Maintaining headroom with 2×4 rafters
Hi all. In the process of purchasing a 1905 home in KY (zone 4) with an attic we plan on finishing into a master suite. Currently, the attic floor joists are 2×6 (true 6″) with wood lath and plaster ceiling below, blown-in cellulose insulation filling the floor cavities, and rough 1x plank flooring. Rafters are 2×4 (3.75″) on 24″ center with gapped 1x plank sheathing, Tyvek underlayment, and 10-year-old shingles. Roof is hipped with a front dormer, we’ll be adding a dormer on the side for staircase headroom, and there are several skylights (one actuating).
My current plans for insulating are to remove the cellulose and use ccf to seal the roof deck, but I’m not sure what will be sufficient. I will be adding horizontal 2×4 furring on 16″ centers to hang drywall at the finished span of the rafters between knee walls and flat ceiling.
用闭孔喷雾泡沫填充2×4腔是否足够，还是需要更多？我想保持尽可能多的净空。如果需要更多的话，我可以让它们过度规划完成的跨度上方和下方，在完成的跨度上，我可以在毛茸茸的条之间添加1.5英寸的刚性泡沫板，除非由于泡沫/泡沫之间的气隙导致问题。
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Replies
Have you determined that the attic floor joist will support the new use? They may only be suitable for light storage.
I haven't. We're in the inspection period, I'm going back to look into spans. Might be possible to sister joists or introduce a beam below. Based on the insulation requirements, might be better to back out and look for something else at this point.
The master bedroom is one of the most important rooms in a house. Ground floor is the only way to go. Attics are for kids. I live in a similar house to there one you are looking at. Added a bedroom suite on the ground floor. Keep looking.
4" of closed cell spray foam is only going to give you a realistic R24 or so. That's not code, and not really all that much. 4" of polyiso on the exterior would bring you up to about R50, which would get you just over R49 required by code.
过度填充和跳闸闭孔喷雾泡沫很难，因此您可能会得到少于满足的东西。根据您的2x4毛茸茸的定向，您可能会在其中再获得3.5英寸的绝缘材料，然后可以将闭塞单元安装为“正常方式”（填充），可能是大约6-7英寸左右。7英寸将是Abour R42，6英寸左右。当前的代码也不是，但是您的R屋顶上的R值不会真正较低，并且在外部上没有任何操作的情况下可能还可以。
Bill
卸下屋顶以添加外部泡沫并不是真正可行的。看起来将斜坡墙进入代码并不是真正可行的，而没有丢失大部分成品区域（如果我们站在尾声上，大约15％，也许要多一点，以制造7.25英寸的海湾）。我会与之交谈。检查员在我们退出并走开之前，也许他会让我们在斜坡墙上少一点。
Check with your local building department on how much insulation you actually need.
根据R值，您可以做的是将屋顶与垂直于after骨的边缘上的2x3 2x4或2x6交叉绑带。喷洒after腔泡沫加上这个额外的空间。由于现在喷雾泡沫完全覆盖了after剂，因此它将减少热桥梁，并显着改善您的整体组装R值。您也可以以16英寸的OC安装绑带，以简化干墙。
Since the assembly R value is much higher in this case, you can comply based on U factor, chances are you can get away with only 5" of closed cell SPF with this type of roof.
Converting an unconditioned attic to finished space isn't necessarily simple. As others have noted, ceiling joists aren't the same as floor joists. I'd also worry about 2x4 rafters being able to take the additional weight of drywall. Plus insulation and air sealing can be tricky if the house wasn't built to accommodate them in the first place.
We renovated an old house with 2x4 rafters. As it turned out, they were undersized by at least two sizes. The solution was the remove all of the rafters and reframe the roof with 2x8 rafters on 16" centers, filled with FG insulation, and then 4" of rigid foam above the deck. Very expensive way to get conditioned attic space. It's still up on the third floor, and like Jonathan said (#7), attic are for kids. Picture yourself climbing all those stairs every day for the next xxx years. Yes, it can be done but there are probably better ways to spend your $$. Keep looking.