Replies

1. 
   user-2310254 | Mar 13, 2022 12:21pm | #1

   Kevin,

   Closed cell foam is safer. See this article for details: https://www.greenbuildingadvisor.com/article/open-cell-spray-foam-and-damp-roof-sheathing.

   You might want to use the flash and batt method. (See https://www.greenbuildingadvisor.com/article/the-flash-and-batt-method and https://www.greenbuildingadvisor.com/article/flash-and-batt-insulation.) You could install R10 to R15 of closed cell followed by batts or open cell.

1. 
   kevinkeegan | Mar 13, 2022 04:35pm | #2

   Steve, thank you for your fast response. The roof has solid wood sheathing with asphalt shingles. the construction before the demo was fiberglass insulation installed between the ceiling joists and the space above insulation sealed with no access, no soffit vents and no ridge vent. It seems to have performed reasonable well. I see no signs of rot. One thing that does concern me is if I ever do have to remove the original roof sheathing and it has spray foam on the underside the spray foam will come out with it. The flash and batt idea is good but the rafter bays are irregular so it is a lot of custom fitting of the fiberglass. I was also wondering if you could recommend the type of mask / respirator I would need to be in the same space as the spray foam installer? I would like to make sure if I do need that first layer to be closed cell SPF that it actually is. Thank you very much, and I look forward to your further advice.
   Best, Kevin

1. 
   user-2310254 | Mar 14, 2022 09:39am | #4

   Kevin,

   I'm not an expert, but I think you should stay out of the structure while it is being sprayed. I'm not even sure the contractor (or its insurance) will allow you to be present while they are working. Your best option is to pick a reputable contractor and trust that it will do the work correctly.

2. Expert Member
   Michael Maines | Mar 14, 2022 09:11am | #3

   Kevin, no matter how much open-cell foam you install, it won't block water vapor movement. It's airtight at around 2".

3. GBA Editor
   Kiley Jacques | Mar 14, 2022 06:37pm | #5

   There’s some good information and further resources for you in this Q&A thread.