Roof insulation and air sealing; advice and input appreciated…
I’m thinking ahead about a room addition to my house. This would be a 16′ x 15′ room with a gable roof of at least 6/12 pitch with no dormers, chimneys or other obstructions. I’d like to do a cathedral ceiling for the aesthetics, and since I can plan to insulate well, I’m okay with the small energy inefficiency of the higher ceiling (not really that high).
I’m thinking unvented with rigid foam above the exterior decking. I live in zone 6, so would need R-25 there. Type 2 EPS has about an R 4.2 value, so I would need 6″ of thickness, a bit daunting but I think unavoidable with this design. I don’t think using XPS to save 2″ of thickness is warranted, but I could do 3″ of polyiso with 2″ of EPS above to keep polyiso warmer.
I know I would need another layer of sheathing above the foam, or could use horizontal purlins since I will be using screw down metal roofing. Between the rafters I’m thinking of 12″ fiberglass batts (6″ batts are short on R-value). I can use 2″ x 10″ or 2″x 12″ rafters that would compress the batts a bit but not enough to threaten my getting more than an overall R-49.
What about below the batts (underside of rafters)? I don’t think I can use taped rigid foam as an air seal here as that would be an inappropriate moisture barrier and limit drying to the interior, correct? (would even 1″ of taped EPS be risky here)? Should I use a smart membrane here for air seal, and what would be good options? The ceiling will be sheetrock. so maybe no need for the membrane, but I’m okay with that insurance unless a complete waste of money.
If I went with all interior insulation (avoiding exterior rigid foam), could it work to have 12″ fiberglass batts (R-38) between the rafters, and then perhaps 2″ of polyiso on underside of rafters, well taped? The felt faced roofing stuff is said to be R-13, giving me R-51. Must this design become a vented one?
Thanks for listening!
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Replies
Howard,
It is good that you are asking these questions at the design stage. Too many posters seem to start thinking through their options (or lack thereof) after construction begins.
In general, GBA posts suggest that putting all the insulation on the outside of the sheathing is the best option if you can make it work. (Be sure to check into reclaimed rigid foam that can be purchased for one-third to one-half the cost of new material.) But I think you can solve this particular challenge with any of the strategies you have listed.
它可能有助于知道是否存在可能会在特定方向推动您的美学或预算问题。
考虑一个凸起的脚跟剪刀桁架。可以用松散的纤维素隔热，是一种坚固的通风设计。
I agree with both Steve and Jon--it's great that you're asking questions at this stage, and you don't have to use foam if you use a raised heel design. It doesn't even have to be a scissor truss; I've used parallel chord trusses in this application several times, with excellent results. A parallel chord truss is like a scissor truss but the inner and outer slopes are the same. Either way you don't need a ridge beam.
If you choose to use foam, XPS ages to about R-4.2 or 4.4, so you wouldn't be saving any energy there. (EPS' R-value remains constant.) If you want to beef up the R per inch, consider either polyiso panels, or Neopor--graphite-injected EPS that has a constant value of R-5/in. I just visited an architect-designed job that I was the energy consultant for, which used both of the systems above--a raised heel scissor truss with R-120 cellulose, and R-60 walls with 2x6 framing and 8" Neopor nailbase panels on the exterior. The owner/builder/designer was happy with how it was all coming together.
Read this and the related articles listed for more information://m.etiketa4.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
谢谢男性。我已经多次读过Martin的优秀文章Re Cathedral Ceilings，这是我所知道的（我认为）的很大一部分我需要进一步考虑桁架椽子。我确实了解他们在外壁上板上的绝缘井中的好处。
几个具体问题：
1)if I go with the exterior foam and 12" fiberglass batts (unvented), what is a good finish below the rafters? A smart membrane for air sealing, just taped sheetrock, both?
2)with exterior foam, deep rafters and an overhang, can I get adequate insulation above the outer wall without raised heel trusses?
3)is my all interior insulation idea a bad/lesser one? I think it needs to be vented, correct? It certainly is an easier overall application.
4)is there a good link to how the trim is finished to look good with 6" of added thickness resulting from the exterior foam? This seems like a challenge and maybe big expense.
谢谢。
Howard,
1.区6你需要至少51%的热阻on the exterior, to meet code and for a safe assembly without doing a hygrothermal analysis. 12" batts are R-38, so you need at least R-40 on the exterior--about 10" of EPS, 9" of XPS or 7 1/2" of polyiso. If you use less exterior insulation, the sheathing is at some risk of accumulating moisture. With the proper amount of exterior insulation, all you need on the interior is painted drywall. If you want additional insurance or if you skimp on exterior insulation, you could add a variable permeance membrane like Siga Majrex, Pro Clima Intello or DB+, or Certainteed Membrain.
2. Code requires at least R-38 over top plates in zone 6, so it depends on your assembly. It is certainly possible, and sounds likely, based on where you're heading with the design.
3. Vented rafters are a time-tested solution. Read Martin's article on designing a cathedral ceiling.
4.取得成本和各种方法，具体取决于您的整体设计。可以使它看起来很好，很容易让它看起来很糟糕。
I like a parallel chord truss (24") with a dedicated 1 1/2" minimum airspace below the roof deck with fiberboard or plywood as the spacer. Vent at both the eave and ridge. Blown insulation (22") can be used to fill the rafter space, use a warm side air barrier that is regionally appropriate for your area. This configuration gives full insulation throughout the ceiling and very good insulation fill around the roof truss webs. I will attach the ceiling drywall before insulating and leave an 18" or so space at the ridge for the blown insulation installers, they will net this final area and complete the blow. I have used this method for an art studio built over a garage in the Minneapolis/St. Paul area. The ceiling is R-65, the walls R-30 and the floor R-50 and built very airtight (1 ACH50 or less). I was there yesterday to visit the owner and with my infrared thermometer recorded a ceiling temperature at the 12' ridge of 71.5 F and a floor temperature of 70.5F. Well built and highly insulated, airtight structures will have very even thermal comfort.
迈克尔 - 感谢您的具体信息和实际数字。10“刚性泡沫是相当令人生畏的，所以我对这种情况感到不那么自信。我可以吝啬一点（在这里没有代码人和我的房子的低冬季湿度，在这个房间里的小水分来源拯救了人们），但这不是一个开始或思考的好方法。室内绝缘和通风是有利的。我可以做12“击球声，上面的击球通道，通过2”或更多井胶带的聚四亚。不太喜欢我认为，在r-51左右的鲁棒，并且不太能够得到外墙上方的绝缘，但是，我认为是可行的。
Doug - thanks for the info on trusses. That scenario is gaining based on your explanation. Since this is not a dense pack fill, is it a realistic DIY job. I have done blown cellulose with the standard rental machine a few times.