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Cathedral ceiling ventilation baffle

Doug G | Posted in General Questions on

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

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Doug,
    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.

    There's another issue, however: presumably, there is roofing on the old roof (the lower of the two roofs). Let's just say that this hidden roof is covered with old asphalt shingles. If that's what you've got, there is a vapor-impermeable layer on top of the roof sheathing boards you are looking at from the attic side -- so that if these boards get wet, they aren't going to dry toward the exterior, even if the space between the roofs is "wide open to the outside world for ventilation."

  2. Doug G | | #2

    Martin, there is no roofing material on the hidden roof, just 1" boards and large gaps. Gaps big enough that my cat crawled through yesterday and came out at the vent holes under the eave. Trust me, as I've done this project through the winter here in Vermont I've had 20 deg air blowing by me through that roof layer. And it is wide open due to large ventilation openings down at the eave. So I'm confident of the structure. So the original question was whether or not 1" boards are too thick to allow vapor to pass through and dry to the top. I could frame this question another way and just ask if you would build ventilation baffles with 1" thick panels? or do they need to be thin? Also do you know if the 5.5mm "tri ply" hardwood underlayment I'm using as baffle panels will pass moisture through? It says water resistant, but that's all I can find.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Doug,
    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.

    You should consider installing an air barrier, however, since you plan to insulate the rafter bays with an air-permeable insulation (mineral wool). For example, you could install strips of housewrap in the rafter bays (between the roof sheathing and the mineral wool) to limit air leakage.

    Q. "Are 1 inch boards too thick to allow vapor to pass through and dry to the top?"

    A. No. The boards are quite vapor-permeable.

    Q. "Would you build ventilation baffles with 1-inch thick panels?"

    A. Sure -- if they were solid wood. (Not that solid wood panels would be a practical choice.)

    Q. "Do you know if the 5.5 mm 'tri ply' hardwood underlayment I'm using as baffle panels will pass moisture through?"

    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.

  4. Doug G | | #4

    Thanks Martin.

    doug

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