Unvented vented roof with exterior rigid insulation
大家好，
我正在进行一个在第5区的能量改造简单的盒子山墙上。部分呼吁重做屋顶。我卖在外部刚性泡沫的想法上，以减少热桥接，因为我的屋顶坡度非常低，屋檐很浅。
Code calls for R38 but I want to shoot for about R50. The planned assembly going upward is:
1. R20 of fluffy between upper truss cords
2. Structural sheathing with taped seams
3. Air barrier
4. First layer of 3″ polyiso rigid foam
5. Second layer of 3″ polyiso rigid foam with staggered and taped seams
6. Roof underlayment
My question is in venting the underside of a metal roof to add to the performance since ice damming can be a problem around here.
It seems you would need to use horizontal purlins to add a gap between the metal roofing and the underlayment. However horizontal purlins seems to be contradictory to air flowing up to the ridge and water possibly damming.
Could anyone make any recommendations on this scenario?
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Replies
喷射，
It's not possible to install metal roofing with 7-inch fasteners, so you'll need either a second layer of solid sheathing above the 6 inches of rigid foam, or (if allowed by the manufacturer of the metal roofing product that you are specifying) you will need 1x4 or 2x4 purlins above the rigid foam.
不需要金属屋顶下的气隙，但如果您想要一个，您可以随时包含一个。即使没有距离脊架气流，1×4或2x4纸蛋白（平行于脊）之间的气隙（平行于脊）也提供了益处。如果您坚持坐落到岭气流，可以安装垂直于脊的2x4s，每个2x4以上椽子或屋顶桁架。然后，您可以在空中通道上方安装一层OSB或胶合板。
That makes for a nice roof, but it's a little expensive.
The purlins do not need to be continuos, we space them with 2"-6" gaps between them to allow air and water pass though. Peter Pfeiffer, an Architect from Austin, TX, likes to install the purlins at 45° allowing air and moisture to "move" up the roof to a vented ridge cap, I think is a great idea but needs more quality control.. Maybe you can Google his approach.
@Martin - Thank you again for the great information. Most of what i've learned is from this site. Since my siding is also vertical I need to do the 2 layers of perpendicular furring to achieve a proper rain screen so it sounds like the same can be done on the roof as well. Is the top layer of OSB necessary or can the roofing just go directly on the outermost layer of purlins?
@Armando - I have seen this approach in many of Matt Risinger's projects (in Texas) and while it certainly makes a lot of sense I was curious if anyone had negatives on this approach. I'll definitely need to consult the roof panel manufacturer to see if they recommend installs on diagonals purlins. It'd certainly save a lot of labor and materials.
Jetlaggy,
If you install the purlins on a diagonal, you need to figure out how to vent the ones that don't make it to the ridge but end up terminating on the gables (the two triangular areas at each end).