Is this a good way to install Polyiso insulation on roof?
sgba
| Posted in Energy Efficiency and Durability on
We are installing a standing seam metal roof. Was thinking of getting some exterior polyiso insulation under the roof at the same time. This is the installation we are considering –
This is in Climate zone 3c. We have a vented attic with some batts of the pink stuff on the ceiling. Anything to be concerned about?
TIA
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If you have a vented attic then exterior insulation makes zero sense. The vent pathways will allow outside air to completely bypass the insulation, rendering it absolutely useless. It might make the roof a little quieter when it rains, but I can’t think of another benefit. Do you plan on converting the attic to an unvented conditioned attic one day in the future?
Also, as far as the stack up goes, I think I would have done the peel-and-stick membrane on top of the first OSB layer and the mechanically attached and taped on to the second layer to make it easier to access and remove when you replace the roof later.
Thanks Matthew. Good point. I do have plans to go to a conditioned attic in the future since all our HVAC ducts are in it and it gets super hot in the summer.
sgba,
I think Matthew has given you good advice. If you do go with roof top insulation for a future conditioned attic, make sure you put enough up there for your climate zone.
In zone 3 you only need R5 exterior rigid so not all that much thickness. I would reach out to your roofing manufacturer to see if they offer a clip system that can be installed directly over the rigid (clips sometimes need bearing plates to spread the load). This avoid the 2nd roof deck.
+1 on putting the peel and stick under the foam and using regular underlayment under the metal roof.
Thanks all for the recommendations. @Malcom - Plan was to put R30 on the exterior. IRC requires R38 for my zone (3C).
As for where the peel and stick goes, any downside (besides cost probably) in applying the peel and stick to both the first and second layer of OSB/CDX?
The one part I should not have left out, is that, about 1/3rd of our roofing area (~1000 sqft) is low/flat roof, currently with some kind of tar/gravel topping. Also have leaks there, prompting the reroof now. Planning to put standing seam metal there too after building it up to 1/12 with tapered foam. That section of the roof has no accessible attic. There are soffit vents on that side, but I see no obvious exit vents. Also the space between the ceiling and roof deck has pink batts, so I doubt even the soffit vents work, and would be least surprised if there is rot on that section. Assuming putting exterior insulation on that side will address the situation as described here - https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs.
Thanks again for any suggestions/ advice.
Several thoughts:
1) R30 exterior insulation is like ~6" of foam. Do you have a plan to drive the screws straight and hit the underlying rafters accurately? I would buy Rockler's portable drill guide and snap chalk lines on top of the foam where the center of the rafters are.
2) Putting a peel-and-stick layer on the outer sheathing is not necessary. It is not your primary WRB (the inner layer is) and it already has the impermeable foam underneath it which prevents it from drying in that direction. I would put a permeable mechanically-fastened underlayment on top of that layer and be done with it. This will allow for easier repairs if needed in the future. Also, some metal roofing manufacturers or installers are really picky about underlayments. Even if you did a peel-and-stick some of them will say they require a special underlayment on top of the peel-and-stick anyways. In either case, that underlayment under the metal roofing needs to be rated for high-temperatures in your climate whether peel-and-stick or mechanically attached.
3) 1/12 slope is low for a standing seam metal roof. Be sure to check what your roof panel manufacturer requires. Sheffield Metals requires 2/12 minimum slope, for example.
Matthew,
+1 on everything.
Thanks Matthew for your thoughts. Much appreciate it. I will check with the roofing contractor on all 3 points.
On #1, the plan was to use RMAX Nailable Base 3 (which combines polyiso foam with OSB). The manufacturer's installation instructions (https://metrointerior.com/wp-content/uploads/2017/08/Datasheet_Rmax_Nailable_Base-3.pdf) says this -
"Secure the Nailable Base-3 to the structural roof deck with Rmax Nail Board Fasteners or other FM listed roofing insulation screws or ring-shank nails
of a length at least one-inch longer than the composite thickness of the insulation panel. The fastening pattern for securing the Nailable Base-3 shall
be a minimum of 15 fasteners per 4’x8’ board, see detail below for spacing requirements."
So I guess it doesn't have to be nailed to the rafters, but to the bottom deck.
I am meeting the roofing contractor today, so will discuss the other points.
So met with the contractor. On #2, they are open to use peel and stick on the bottom deck too.
Interestingly, we went to the attic and found I currently have no ridge vents or gable vents! So all we have is soffit vents. + vents for bathroom exhaust fans. No wonder it runs super hot - 125F even on a 80F day. So now my choice is
A. Add ridge vents to the new roof (but that makes the polyiso on exterior less useful)
Or
B. Don't add ridge vents, plus close off the soffit vents and make it an unvented attic. But I have an “atmospherically vented" furnace in the attic. So not sure we can go to unvented without also replacing the furnace to a sealed combustion unit (as per (https://www.greenbuildingadvisor.com/article/sealed-combustion-appliances-and-hot-tub-parties). More questions/investigations! Open to any and all recommendations/ideas.