispensing. If you use canned foam to seal around the edges of XPS, dispense it with a professional gun because it is difficult to use the consumer canned foam with straws. I found that I needed a short barrel and long barrel gun to get into all areas of the roof. It's very difficult to tight fit XPS between rafters especially if you cut it with a hot knife (uneven sides), or the rafters are twisted. So leave a gap of about half an inch or more between the XPS and rafters so that you can get the gun in the gap and fill it, otherwise it will be difficult to do a good job of sealing around the XPS.

   If you skip the rigid foam and use only XPS, you should ensure that the existing ISO is preventing air leakage otherwise you should flash the underside of the roof deck with a layer of spray foam. You would need a two-part system for this task because it would be tedious to cover the underside of the roof deck with bead after bead of canned foam.

   Finally, when you apply drywall, use some of the airtight drywall techniques that have been suggested in several Fine Homebuilding articles and videos.

1. 
   hobbitrock | Feb 18, 2015 09:52am | #2

   Thanks so much for the detailed response. I was looking for something I could do relatively easily and this sounds like it. I wasn't sure about what type of rigid foam to use. I was picturing the foil faced stuff but the XPS you're describing is more like the 2 inch thick pink  kind they have at the box stores. Cool. I've also cut that on a table saw. Definitely the way to go. And then filling the remainder of the bay with rock wool sounds like exactly what I had in mind.

   I'm a little confused on the use of a vapor barrier. Never used the smart vapor barriers before. That would probably be a special order item from the lumber yard. Is this a critical step in this sort of assembly? I feel like I remember an article about a warm roof being able to "dry to the inside" and therefore having no vapor barrier. Does that sound right? Thanks again for your help.

1. 
   schrickker | Feb 19, 2015 09:52am | #3

   Thanks So Much

   Hi. With respect to your vapor barrier question, my understanding from engineering reports and depending on your geographic location, 50 - 75 mm (2 - 3 inches) of continuous XPS acts as a vapor retarder. However, in the case where the XPS is going to be against the roof deck and then rock wool towards the drywall side, the vapor retarder (depending on how thick the XPS is), would be inside the insulation structure rather than on the warm side of the building (I'm assuming you are in a colder area of the country). In my area building inspectors insist on a vapor retarder such as plastic sheeting even if the insulation is 6 inches of closed cell foam, which is why the engineer suggested a smart vapor retarder (Google 'smart vapor retarders').