Design for a ventilated low-slope roof
We are building an energy efficient house in central Vermont. Part of the house is one story and has a “shed” roof with a 3/12 pitch in which the ceiling is parallel to the roof (the highest point of the cathedral ceiling is 12.5 ft). Our architect wants us to build a cold, ventilated roof based on ideas from Joe Lstiburek.
The roof section looks like this (from inside out): tongue and groove wood ceiling, cross-strapping, pro-clima DB+ smart vapor retarder, 2×12 rafters at 16″ o.c. filled with dense-pack cellulose, 1/2″ thick sheathing, 2″ of foil-faced polyiso insulation board, 1/2″ sheathing completely covered with ice & water membrane, 2″ thick strapping to form an air gap (with vents at bottom and then into a vertical wall above), 3/4″ thick sheathing, synthetic underlayment (e.g. Grace Tri-Flex) and a standing seam roof. (this should give an approximate R-50 value – we cannot make the roof thicker for various reasons).
I question whether we need all three layers of sheathing. I would rather omit the sheathing applied to the rafters and instead install the polyiso board directly on the rafters. I would then increase the next layer of sheathing to 5/8″ thick (e.g Advantech tongue and groove) and install this with long screws into the rafters (and then cover it with the ice & water membrane). I would also reduce the final layer of sheathing to 5/8″ thick (again Advantech T & G).
Will I be OK with putting the polyiso directly on the rafters and then having just two layers of Advantech sheathing?
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Replies
Timothy,
There are several issues here.
最重要的问题是Polyiso的2英寸层是错误的。对于此应用，这是错误的刚性泡沫的错误选择 - 因为Polyiso在低温下表现不佳 - 并且太薄，无法在冬季将屋顶护套保持在露点上方。这是冒险和违反代码的风险。
Here is a link to an article that discusses this type of roof assembly:How to Build an Insulated Cathedral Ceiling.
In your climate zone (Zone 6), the rigid foam layer above the roof sheathing needs a minimum R-value of R-25. This could be accomplished with 6 or 7 inches of EPS. (It makes more sense to install EPS at this location than polyiso. To learn why, read this article:In Cold Climates, R-5 Foam Beats R-6）
For more information on installing rigid foam above roof sheathing, see this article:How to Install Rigid Foam On Top of Roof Sheathing.
就您而言，您可能需要考虑安装6英寸EPS Nailbase。
This type of roof doesn't need ventilation channels above the rigid foam, although the ventilation channels are an excellent idea and make for a great roof.
One final point: If you can't accommodate the rigid foam because of height restrictions, you can switch to a vented roof assembly. From the bottom up, one way to do that would be: finish ceiling, air barrier membrane, rafters filled with dense-packed cellulose, plywood roof sheathing, vapor-permeable roofing underlayment, 2x spacers to create ventilation channels, another layer of plywood or OSB roof sheathing, roofing underlayment, and roofing.
感谢马丁。实际上，我误读了我所拥有的部分图纸，我的建筑师要求4英寸的Polyiso，如果您指出的冷温度不适合R值的抑郁症，那就足够了。没有任何泡沫。不会给我们带来必要的R值，因此我想我们有两个选择：a）找到某种方法将外部泡沫增加到6“或b）将polyiso移入afters中，它将保持温暖。如果我们使用选项B，我们将Polyiso接缝胶带使其成为空气级，并在第一层护套（例如Pro Clima Mento）上方使用可渗透的底层（例如Pro Clima Mento），以便纤维素可以干燥到我们的侧面。
Timothy,
您可能会考虑一个外部泡沫三明治，底部有2英寸的polyiso和3英寸的EPS。这样，每股收益可以使多丽莎保持温暖。
EPS和Polyiso相同的跨界点大约为43F。对于Martins 2+3溶液，从Polyiso Out的中间，您拥有polyiso的R6和EPS的R12。在温暖的一面，您有R6的Polyiso和R38的纤维素。因此，您的R18在寒冷的一面，R44在温暖的一面。polyiso的平均温度，内部温度为10 f，内部为70 F的温度为：
10 + (18/62)60 = 27 F. That's low enough that the EPS is still a better choice than polyiso.
我喜欢外面的EP和内部的EPS的想法，但是当您的内部也有那么多纤维素时，Polyiso在佛蒙特州的寒冷一天仍然很冷。