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Community and Q&A

Cathedral ceiling and ridge vents

Duane Otto | Posted in General Questions on

I have a variant of what seems to be common questions about cathedral ceilings.  I’m new to the site and tried to research as much as I could.  My plan is to air seal the rafter bays – the 4 electrical boxes, 1 plumbing vent, caulking all wood-to-wood joints, reinforcing any tape joints I see in the plastic plus run a bead of open cell foam along the plastic sheet and rafter to air seal if any joints on the rafter, insulate the top plate with rigid insulation (which will also cover any missing sheathing), and install a soffit baffle using rigid insulation (about 1 to 1.5” gap).  In the attic area, there are 4 fans and 2 recessed lights plus the chimney to air seal around and of course check the plastic.

Am I missing anything?  Should I change our gable vent to a ridge vent?  If so, what kind?  I’ve read about baffled ridge vents – are they preferred?  Ridge vents are new for me as always used other kinds.  Should I add a ridge vent to the part of the house with no upper vents?

I live in Lansing, NY (climate zone 6).  We have a ranch built in 1992 with a loft and partial cathedral ceilings (12’ from the eave to a flat section that is 8’ across).  Our current profile is tongue & groove ceiling, plastic sheeting, and unfaced fiberglass insulation in the 2×8 rafters (guessing R19).  The shingles are due for replacement, so am trying to take care of some of our air leakage.  There is no attic access so I will remove some roof decking (well, probably most – one at a time) to get access.  No evidence of moisture problems.

Current ventilation is soffits vents and gable end vents (the vinyl siding is louvered but I haven’t made it up to look at them yet) in the main part of the house (with a loft).  The baffles looked positioned more to force air through the fiberglass than to keep it next to the decking.  There are soffit vents, but no roof vents in the shorter part of the house (living room over the garage).  We hear a lot of popping (assuming the truss) from this attic space when the temps get real cold.

I have looked in some of the soffit vents and removed a half sheet of roof decking to get a peak at what was inside.  Found out our cold wall had sheathing installed 1.5” short of the top plate, so that is likely the cause of our air moving straight in from outside where it goes in the soffit vent and down the wall instead of up the rafter bay.

Not going to do anything with the ceiling, am going to avoid adding insulation above the rafters which will affect the skylights, and can’t afford to spray foam everything right now.

I have read several articles on here about cathedral ceilings and air sealing. I know I will be short on insulation in the rafter bays – should be about R25.  So there will be some heat going to the roof deck, but I haven’t seen evidence of ice dams but our roof is clear of snow relatively quickly.  I will add insulation to the flat area to ensure we are at least the R49.

Sorry for the length of the details.  Thanks for your input.

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Replies

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

    Duane,
    Your roof assembly is poorly detailed, and it sounds as if you don't have the budget to fix everything. Your plan is imperfect, as you admit: "I know I will be short on insulation in the rafter bays – should be about R25. So there will be some heat going to the roof deck." It's hard to advise you under the circumstances.

    Some of your details are worrisome. Other details are unclear.

    Particularly worrisome are these details:

    1. "In the attic area, there are 4 fans and 2 recessed lights." What type of fans? Where? If you've spent much time on GBA, you know that the recessed lights are a very bad idea.

    2. "The baffles looked positioned more to force air through the fiberglass than to keep it next to the decking." I'm not sure what that means, but it sounds bad.

    3. "Our current profile is tongue & groove ceiling, plastic sheeting, and unfaced fiberglass insulation in the 2×8 rafters (guessing R-19)." As you may or may not know, tongue-and-groove boards are extremely leaky, and the polyethylene sheeting is almost certainly riddled with holes. In all likelihood, you have major air leakage through your ceiling -- and the best way to solve this is to install gypsum drywall.

    In short, your roof assembly is probably leaky, and certainly poor insulated. If you are installing new roofing, you have a golden opportunity to plug up all the ventilation openings, and to install rigid foam above the existing roof sheathing. But getting the details right on this roof assembly will be expensive, and it's unclear whether you are committed to a total fix.

    1. Duane Otto | | #3

      Martin,

      Thanks for the reply and to answer your questions:

      1) The fans are ceiling fans mounted in the flat ceiling area. I was mentioning them as a point of intrusion into the attic that was going to be air sealed. These and the recessed lights are fortunately in the flat attic area, so plenty of space to box around them. But, the lights might be able to be replaced.

      2) Yes, the current baffles are positioned very bad, which is why I want to fix that. The attached picture isn't very good (I took it as a picture of the wall sheathing not extending up to the top plate), but you can see the bottom of the existing baffle at the top of the picture. The baffle is at the top with no air blocking underneath it. Which is why it looks positioned more to force the air through the fiberglass.

      3) Yes, I'm aware that the tongue-and-groove boards are leaky. Being able to see through a couple knot holes showed me the plastic. I hadn't considered the plastic being riddled with holes. I understand installing gypsum drywall would be best and that most of the other options have a good chance of failing over time. I was trying to provide an air barrier around the fiberglass in the rafter bays that isn't vapor impermeable, thus trying to make sure it is vented.

      I guess maybe I could rephrase my questions. Will air sealing all that I can cause a problem? Will adding rigid insulation as an air block for the fiberglass at the soffit vent and also for a soffit baffle (going completely rafter to rafter) create a system that won't dry if/when moisture does come through (knowing that I'm don't have the funds to do the total fix)? Should I remove the plastic from above the ceiling and replace with something more forgiving if moisture gets into the bays? Or use a different material for the baffle?

      I'm trying to take advantage of this time to make improvements but definitely don't want to do something that makes things worse. My goal was to minimize the air leaks into the attic space and make sure the ventilation is functioning.

      Duane

  2. User avatar GBA Editor
    Peter Yost | | #2

    Hi Duane -

    I am just adding to what good questions and guidance Martin has already provided but on comparing gable venting to ridge:

    Air flow from soffit to high on the gable or to the ridge is created by two driving forces: wind and stack effect. In cathedral roof assemblies, I am not convinced that we adequately understand when and how much air flow actually takes place but I am working on this (https://www.greenbuildingadvisor.com/article/wingnut-testing-soffit-ridge-roof-venting).

    However: way more important than which soffit to gable/ridge system gets you better air flow is getting the air barrier in your roof assembly as continuous as you can. Focus on that and if I had to pick one at this point in time: install ridge vent and so far, one of these: Benjamin Obdyke Xtractor, GAF Cobra vent, Lomanco.

    Peter

    1. Duane Otto | | #4

      I assume the air barrier you are referring to is the one at the ceiling to keep the air inside the house. That I will try to do as much as I can. If there isn't any difference for the gable vs ridge in my situation, I may leave the gable in the main part of the house but install the ridge on the other part. I would like to make the gable end able to open so that I can check on the attic space once in awhile.

      Interesting study you are doing. In my case, I think there would also be some heat from the house escaping through the insufficient insulation. Do you think this will help drive the ventilation? Of course that may mean I lose more heat as cold air comes in too, but with the air sealing, it shouldn't be sucking much air out.

      Thanks for

  3. User avatar GBA Editor
    Martin Holladay | | #5

    Duane,
    Q. "Will air sealing all that I can cause a problem?"

    A. No, air sealing will not cause a problem. But roof venting issues are complicated. Your roof assembly has existing problems, and if all of the problems aren't addressed, some of these unaddressed issues might come back to bite you.

    To provide one example: Sometimes, increasing the size of a ridge vent has the result of pulling more interior conditioned air through ceiling cracks, making moisture problems worse.

    That said, sealing air leaks in your ceiling is a good idea. The best approach (as I wrote in an earlier response) would be to install drywall on the interior side of the existing board ceiling. It's also always a good idea to seal air leaks at ceiling penetrations.

    Q. "Will adding rigid insulation as an air block for the fiberglass at the soffit vent and also for a soffit baffle (going completely rafter to rafter) create a system that won't dry if/when moisture does come through (knowing that I'm don't have the funds to do the total fix)?"

    A. I think to are talking about the installation of a wind-washing dam between the top plates of your exterior walls and the underside of the ventilation baffle. Installing this type of wind-washing dam is always a good idea. For more information, see this article: "Site-Built Ventilation Baffles for Roofs."

    Q. "Should I remove the plastic from above the ceiling and replace with something more forgiving if moisture gets into the bays? Or use a different material for the baffle?"

    A. No, you don't have to remove the polyethylene from the ceiling. But the addition of a robust interior air barrier (new drywall) is recommended.

    Certainly the existing Proper-Vent polystyrene baffles shown in your photo aren't recommended, because they are flimsy and they aren't airtight. I'm not saying they should be replaced -- but you should know that they are part of the problem. A good solution would be to abandon the existing venting system (by blocking the ventilation openings at the soffit and ridge vent) and to install an adequately thick layer of rigid foam on the exterior side of your roof sheathing. For more information on this approach, see "How to Install Rigid Foam On Top of Roof Sheathing."

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