Air Space Above Roof Sheathing – Cathedral Ceiling?
I have read about the different options for Cathedral ceilings, and I was curious about something. Let’s say my roof is 16″ I-Joists and I’m here in climate zone 5 (Detroit). One option would be to run site built baffles made from plywood/osb under the upper chord of the joist all the way to the ridge vent. On the same concept, would it be appropriate to lay down roof sheathing, then a 2x and a second layer of roof sheathing (assuming only one of the two layers needs to be structural, the other could be 7/16)? The lower layer could be zip or covered in a house wrap in order to tie your air barrier to the walls very easily, and not have to worry about interior penetrations. I really like Josh Salinger’s design where the wrap just sits directly on the rafters with an air gap above that. But I’m planning for a steeper roof, 10/12 or 12/12, and I don’t know that it’s a safe way to go. Vs the site built baffles option, I don’t see this costing much more, and it may save you time since you don’t need to rip the boards down for every rafter bay. As far as drying, the lower roof layer would have the same ability to dry as the site built baffles. Gains you an extra 1.5″ of insulation space too! Definitely cheaper than spray foam or exterior rigid foam.
Maybe this has been brought up before, I couldn’t find it. Thoughts?
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I came upon the thread with Josh's video that I hadn't seen before for some reason. He addresses my question pretty well in there. The plywood/osb sheathing has too low of a vapor permeability, so it would still condensate. I think this would make it a poor choice for site built baffles as well?
superman22x,
I don't think it's a risky assembly at all - and less so than impermeable sealed baffles. It doesn't have the drying capacity of Josh's roof that uses a membrane, but as long as the exterior is more permeable than the interior (which would likely preclude using Zip as the lower sheathing), and there is some attention paid to air-sealing, it will work fine.
The big knock on it is you end up using almost twice the (expensive) materials a typical roof does.
Thanks for the feedback, Malcolm. At the moment, 7/16 OSB is only $1/sheet more than 1/4 wafer board. You'll use about the same amount of material doing full site built baffles I would think. This was mainly why I was thinking that it would be easier to do the second roof deck for roughly the same price, the 2x4s being extra.
If Zip is too impermeable, plain OSB could be your air barrier? My thought was air sealing on the exterior would be much easier as you aren't really working around anything. Rafter tails would sit on top of the air barrier.
superman22x,
" You'll use about the same amount of material doing full site built baffles I would think."
True, and the baffles represent a lot more work than an extra layer of sheathing if you are using similar materials.
While I do think the assembly you suggest is safe and workable, I'm with Akos - both in his advice that baffles should be made from something like house-wrap, and that if you have enough depth that baffles are best eliminated. Our code here requires a 3" ventilation gap. Unless the installers are really sloppy that precludes the possibility that the batts will block or impede the air-flow. It's the standard way of insulating vented cathedral ceilings here.
I should note that it goes against Martin's advice that any baffles need to be air-sealed. I'm just not as convinced as him that using the baffles as a secondary air-barrier is necessary, or that the loses from wind washing justify the extra labour involved.
You have to be careful with site built baffles to avoid exactly the issue you bring up. Top venting CDX is not the same as venting bellow since all the moisture first has to move through the plywood, CDX is not the best for baffles. You want something highly permeable such as cardboard, fiberboard or house wrap.
House wrap is the simplest. Take a 3' roll and on a miter saw cut it to a bit under the spacing of the I-joist webs. Hold against the bottom flange and staple and unroll as you go along. Too wide and it will jam, too narrow and you won't have area to staple, much easier with wide flange I-joists.
The better option is to avoid any baffles at all and use high density batts as these don't suffer from wind washing.
The monopoly house type of build only makes sense if you are building a home with no overhangs (which is a bad idea for many reasons).
It is pretty simple to tie the sheathing air barrier to the ceiling air barrier as long as it is part of the design. Not need to get the roof deck involved. Way less work than bolt on overhang, no need for a 2nd roof deck and you now have a simple vented assembly that we know always works.
Thanks, Akos. I like the house wrap technique. I'm not sure I'm following the high density batts version. What keeps the batts in place to keep from getting pushed up to the roof deck?
I see the advantages to keeping the air barrier inside. We are considering a roof with no overhang on the sides, but nothing finalized there. I'm aware of the pitfalls, and we may avoid it in the end, we'll see!
High density batts are much stiffer than regular batts. Provided you get the right size (you want ones for metal studs when dealing with I-joists as they are slightly wider) they are friction fit and stay in place. As long as you keep them flush on the ceiling side, they won't block the vent space.
One challenge is with HD batts and wide flange I-joits, the batts won't squish enough to smoothly wrap around the flange. The simplest if you have the height is to only install batts between the OSB webs and keep the 1 1/2" gap to the ceiling. If you need this extra space, you need to notch the batts to make room for the bottom flange. Bit of extra work but not too bad.
Zero overhang roofs only work if you have metal siding. Anything else and you are looking at issues in no time. Generally zero overhang also means no gutters, all the water dumping to the base of the wall, so your foundation water proofing has to be perfect. Not matter how you look at it, it is a bad idea.
On one of the projects I had the option of eliminating the overhangs. In the end I kept it to protect the existing 100 year old brick and glad I did. The one section that got exposed to extra rain during construction had enough damage after one year that I'm pretty sure the wall would not have survived without the overhangs. After it was all said and done when you look at the whole house, the overhangs are also barely noticeable.
I've heard mixed opinions about impermeable baffles. I follow the logic that it makes sense for them to be permeable, but then I see products like these: AccuVent Cathedral Ceiling: https://www.brentwoodindustries.com/construction/accuvent/ and DCI SmartBaffle: https://dciproducts.com/smartbaffle/ both of which the manufacturer recommends for cathedral ceilings. Are these defective by design? Or does the vapor permeability of the baffle not actually matter? I know the existence of a product doesn't mean the product is a good idea, but it makes me curious.
Brendan,
Martin's take is that while permeable baffles are probably preferable, impermeable ones work with good air-sealing as any incidental moisture can diffuse through the framing.
https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs
I guess it comes down to how robust you need the roof assembly to be. How much moisture you think will need to be removed.
I think the difference here is that typical vent baffles are not installed in an airtight manner. As long as there are air leaks around the baffle, permeability is moot as all the drying will happen by the air flow.
Proponents of site built baffles generally recommend installing them in an air tight manner. In this case you are relying on permeably for all your drying capacity and this is where you can get into trouble with low perm products.
If you look at the extra material cost, fussy install and possible moisture issues, I don't see what site built vent baffles get you. Maybe if you are dense packing it makes sense.
Thanks for all the feedback! It helps clarify what are good/bad options going forward. And I will definitely think twice about the overhangs, we would not skip gutters either way.