Cathedral ceiling ventilation baffle
I have a 12/12 roof over an attic that I am making into a storage room. I added 1/2″ plywood onto the joists to make a rough floor. On one half of the roof a second roof was built over it at about 10/12 to extend the roof out over a porch. All three “roofs” meet at the ridge. I am building ventilation baffles between the rafters with 1×1’s as the spacers and 5.5mm underlayment plywood as the baffle panels. The sheathing is old 1″ boards on the rafters with 1/2″ OSB over that on the single roof side. On the double roof side it is 1″ boards throughout with OSB only along the upper 24″ or so. Below that it is just the 1″ boards with lots of large gaps for air. There are lots of gaps between the old 1″ boards throughout the roof. It will be 3-1/2″ Roxul between the rafters and then 2″ of XPS under that and then a 6mil poly sheet. Approximately R-25. I’m in central Vermont and I know that is below standard, but it is for storage and is too low to ever be a living space. The floor of this space has R-19 fiberglass in it covered in plywood now since it used to be the ceiling. Here is my question: On the side without the added roof I’m doing it as shown on one of your pages, but on the side with the added roof it seems that no baffles would be needed under the original roof layer since the space above that (the space between roof 1 and roof 2) is wide open to the outside world for ventilation. Is that reasonable? If any moisture leaks up into the 1″ boards I would assume they will dry out from the top. Or, is 1″ wood too thick to allow adequate drying. Lots of air on top so it will not be stagnant.
I hope I made the structure clear, what do you think?
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Replies
道格，
Whether the space between the two roofs is "wide open to the outside world for ventilation" (as you describe it) or a confined space bound by impermeable roofing on the upper side depends on construction details that we don't know. These details maybe something you are intimately familiar with, or they may be details that are hard to assess from inside your attic.
So, the first issue concerns the question of whether or not you are confident that the space between the two roofs is very well ventilated.
但是，还有另一个问题：大概是旧屋顶上有屋顶（两个屋顶的下部）。可以说，这个隐藏的屋顶上覆盖着旧的沥青瓦。如果那是您所拥有的，那么您从阁楼侧看的屋顶护套板上有一个蒸汽的层，即使屋顶之间的空间“向外界开放，要进行通风”。
马丁，隐藏的屋顶上没有屋顶材料，只有1英寸的木板和大缝隙。缝隙足够大，我的猫昨天爬行并在屋檐下的通风口孔出来。相信我，因为我完成了这个项目整个冬天，佛蒙特州的冬天，我在屋顶层上吹了20度的空气。由于河口的通风开口很大，它敞开了。所以我对结构充满信心。因此，最初的问题是1英寸板是否太厚，无法使蒸气通过并干燥到顶部。我可以用另一种方式构建这个问题，然后问您是否会用1英寸厚的面板来构建通风挡板？或者它们需要稀薄吗？还知道我是否将我用作挡板的5.5mm“ Tri Ply”硬木底层面板会通过水分吗？它说耐水性，但这就是我所能找到的。
道格，
Thanks for answering my questions. It sounds like the area between the two roofs is, indeed, well ventilated. If there is no roofing on the lower roof, you're good -- you don't need a ventilation channel there.
但是，您应该考虑安装空气屏障，因为您计划用可渗透的绝缘材料（矿物质羊毛）将after湾隔热。例如，您可以在after架（屋顶护套和矿物质羊毛之间）中安装房屋包裹，以限制空气泄漏。
问：“ 1英寸的木板是否太厚，无法使蒸气通过并干燥到顶部？”
A. No. The boards are quite vapor-permeable.
Q. "Would you build ventilation baffles with 1-inch thick panels?"
答：当然 - 如果他们是实木。（并不是说实木面板是一个实用的选择。）
问：“您是否知道我正在使用的5.5毫米'Tri Ply'硬木底层，因为挡板面板会通过水分吗？”
A. I'm sorry, I don't know the vapor permeance of tri-ply hardwood plywood. However, the vapor permeance of ventilation baffles isn't particularly relevant or important. To learn why, see this article:Site-Built Ventilation Baffles for Roofs.
感谢马丁。
doug