Insulating porch ceiling
Good afternoon!
我有一个问题，我似乎无法弄清楚一个明确的答案。我在芝加哥有一个1890年代的砖房，在某些时候加入了一个封闭的门廊，（可能100年前）。它不是一个大门廊，也许在18英尺深度深处。门廊总共是3个故事，走出了我们公寓的类似尺寸的第二级等级的下部，以及漫步阁楼的截面。我想完成Porch的两个顶级水平，为我的成长家族添加额外的生活空间。我对与阁楼天花板有关的困惑。屋顶被前任所有者替换为4-5年前，良好的形状。它在护套上有沥青带状疱疹，这是铺设在原始的旧屋顶板上。整个屋顶有4英寸的椽子状况良好。屋顶在房子的末端结束，没有拱门或悬垂，屋顶不打开。 My original idea was to Cut and cobble 4” rigid foam in between the rafters since they are pretty straight. Foam and tape that, then take large uncut sheets of 2-3” rigid foam and go over everything horizontally. Tape that, then hang drywall over the foam. I also thought about adding a soffit box down the middle of the ceiling to add lights, and by doing so would be able to add insulation above it. I’m worried of course that I will damage my roof, or somehow compromise the porch. Is this a bad way of going about this? I apologize if I am not using the correct terms for everything. I’m relatively new to home ownership and looking to learn as much as I can.
thanks!
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Replies
Hi Cpros55.
从房屋内部构建的瓦文屋顶装配通常需要直接在护套以下的空中渗透绝缘层。然而，通常不推荐用于护套以下的刚性泡沫和开孔喷雾泡沫。常见的选择是足够的刚性泡沫层，通常是透气绝缘。当然，材料选择取决于您的r价目标。我建议你读到这一点：How to Build an Insulated Cathedral Ceiling.
Thanks Brian,
I'm a bit confused by your answer. "rigid foam and open cell sf are generally not recommended" "the common method is an adequate layer of rigid foam and an air permeable insulation..." Is rigid foam considered air permeable or air impermeable in this application? why is it not recommended to use rigid foam in your first scenario, but it is the common method in the second scenario? Sorry I just want to make sure I am understanding.
Also thanks for the article, I think the only real option I could glean from it was I was going to have to extend the width of my rafters significantly to then add several inches of closed cell spray foam, both of which sound a bit more expensive than I probably can afford
-克里斯
I'm an amateur who spends way too much time in these parts, so take it for what it's worth...
I'm pretty sure Brian made a mistake there. What I believe he meant is that rigid foam is not generally recommended in that application; the common choice is an adequate layer of *closed cell spray foam* followed by air-permeable. That's one of the approaches in the article he linked to. The reason for not using rigid foam (foam boards) in that approach is because cut-and-cobbling is risky for air leakage past the boards, which would create a rot risk at the roof sheathing. That's discussed here:
//m.etiketa4.com/article/cut-and-cobble-insulation
您可以在文章中看到Brian引用的，在此应用中使用刚性泡沫，Martin提到“可以说”。即，它是可能的，可能是代码允许的，但可能不是一个好主意。
As far as your additional layer of continuous foam below the rafters, that prevents drying to the inside. So now you've got 2 risky situations.
就我个人而言,我可能会冒这个险cut-and-cobbling the foam against the sheathing, and be very careful about doing it. Then, I'd do the air-permeable below that (eliminating one risk). Making the rafter bays deeper to accomodate isn't that costly. This can be done with cheap furring strips run perpendicular to the rafters. Stack a few 2x3's and you're good.
I believe you're Zone 5, so you're supposed to get R-20 from the foam, then the rest of your required R-49 from the air permeable (I'll just say batts, but doesn't have to be). I think you can pull that off with Polyiso as the foam in just the existing space, then the batts would go in the furred-out space. But obviously pretty thick batts. Check out this article which discusses flash-and-batt, and maintaining the minimum ratio of foam to batts.
//m.etiketa4.com/article/combining-exterior-rigid-foam-with-fluffy -sulsulation.
But course if you're not so worried about code, you could get away with less (e.g., EPS and thinner batts). Additionally, I'm not really sure why the thickness of rigid foam is important using that approach. With foam on the outside, it has to be enough to keep the sheathing warm. With everything inside, it seems like as long as you form an adequate air control layer, it's done it's job. Perhaps just a Code oversight. Maybe an expert can weigh in on that.