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Hello! To start, my main questions are:
-Can I install standing seam metal roofing directly over 2" or 3" semi-rigid mineral wool panels (e.g., Rockwool ComfortBoard 80) installed over the roof sheeting? The roof in question varies in pitch from 7/12 on the steeps to 4/12 on the dormers. I understand the fasteners would have to be long enough to penetrate the insulation into the sheeting.
-Would an installation like this support light (maintenance only) foot traffic? Or would the insulation not be supportive enough to keep the roof from being damaged by even a few footsteps?
-Are there better recommended products (specific insulation, roofing, etc.) for this scheme?
-Should I put another semi-permeable membrane (e.g., Tyvek Protec) on the existing plywood roof sheeting, under the new mineral wool panels, with a water flow path into the gutter, in case water ever penetrates the metal roof and trickles down through the mineral wool fibers? Or is this step unnecessary overkill if the metal roof is installed correctly?
I have a tricky project in a 100+ yo house in Seattle. It's a tall (3+ story) house with a complicated roofline, one rental unit on each floor, with the top floor unit up within the roof (angled ceilings + small attic). Angled roofline ceilings are currently not insulated, finished with old lath & plaster, framed with 2x6 rafters. The old 3-tab roof now needs to be replaced. Currently there is not eave or ridge venting. Previous 3-tab roofing job installed 1/2" plywood over pre-existing skip sheeting which had been there from the original shake roof.
So, I want to insulate without venting in conjunction with the roof replacement, and I want a metal roof. On the interior, I'm resigned to removing lath & plaster (ugh). I'm thinking of using a vapor-variable membrane (Intello Plus) on the interior, and blowing in sheep's wool in the rafter bays, then putting up new sheetrock. That brings it to about R-23. With 3" additional mineral wool insulation on the outside, I get close to the required R-38 for the Puget Sound lowlands.
Thanks for your questions. To a large degree, I have to refer you to your roofing manufacturer and also to Rockwool. What you are doing has been done before and I'd say it's not real concerned but ultimately your roof manufacturer needs to be in agreement. Additionally, I believe that RockWool is pretty concerned about moisture and you're taking some steps to keep it dry from moisture driving out of the living space. However, they also may not want the roofing placed directly on top of it (or underlayment) but they may want it on spaced boards. You will need to confirm with them. You also may ask them about whether a breathable roofing underlayment would be appropriate.
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