A-frame roof insulation
Hello all,
We recently purchased a small a-frame in zone 4a as a vacation home.
The construction of the cabin is fairly rudamentary. The roof is composed of asphalt shingle on top of plywood sheathing, which is supported by sistered 2x8s rafters, with a four foot span between rafters. There is currently no insulation or ceiling. Several of the rafter bays are interrupted by skylights.
From reading the articles and comments on this site, I understand the best practice would have been to insulate using rigid foam on top of the roof sheathing. The roof has been recently replaced, so that is not an option at this time.
I understand the next best option would be to do an unvented roof assembly with closed cell spray foam. We are concerned about potential health effects and enviromental impact, and have largely (and reluctantly) ruled out that option.
这似乎使我们只有发泄的选择。我的承包商要么提出：（1）在现有的after子之间添加新的after子，以支持R-19 Batt玻璃纤维绝缘材料，并发泄挡板；或（2）与刚性泡沫切割并鹅卵石，并在泡沫后面用通风空间。我目前不确定我们将如何在天窗上排气。
Neither these options would get us to code, so we may add rigid foam on top of the rafters to add r value and prevent thermal bridging from the rafters. Our preference would be to use EPS foam.
从桌上取出最佳选择后，还有其他我们未考虑的选择吗？
Thanks for any help you all can provide!
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Replies
如果您查看新的框架和内部干墙工程的成本，则可能正在寻找与重新屋顶相似的成本。我知道扔掉一个新屋顶听起来不错，但这可能是最简单的。这将使您用T＆G覆盖内部，看起来比石膏板好得多。
If you are fine with the interior work, I would not do cut and cobble. Because of the thermal bridging of the rafters, the extra R value from the foam doesn't do anything to increase of the R value of the whole assembly. Lot of extra money and work for minimal gain. Stick to batts.
If you want slightly higher R value assembly, go with 6" R24 mineral wool batts (can also go with 2x4 batt+ layer of safeNsound ~R26).
You can vent around skylight by drilling holes into the rafters and creating an air path to the bay beside it. Also since the building is not occupied full time, a couple of unvented cavities won't matter much, they will be able to dry through the rafters while you are not there.
Akos- Thanks for your response.
I am wondering if the rigid foam might still be a preferable option, since we could put it in the existing four foot span without doubling up on the rafters, which I would think would have a pretty significant impact on the amount of thermal bridging. Skipping on the new rafters also reduces the cost differential somewhat, though the foam is still $1-2k more expensive, as quoted. Does that change the analysis at all?
Thanks again.
It could work. The challenge is the existing sistered 2x8. Even a small gap such as 1/32" between them times 12' length is a big hole. By drywalling over the whole thing, you can ensure a continuous air barrier. In taller structures such as an A frame, air leaks could be a much larger source of heat loss than insulation.
可以在the子之间做泡沫。您仍然可以选择板条通道，但我认为这可能会更糟，因为它将更多的外部空气变成了一个很难密封的3D空气泄漏路径的结构。
一种选择是，在现有的after子之间使用泡沫，使用未通风的屋顶。在这种情况下，请确保使用可渗透的泡沫（未透明的EPS或可渗透/纸面向Polyiso），因为屋顶现在只能朝内部干燥。只要您在冬季大部分时间都不在那里，则组件应该有足够的时间向内部干燥。您需要进行一个不错的工作空气密封（胶带泡沫的接缝，用罐装泡沫填充边缘周围的间隙，然后将边缘粘贴在after上），它应该很好地工作。这个组件的好处是泡沫是空气屏障，如果您不想要干墙，则可以用T＆G覆盖它。