Roof Membrane/Underlayment Recommendations
嗨，我们将把我们无人命的阁楼变成条件空间。它是1900年时代的农舍，位于马萨诸塞州西北马萨诸塞州，用董事会护套，超过真正的2×6椽子。12:12球场。Current plan is to remove existing slate (it’s in pretty rough shape) and put down peel and stick membrane as air barrier, then 1.5″ polyiso insulation between horizontal 2x4s, another layer of 2.5″ polyiso, then another layer of 3″ foil-faced insulation. (Total 7″ of rigid insulation outside sheathing.) This will be covered by a layer of plywood sheathing screwed through to the 2x4s, then underlayment and finally standing seam metal roof. We’re also planning to put batt insulation between rafters and cover with shiplap or bead board.
我正在寻找1）剥离和粘膜的建议，以覆盖板护套，2）为底层走在金属屋顶下面。我们需要在金属屋顶下高温下划线（计划使用普通的Galvalume，这应该是非常反光的）吗？界定是否应该渗透？对界定来说是更好的吗？
我计划在屋顶底部的玻璃板护套和天窗山谷底部运行3'，然后在底部落下底层。这是一个好主意吗？
任何建议都很欣赏。
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Replies
Not sure if 7" of rigid+~R22 of batts is worth it in your climate. The overall R value is impressive but the ROI is probably measured in centuries.
I would stick to only enough rigid insulation for condensation control, take a look at Table 1 here:
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
For the board roofing you want a peel and stick with either acrylic or butyl adhesive. The one you want to avoid is SBS ashpalt as it can react with sap and ooze.
The underlayment under the metal roof can be any of the many synthetic roofing underlayments. I prefer to use a permeable one (ie Deckarmour or BreathX) as it is minimal extra cost and does allow for a bit of drying of any trapped water. The underlayment does not need to be self adhered. A strip of high temperature peel and stick (ie Grace HT) is a good idea around the roof valley.
Unpainted galvalume does not hold up all that well. You are better off with one that is pained to look shiny if that is what you are looking for.
P.S. For your attic ceiling, you still need a decent warm side air barrier. The simplest is either OSB or drywall but can also be one of the variable perm membranes.
Thanks very much for your feedback. I've already got a lot of recycled insulation for this project and figured 7" of recycled rigid plus batts would get me well over R49. Why do you suggest a warm side air barrier? I was assuming any moisture that gets into batt insulation between rafters could dry to inside and 7" of rigid insulation above sheathing will prevent condensation on interior of sheathing. I'm planning to spray foam around perimeter where ceiling meets floor to air seal top plate and connect walls to air barrier on top of roof sheathing.
The warm side air barrier is still needed for the hybrid assembly I linked to earlier.
With 7" of exterior iso, you might be fine. If I remember correctly you need somewhere around 1.5x interior insulation R value on the exterior to skip interior barriers.
If your board sheathing is in good shape, with the amount of iso you are putting above the deck, you can also skip the interior fluffy. For interior finish you can paint/stain the boards and rafters and skip any wood paneling. I did this for my wife's studio (mostly to save time/cost) and it turned out pretty good. As a bonus the extra rafter space also makes the space feel bigger. You do have to take a bit more care with running wires as they are now visible. MC/AC/EMT pained tends to blend pretty well.