2:12 Roof – Rigid Foam On Sheathing Top
Hey everyone!
Our builder is having our black tin roof removed (low slope), adding rigid foam to the top side of the sheathing and then put the metal roof over top and screw down through the tin to the roof deck. We plan to rockwool the interior. I found an amazing deal on used good condition ISO and XPS R10 Rigid Insulation. We are in Okanagan, British Columbia. Do you see an issue with doing this used R10 XPS (tapped seams) then black metal roofing on top, with interior insulation? The Facebook marketplace ad doesnt list roof sheathing application – just insulating foundation walls and under foundation slabs but assuming this is a product that can be used for roof insulation. I want to try to buy used material for cost savings and keep the planet safe.
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Replies
Tlynn,
I see that your region has winerys and skiing so I am guessing that the R10 above your sheathing will be inadequate to protect the sheathing from condensation on the underside. Attempting to face screw metal roof to the sheathing with three inch screws is not a good idea either. At minimum you need sleepers to screw the metal roof into. Better if you have a full new sheathing and high temp water barrier like Grace High Temp Water Shield.
If you have both Polyisocyanurate and XPS recycle foam great. I used reclaimed XPS so the karma hit went to someone else first. XPS is fine below ground on foundations and under slabs. Polyiso not so much. It can hold water, which also makes me caution you about the reclaimed Polyiso. If the roof it came off of was pooling water and leaking the sheets that got wet are of no use to you. Find out what a dry new panel weighs and proceed cautiously.
You are probably in zone 5 there (3000-4000 HDD Celsius). In that case, you want 40% of your roof overall R value as exterior insulation. If you are elsewhere use the roof ratios from here:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
If you are trying to hit R31 overall, that means R12 above deck and R19 bellow. If you add more insulation bellow, you need to bump up the R value above the roof deck as well.
Any type of rigid will work. Reclaimed is not an issue as long, as Onslow pointed out, it is not saturated. XPS, Polysio, EPS, no problem.
You want the air barrier to be your existing roof deck. I would that as the main air barrier and tape the roof sheathing seams there. If there are a lot of holes, a layer of peel and stick over the whole roof might not be a bad idea either.
The rigid can be installed and held in place with cap nails/screws. No need to tape any seams on it. Two staggered layers is the better option if you are doing thicker install.
Some standing seam roof panels with clips can be directly installed over rigid but most need a solid substrate. This can be another roof deck or horizontal 1x4. You'll also need a synthetic underlayment here (either over the new roof deck or under the 1x4 strapping). This doesn't have to be anything fancy, anything rated for high temp will work. No need for peel and stick except where your local code requires it.
Make sure to picture frame the edges of the rigid with 2x lumber ripped to the thickness of the foam to keep critters out.