Replies

1. 
   mfredericks | Jul 25, 2013 12:03pm | #1

   I think you first need to establish an air barrier before you apply insulation on the exterior of the roof. In the unvented roofs I've studied, the air barrier is most often at the sheathing plane. I expect removing the old shingles would provide you the chance to apply a peel and stick membrane or something similar to create this air barrier before the foam goes on. An advantage to a membrane over the roof sheathing would also be its ability to act as a back up waterproof layer covering the structure of the roof, if anything above ever leaks.

2. GBA Editor
   Martin Holladay | Jul 25, 2013 01:00pm | #2

   Mike,
   Here's another vote in favor of removing the existing shingles. That will allow you to inspect the existing roof sheathing to see if it has any rot.

   There is no need to vent a roof assembly with 8.5 inches of polyiso. But you will need to buy a lot of long screws.

3. 
   mykey | Jul 25, 2013 06:49pm | #3

   OK, thanks for the comments on the roof assembly.
   I understand the best way to assess the current condition of the roof sheathing would be to strip the shingles, and that would also allow for a membrane to be laid down cleanly as an air barrier.
   If tear off and disposal of the roof is not in the budget on this project is there another way to establish an air barrier on top of the shingles? Wouldn't the felt layer + asphalt layer act as an air barrier or is it too permeable?
   Thanks,

   Mike

4. GBA Editor
   Martin Holladay | Jul 26, 2013 05:59am | #4

   Mike,
   Asphalt felt plus asphalt shingles is not an air barrier.

   Polyiso can be an air barrier, if the seams are carefully sealed. That's easier to do with new foil-faced polyiso than with recycled polyiso; I would say that it really isn't possible with the recycled stuff.

   One option is to use one of the European air barrier membranes from Four Seven Five.

   You are planning to install a standing-seam metal roof, which is one of the most expensive roofing options. That's great. But I think it is false economy to skip a simple step (stripping one layer of asphalt shingles) when you are investing in a quality roof system.

5. 
   mykey | Jul 26, 2013 01:47pm | #5

   I guess I am not understanding what an air barrier is exactly and how it performs in a roof assembly such as the one I want to build. I have been reading through this site; other people's questions about air barriers and all the links and good info there is on and connected with this site.

   Martin, here is a quote in your Musings of an Energy Nerd - Q & A About Air Barriers: "To make a good air barrier, a material not only needs to stop air flow; it also needs to be relatively rigid and durable. If you want to determine whether a material is an air barrier, hold a piece of the material up to your mouth and blow. If you can blow air through it, it’s not an air barrier."

   I guess this was one of the reasons I was thinking the asphalt shingles + felt would be an air barrier in my roof assembly.

   Here is another one from an answer to someone else's air barrier question:
   "Bill, If you are insulating with rigid polyiso, it doesn't really matter whether your air barrier is on the interior side of the insulation or the exterior side of the insulation. There is a slight advantage to installing an air barrier on the interior side: to prevent convective air currents in the (tiny) gaps between the polyiso sheets. However, since you are planning to install two layers of polyiso with staggered seams and taped joints, that issue shouldn't arise.
   Here are some articles to read:
   Questions and Answers About Air Barriers
   Airtight Wall and Roof Sheathing
   ANSWERED BY MARTIN HOLLADAY, GBA ADVISOR
   Posted Thu, 05/23/2013 - 12:34"

   That last quote leads me to believe that I could potentially lay down my recycled polyiso directly on the shingles and then wrap it with an air barrier paying attention to the perimeter, before the plywood goes on.

   So I'm still confused about the best way to go about doing this assembly and what the best material is for the air barrier. I am trying to do this renovation in a green manner and money IS a factor. Keeping as much waste out of the landfill is important to me but I don't want to jeopardize my own project in the process, hence all the questions. Yes a standing seam roof is expensive and should last a good long time, but if I have to put more money into other aspects of this project the standing seam roof seems less attractive.

   Thanks again for all the help and insight into a project that is perplexing me at the moment,

   Mike

6. GBA Editor
   Martin Holladay | Jul 26, 2013 01:59pm | #6

   Mike,
   You're right: you can't blow air through an asphalt shingle (at least when it is new). The asphalt shingle isn't the problem. The problem is all the seams between the shingles.

   If you could come up with a high-tech tape that could be used on an asphalt shingle -- good luck with that -- and you were willing to patiently tape together hundreds of asphalt shingles, then the whole assembly might be an air barrier. Clearly, that's not going to happen.

   If you want, you can create an air barrier at the ceiling plane; at the roof sheathing plane; or above your polyisocyanurate insulation. In my opinion, it doesn't matter very much where you put your air barrier, as long as the air barrier is adjacent to the insulation layer, and as long as it is continuous and uninterrupted.

7. 
   wjrobinson | Jul 26, 2013 03:34pm | #7

   Foam board roofs fail from interior moisture around this neck of the woods. Often.

   Solution. Vent above the foam.

   I have nothing further to debate on this subject at this time.

   Been there and have removed such failings.

   ;)