GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Cathedral ceiling insulation question

JPanzer | Posted in General Questions on

I’m in the process of replacing the 100+ year old slate on my home with EcoStar synthetic slate. The original slate was installed on traditional skip sheathing – true 1”x5” boards with a 1” gap between. Somewhere along the way, batt insulation was installed, along with a plastic vapor barrier and drywall in part of the attic. The roof is 12/12 pitch.  Not a good situation but at least the system could breathe outward. The new installation required a full plywood overlay, which would eliminate this ability, so I decided to create ventilation channels over the skip sheathing by placing 2x4s flat on top of the old sheathing, directly over the true 2”x5” rafters. ¾” plywood was laid over those, with appropriate underlayment on which the EcoStar will be installed.  Not that it’s particularly relevant (just a source of pride) but I’m also having the long-gone stop gutters rebuilt as part of this project.  Here’s the problem/question – how best to insulate this thing, as clearly the 5” of deteriorated fiberglass will not suffice and is probably unhealthy in its current state.  Unfortunately, insulating over the roof was not an option, owing to the constraints of living in an historic district. (Just getting the 2-1/4” added thickness was a heavy lift!) Ideally, I’d use high-density foam, but the spaces in the skip sheathing would allow the foam to block the ventilation channel and blocking the spaces would be a fool’s errand. So far, cut and cobble (a 2” layer and a 3” layer of foam) to fully fill the cavity seems the best option.  An airtight R25 would be a lot better than what’s there, but I’m still wondering if this is the best solution. Lastly, this area is an attic, that is mostly used for storage, but it does need to be semi-conditioned.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Peter Yost | | #1

    Hi James -

    Sounds as though you just need a sheet good stop of some sort to receive/be the substrate for the spray foam?


  2. Expert Member
    Akos | | #2

    You can staple house wrap along the bottom of the skip sheathing and spray foam over it. Since the roof is vented, you can use open cell foam.

    Cut and cobble is generally a huge waste of time only to end up with a roof that is marginally better than high density batts.

    1. JPanzer | | #3

      Thanks. I had considered this but was wondering if the foam would stick to the house wrap.

      1. Expert Member
        Dana Dorsett | | #4

        Spray foam is very similar to Gorilla Glue- it sticks to just about anything. Even if it doesn't adhere to the housewrap in an a very aggressive a way, once it's in place it will stay there- it's fairly rigid. The housewrap serves to stop the droplets before they expand, flowing/expanding into adjacent expanding droplets to form a solid block perfectly fitting the enclosing space.

      2. Expert Member
        Akos | | #5

        Spray foam stick pretty well to the backside of Tyvek. It sticks, but not well to the front.

        About the only thing spray foam doesn't stick to is 6 mil poly.

        I've had no issues with it in the walls of a garage. To make sure, it would be good to check with the spray foamer before starting.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |