Roof-Siding Details on Restoration Project
Dear GBA Community, I am working on a roof/siding detail (see attached) for a restoration of an old 1840s 2-story building on the east end of Long Island, NY (zone 4A – though very close to 5A). My questions are as follows: 1) Do we need an air barrier above the pre-existing roof sheathing (assuming it does not need to be replaced after initial demo)? We will be spray foaming the rafter bays with 3″ thickness. 2) Do we need a roof underlayment atop the exterior rigid insulation as long as we have staggered joints and taped seams? If so, do we need to be concerned with permeability and does it need a layer of sheathing between the underlayment and insulation? (the need for plywood seems only mandatory if we decide to go with cedar breather vs furring strips as it would act as a continuous nailer- see question #4). 3) Which type of rigid insulation does the GBA Community recommend in this scenario? Polyiso vs XPS vs EPS Type II (is foil-faced necessary?) 4) We are using Cedar Shingles, and are attempting to do the installation method which prioritizes longevity (aka drying). I am more trusting of a lath/furring strip (wooden grid) install than I am of the Cedar Breather product, but I have no data to back that up. Does the GBA Community have a stance on this? As it pertains to question #2, this might affect the necessity of having a layer of sheathing atop the exterior insulation. 5) Feel free to mention any comments/concerns about other aspects of the attached details. Thank you so much in advance for all your help. I am a huge fan of the site and all the great info provided by Admin and the Community! -Isaac
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Replies
3) Polyiso. Definitely NOT XPS, even the new HFO-based formulations are far worse than polyiso and EPS. EPS is fine but polyiso has a higher R-value. Might be able to find salvaged roofing polyiso from commercial flat roofs that use them. You can tape the seam of the foil faced polyiso as a secondary water barrier but the foil facing is not necessary, fiberglass facing is another option that has some perm rating.
Thank you @matthew25! Does this mean that we do not need to put sheathing and underlayment/water barrier on top of the taped seams poly iso? That means we would have polyiso (foil faced with taped seams) then lathe, then cedar shingles, as opposed to rigid foam, then sheathing, underlayment, lathe, then cedar shingles.
I think you have existing roof sheathing? From inside to outside I would do
1. existing sheathing
2. WRB
3. Polyiso (taped seams is optional but won’t hurt)
4. Furring strips
5. Cedar shingles
Technically the polyiso alone can count as your WRB but they will expand and contact a little so unless you have two layers of foam I would personally feel more comfortable with a WRB on top of the sheathing.
Great - yes that is exactly what we are hoping to do -- Thank you very much @matthew25
4) I also don't have any data on this, but I've never liked working with Cedar Breather. I felt like it's always a bit too squishy, which made the install a pain. Also seemed like it was easier to accidentally split shingles when nailing/stapling through Cedar Breather. Might be a bit more work, but I'd much rather go over wood furring strips.
Thank you @Paulmagnuscalabro, we agree about Cedar Breather. Our chief concern with the lathe was how it thickened the roof construction (3" of rigid foam doesn't help) of a historic old building that we are trying to retain the original character of. I know for ventilated siding that 3/8" plywood lath (two perpendicular layer totaling to 3/4") is sufficient, but does the same ring true for ventilated roofs?
If you want a thinner roof line, you can cut the old rafter tails off plumb and place dummy tails in the foam layer. Reference this video around 1:20
https://www.youtube.com/watch?v=6e1Swd2_Tvs
3/8" may be ok for the layer running along the rafter, but I would want to nail the shingles into 3/4 material or thicker.