Insulated roof design options – thermal bridging & EPS foam creep
Working further through my insulated roof design after getting some help in an earlier thread. I’m now debating two possible designs as I try to come to terms with the madness of analysis paralysis.
Context on the project
I’m working on figuring out some of the remaining details of my design for an insulated roof assembly retrofit to an old cabin. The home is in Zone 5a and currently has no insulation at all in the roof so there is a condensation issue on the underside of the roof decking. I’m looking to address that plus improve my thermal performance by adding roof insulation. The roof is a gambrel with one side having a porch overhang (the drawing included shows this side).
My current assembly is as follows
Interior (from inside to outside)
-Pine T&G Paneling
-Intello smart air barrier over rafters/Rockwool
-2×4 Rafters on 16″ filled with R15 Rockwool
Exterior (from inside to outside)
-1×12 original roof decking boards
-Membrane (475 Supply Adhero 3000)
-EPS Foamboard (25PSI) 1.5″ + 4″ Staggered and taped seams
-19/32 Plywood
-Synthetic roof underlayment
-Standing Seam Metal Roof
Roof Design #1
My original design had been to build a 2×6 frame on the roof deck over the membrane layer and then fill this in with two layers of EPS foam. The thinking was that the 2×6 frame would give a solid base for the plywood sheathing over the foam as I was worried about the EPS creep over time causing the plywood to sag as the EPS shrunk. The downside to this design is that it creates a thermal bridge that goes from the interior rafter side all the way to the sheathing, so now I’m wondering if these thermal bridges are going to cause issues (both inside and out).
Roof Design #2
This version replaces the 2×6 interior framing for 2×4 laying on their sides over the rafters. This lets my first layer of 1.5″ foam sit flush with this framing, then I’ll lay over that lower layer with my upper 4″ foam layer. This layer would totally support the plywood sheathing that would be screwed through to the rafters and 2×4 below. This fixes my thermal bridging issue, but I’m back to worrying about the foam shrinking over time and that causing the sheathing to not be flat which will then show up in the metal roof.
Questions
1. Which design is preferable?
2. Is EPS shrinkage and creep enough of a concern to where it might cause the sheathing to collapse enough to make the roof flatness wonky and distort the metal roof in design #2?
3. Is the thermal bridging of design #1 enough of a concern to cause issues both on the roof with melt and on the inside with condensation at the rafter attachment?
Thanks!
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Replies
Metal or asphalt roof?
I don't like either of your designs. Given your preferred foam depths:
NO framing at the original roof deck. Reverse layers: 4 inch first, followed by 1.5 inch. Embed 2x4 strapping at 24 inch centers into the 1.5 inch foam. This'll slightly reduce the performance, but, allow for simple fastening of the new roof deck. Counter sink structural screws through the 2x strapping into the rafters.
You'll need fastening for cornice trim- secure a 2x6 on edge around the perimeter of the roof.
Edit: I see standing seam. Excellent! I have used this tactic successfully on several metal roofs.
Thanks climbing_carpenter!
I threw together a quick version of what I thought you were describing.
So in this version I'm countersinking the 2x4 strapping and attaching through to the rafters, then I attach my plywood sheathing to the 2x4 strapping with much shorter screws (2").
This does reduce the amount of insulation vs. strapping in the 1.5" layer, but I guess maybe that is offset overall by having a much cleaner 4" thermal break between all the strapping and the rafters, so the overall performance is still better.
Question- What about eliminating the middle strapping all together and only strapping at the edges of the plywood sheets? All the edge strapping would follow the same method as described above, but the middle strapping would be eliminated and I'd just screw all the way through the whole assembly into the original decking and rafters with something like a 6-8" screw? This would save a lot of cuts on the 1.5" foam layer and replace all that strapping with more foam for more insulation vs. wood. The downside would be of course having to try and guide long screws through the whole assembly and hit my target.
Oh, and one other question, the strapping method shown in the illustration would mean I don't have any solid framing between the outer sheathing and inner sheathing, so the foam is supporting the whole thing (except at the 2x6 perimeter). This brings me back to that question of EPS foam creep over time and if I'd lose height due to shrinkage and that would be enough to make the sheathing sag/metal roof look bad?
Thanks!