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

Efficiency+available products is causing unsafe build practice or unreal cost on cathderal ceiling

Ryan_SLC | Posted in General Questions on

Hi there.

I have a 1980 house with cathedral ceiling in main area. We added an addition with matching roof line to this area. Problem is, 2x6s was used as rafters, then 2x4s attached as furring. These 2×4 are not structural of course.

That’s pretty structurally sufficient.

Now get me R-38 in 8” with 1 inch air gap vent. I can’t figure it.

Cathedral fluff foam is R-38C is 10.5″ thick. Too much.
Rockwool or Thermafiber R15+ R23 is 3.5″+5.5″…too much.
R30 rockwool is 7.5″…too much.
R21 rockwool + R15 either fluff or rockwool is 2″ too much.

I challenge anyone to find Rockwool R23 or Thermafiber R-24 in single bags instead of pallets. A home owner just can’t do it without a 12 bag minimum.

Now try find R23 + R15 at above 16 OC. Can’t, and astronomical if going pallet.

I approached my city building inspector and ask if he’d fail my idea of iso R-6 Rmax rigid foam on the rafter face. He said no he wouldn’t fail the interior foam, though he’s never seen it this way, but couldn’t see how that wouldn’t be a vapor issue for me. I’d still have to cheat compression to have 1 inch for vent.

Short of expensive spray foam that is kinda questionable when you start digging into claims…my only option is to fur down the fur down either again or perpendicular.

I want R38. I do. From the bottom of my concerned planet soul. But lest we forget, my house was built with R13 and R21 throughout. It’s unlikely any house like mine will ever get a total insulation replacement.

So…now I am stuck in a situation where the solution is foam that is a significant percent of the total construction or two fir downs.

This isn’t adding up as good outcomes for people with homes. What am I missing?

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. Ryan_SLC | | #1

    Now I am also wondering the construction of furring down as the rafters are below the wall top plate.

    I believe drywall performs some structural performance, yes? Suggestions of furring down don't ever seem to mention integration at top plate is moved to the wall. Maybe because it's not a thing...but again, that's not normal build.

  2. walta100 | | #2

    Consider using one of the five strategies in this article.


  3. Expert Member
    Akos | | #3

    There is nothing wrong with interior polyiso bellow the fluffy insulation in a vented roof. I would size the fluffy to get you the code required vent gap and pick the thickness of polyiso to make up for the R value difference.

    Commercial drywall places sell drywall screws up to 5" long if you want to directly attach the ceiling drywall through the foam (part of my roof is built this way). You can also install 1x4 furring under the rigid and screw the drywall to that.

    Also check your local code as all have a U factor based compliance option. Since you have the continuous insulation bellow the rafters you get better performance. You can use an on-line calculator like this to check the assembly if it complies:

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |