Roof Insulation Plan…help?
Hi everyone! I’ve come across this forum numerous times while researching insulation techniques and figured I would be wise to submit my roof design for critique before moving forward with it in the coming weeks.
The structure is a timber frame located in southern Vermont, 12/12 pitch roof.
Layers as follows:
1 – 1×10 shiplap roof boards
2 – epilay synthetic roof underlayment
3a – 2×6 “rafters” 2ft apart to create bays for foam board insulation
4b – 5.5″ of foil faced polyiso consisting of 2 layers of 2″ and one layer of 1.5″- taped seams
5a – 2×4 “stringers” layed on the 3.5″ face, running perpendicular to the 2×6’s, 2ft apart
5b – 1.5″ foil faced polyiso within the 2×4 grid
6 – zip 22/32 roof sheathing
Roofers will do the rest:
Another layer of epilay (?) + standing seam metal
My concern is an air gap.
The 2×4 layer seems like the logical place for it, but then we lose 1.5″ of insulation.
Any thoughts or suggestions are GREATLY APPRECIATED as we are moving forward with this build this weekend (9/16-17/2023).
Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I'm assuming you're not just laying foam in sheets to avoid using really long screws?
That would be the 'easiest' from a design standpoint: lay the foam in continuous sheets, run long screw through vertical strapping to the timbers. Using the U-factor approach, you may not need as much foam as you think even in VT.
If sticking with the rafter bays, have you considered using fluffy insulation there instead of cobbling foam? Fluffy stuff is meant for bays, rigid board is not (more work cutting all that foam. No fun and for what gain?).
You didn't mention if the timber frame had common rafters or purlins, but could your first layer of 2x6 (for the bays) be run horizontal so that you could then strap vertical? If you want the venting. Although hitting these with the screws for the strapping through the foam above could be a bit trickier.
Has the frame been designed with proper deadload in mind?
Not sure what I was thinking re rafter/strapping orientation. They can both run vertical... no?
Hey Tyler, thanks for the reply.
The frame has 8x10 rafters with 5x7 purlins so running the vertical 2x6's with horizontal 2x4's to screw the roof sheathing into made the most sense to me structurally.
I considered running the 2x6's 4ft apart to minimize thermal bridging but was afraid that it might be giving up too much structurally, as the 2x6's will be the roof's primary point of attachment to the frame. Each set of 2x6 rafters will also be attached to each other over the peak of the roof via a gusset.
Would I be insane to just add another 1x4 layer under the zip roof sheathing to create the gap we need?
For better or worse, I've designed this frame myself with the help of a friend who is a professional timber framer...so we don't have an engineer's 2c in the mix...just common sense and good ol fashioned over-building ha
Thanks again for your input!
Just to be clear, you did consider attaching foam with long screws and decided they would be too long? That's the typical way to handle rigid foam over roof.
Even with the 2x6 over rafters, I'm not sure why the second layer of strapping needs to run horizontal if you're putting sheathing on. You could just run it vertical above the foam and get your vent space.
A completely different approach would be deep I joists that accepts deep fluffy insulation and vent it. No fussing with cutting foam or screwing through strapping/foam into buried rafters. Would be a thicker roof though.