答复

1. 
   ethan foley|Jun 04, 2018 11:00am|#1

   You will definitely need a structural engineer to review that. All your roof load will be resting on 3.5" of bearing on foam. Not to mention resisting wind uplift. Not to mention it will be difficult to figure out where to step when the foam is down. You might end up with a few boots through your foam/ceiling! You would be much better off to just create a tall heel so you can get lots of insulation over the top plate and accept the very small amount of thermal bridging where the rafters contact the top plate. Remember, wood has R-value, so if you made a 12" heel, you would still have around R14 where the rafter contacts the stud plate due to the 12" of wood. Over the length of a 10' wall, that's 0.18 ft² of R14. Pretty inconsequential.

2. 
   liam456|Jun 04, 2018 11:10am|#2

   看起来很好。

3. Expert Member
   Dana Dorsett|2018年6月4日11:48 AM|#3

   A 2x4 joist doesn't hold much weight. Are you sure it can support the sheet foam and 20"+ of fluff over the intended span?

   If you're burying the foam layer in up to 20" of fluff, there's no advantage to the full attic floor area of foam- 20" of fluff is a GREAT thermal break over the joists. If the concern is the thermal bridge where the rafters are supported by the top plate that is the only place that needs anything like foam or something more structural, (such as AAC or something) and less likely to compress or wear under wind/snow loading or flexing of the roof to provide the thermal break to the rafters. The wall foam can be extended up to the baffles.

   我希望墙上的所有2“R10泡沫不是XPS。XPS被吹入HFC汤，所有这些都是强大的温室气体。混合中最大的HFC组件是HFC134A（汽车交流制冷剂），具有全球性〜1400x CO2的变暖潜力（GWP）。由于HFC在几十年中漫射泡沫的几十年来，它的性能下降 - R10最终达到稳态R8.4-ISH。英寸为英寸多异氰尿酸，将是一种更环保的选择。Polyiso吹入碳氢化合物的混合物最占戊烷变体，具有〜7倍的GWP。

4. 
   David D|Jun 04, 2018 01:10pm|#4

   谢谢你的回复。以下是更多信息：

   - 泡沫和椽子支撑之间的1/2“CDX（和低于蓬松的绝缘），我忘了画画。
   -The roof will be stick built (2x6s rafters) so no raised heel trusses.
   -The area of the addition is 8'x16' so the 2x4 ceiling joists will span 8' and the rafters about 10'.
   - 蓬松的绝缘绝缘将是彼此垂直安装的击球（因为它们是污垢吱吱声）。
   - 这是在一个非常小的预算上完成的。
   -The foam is xps because I already have it
   - 我们在芝加哥之外，所以有雪负荷
   -I计算每个平方英尺的100磅将在2x4椽子上涉及大约22psi，可以更改为2x6以更好地分配重量

   I guess my main question is your opinion about the foam underneath the rafter plate. They're only 5.5" deep so I'm try to better thermally break them and that area of the roof/ceiling.

   谢谢！

5. 
   ethan foley|2018年6月04日01:39 PM|#5

   With some quick back of the envelope math, using local snow load here, I figure about 35 psi easy on each rafter/top plate connection, assuming 24" c/c and a full 3.5" birdsmouth cut. Also, keep in mind the ultimate compressive strength is rated at 10% deformation, so if you max out your strength you can expect a 1/4" settling on 2" foam.

   如果你想要凸起的脚跟，你总是可以像附图一样框架。
6. Expert Member
   Dana Dorsett|Jun 04, 2018 01:48pm|#6

   So if I'm understanding it corrrecly there is going to be a 1/2" CDX deck on top of the 2x4 joists? If yes, make that your continuous primary air barrier, along with the wall sheathing.

   Without trying to figure out the math by which the 22 psi was determined, assuming it's correct you don't have a lot of margin on the compression specs even on 1.5lbs, XPS and it's over spec for 1.3lbs XPS. There is a high likelihood of permanent deformation/compression over time. For design purposes the manufacturers generally recommend the live loading be no more than 20% of the spec, dead loading no more than 33% of spec.

   Changing the rafter sill to a 2x6 doesn't help, since the 2x4 studwall's top plates aren't any wider- it doesn't really mitigate the problem.

   If you're going to go ahead and do it anyway, cut the XPS into a 3.5-4" wide strip that just covers the top plate. That way any compression (permanent or dynamic) can't create a bow/arc thermal bypass for the batt layers due to the mechanical distortion of the foam board.

7. GBA Editor
   马丁·霍洛拉|2018年6月05日05:50 AM|#7

   大卫，
   我投票支持类似于yupster的建议的解决方案，让您的阁楼举起薪水。

   Using 2x4s for ceiling joists isn't typical, unless the room is very narrow. In most cases you'll want 2x6 or 2x8 ceiling joists.