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Question DetailsAsked on 2/10/2015

I have a cathedral vaulted ceiling, need advice on proper method of baffling/insulation for T&G ceiling, plz help!!

I have an apartment condo I just remodeled. The ceilings used to be at 8 feet, I had a contractor come in and install a 1 3/4" x 16 lvl to run the length of the condo, properly supported to support cathedral ceiling. I am on the 2nd floor, I had a new roof installed, I have soffit vents now and a new ridge vent. My rafters were only 2x6 before, but I furred them out with 2x4s, now I have 2x10 space. I purchased kraft faced R30 high density in order to have proper air flow behind I plan to install baffling / vent chutes. My questions are:

1. Do I need to install the vent chutes & run them the entire 15 ft from the soffit to ridge vent, or just at the soffit?

2. I plan to install T&G wood or the mannington T&G vinyl planks to the ceiling, can I tack right into the rafters or should I sheet the entire area with OSB / Plywood? And do I need some sort of plastic wrap to ensure air won't leak before I put the wood up? Please advise, want to do it correct, thanks in advance!

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2 Answers


Also I could use some help picking out an inexpensive yet durable T&G material for the ceiling, I just don't want to break the bank, as I have on everything else on this project, but don't want to put in something that will fail. Also if someone can recommend a good vent chute, I found accuvent and provent as 2 options that seem like they could be nice. Would it help at all sealing the vent in the seams between the rafters in order to keep extra air flow out. I also have IC7 recessed cans in those bays, so I don't know if I should cut the insulation out around the lights to make room, or just compress the r30 in those spots.

There are so many opinions on the topic of vapor barriers, proper venting, etc. that I just want to get the right information and I have always gotten good advice on this forum. Thanks again.

Answered 4 years ago by jamilhreglus


How about taking this in a couple of small bites, and see who responds to you with other comments also - you can respond back using the Answer This Question button right below your question, to keep all comments and replies back by you in the same thread.

My thoughts - in sort of stream of thought order so a bit jumpy maybe - but hey, it is zero dark thirty here so give me a break:

You DEFINITELY need a vapor barrier under the interior finish - usually stapled across the bottom of the rafters/joists/trusses, right under the finish T&G or drywall - some would say on TOP (towards house interior) of plywood if used as a nailing substrate, others like me would say vapor barrier under (toward cold side) so nailing substrate is exposed to house ventilation so it can dry out if it gets damp from condensation in cold weather. The problem is if a nailing substrate is used, in extreme cold the freezing front might move that far in (especially at rafters), so condensation from the air could form within or on the cold face of the plywood, then wetting it when it gets warmer. Hence why I would put it on the warm side of the VB.

Having a nailing substrate, by evening out the temperature difference at the inside face, also reduces the visual gray striping on the interior finish from dust sticking to condensation at the rafters - "shadow stripes" or "ghosting". Wood works better for this than drywall, but also absorbs moisture better. I have even seen thick rigid Dow HI-60 insulation board used for this purpose - more insulation and can be easily nailed or screwed right through to the rafters - though in many areas would have to have Type X drywall over it in that application, which works too (with long screws) but you are then looking at VERY long fasteners or gluing the architectural finish on. One problem with that is avoiding multiple rows of fasteners in layers - to avoid finish surface fasteners hitting underlygin fastners because that can cause marring of the finish surface if a nail hits an underlying fastener and bends or cuases a nailed double-bounce. Usually a pro will hand-nail in this instance to avoid naill gun marring of the final surface.

I would certainly use an underlying drywall or plywood sheathing unless your finish is a full 3/4" thick for T&G, so probably necessary with the vinyl because it is pretty thin and commonly shows waves in ceiling use, as will thin wood with standard rafter spacing. And of course, without a nailing substrate you are limited to interior finish running crosswise to the rafters, not up and down slope if you wanted that. If using drywall then you have to be sure your finish surface attachment nailing is long enough to penetrate through the drywall and well into the rafters. It is an ugly sight (wish I had taken pictures) when a T&G cathedral ceiling peels off due to using too short of nails. Also - in many code areas, the T&G (with either material) HAS to be underlain by Type X fire rated drywall - depends on code area and how your attic/living areas are configured. If using drywall substrate I would make it water resistant type, like used in bathrooms, for this application - on the ceiling, and even for top piece on walls if not drywalled yet.

OSB/particle board - in my opinion junk that, especially in an environment like this where moisture issues could arise - swells badly and lumpily when wetted, so could result in lumps and bulges in the ceiling if it ever gets wet, even if it does dry out. And seesm to dry out a lot slower than plywood.

One other possibility you may not have thought of that can reduce airflow into the attic area - sheet paneling with simulated T&G or beadboard or whatever appearance - would reduce joints. And goes up a lot cheaper.

Causes a real mess if it ever has to be taken down, but I like glueing (with waterproof glue) any T&G product going on a cathedral ceiling to reduce airflow and make the architectural finish an airflow and partial vapor barrier. Or caulking joints on panel products - use paintable caulk in case any gets on an exposed surface and has to be cleaned off.

If you use wood substrate up there, I would definitely use treated 1/2" (or 15/32 or 12mm) EXTERIOR rated plywood if used as a nailing substrate. If wood T&G I would go with cedar or redwood to make it more moisture resistant, or treated T&G approved for interior use (finished as desired) if you desired finish will go over it. The vinyl will probably behave better from a moisture standpoint if it gets moist, but is not as good an insulator as the wood so condensation on the vinyl becomes MORE likely - so maybe a difficult choice between preventing condensation (which happens easily on vinyl) and having a material that is pretty much immune to moisture and does not soak it up. My personal choice would be wood, but more from an appearance than performance standpoint.

What you are trying to avoid is condensation of moisture from the warm, moist, somewhat stagnant household air in the vaulted ceiling that makes it through the architectural swurface. Of course, the colder it gets in your area, the more important that factor- though you have the opposite problem in cooler very humid areas where you can get condensation on the architectural finish or VB because they are cold from air conditioned air. This is a sort of problem coastal and rainforest areas have, in addition to cold winter area issues. Certainly vinyl is inherently less vapor absorbing and less vapor permeable than wood.

Of course - if you have a ceiling fan or high-level ventilation duct up there that will effectively move air across the surface that will reduce the condensation concerns a whole lot.

If using wood finish and applying finish to it - a breathable finish will reduce moisture trapping concerns, and prefinishing all but the final coat before putting up generally looks better - avoids the thin areas and striping you get if bare material is finished in place that high up.

BTW - be sure your code area allows vinyl as a ceiling finish - some do not due to the intense heat and smoke generated when it burns, whereas on a floor it generally does not start burning significantly till the house is well involved and the people are presumably out by then.

Since you already have the kraft faced R30 that is probably OK, though I would have used unfaced for this. But I would NOT count on the kraft paper as a vapor barrier, though it might be useful to hold the insulation in place temporarily if stapled to the bottom of the rafters, then covered under with visqueen vapor barrier. Make sure the VB is FULLY sealed - I use vapor barrier adhesive at the seams when stapling it up, PLUS vapor barrier tape on all seams, because a cathedral ceiling has a fair amount of gravity air flow (chimney effect) trying to vent into the attic.

Chutes - I would definitely use synthetic - pure plastic or aluminum, no paper product to avoid degradation if they get moist. And full rafter bay width - the type that fastento the rafters rather than the roof sheathign so there is no unventilated sheathing or top of rafters to condense or frost up. You will find all types of opinions on whether you need them full-length or not - I prefer open air space for free airflow and visibility wherever possible (Afterthe eave area where it is mandatory to holdthe insulation out of the eave area), though cathedral ceilings commonly are built without that space in mind.

With R30HD you are probably 8-1/4 to 9 inches thick with the insulation batt, so I am afraid I see a potential real problem there, because the insulation itself will take up basically all available rafer bay thickness as I see it - your joists are 9" as I see it - 2x6 plus 2x4 = 5-1/2 plus 3-1/2 inches unless you used a custom true dimension finish grade wood, which is darn unlikely.

So - with 9" bay depth, take away at least 1 and hopefully 2 inches for air space under the roof sheathing (above the chutes where you have them), that leaves only 7-8" for insulation, and to avoid condensation and mold on the underside of the cold chutes you should have at least 1/2 airspace there, so that brings you down to maybe 6-1/2 to 7-1/2" available for your 8-9 inch insulation batt - which you do not want to shove up tight to the chutes or compress to fit. Think on that and get back if you want - and hopefully you will get more contractor or insulation types contributing their thoughts here.

One other thought - however you do this, keep line of sight from top or bottom if at all possible for inspection - at a minimum, with soffits you can't see through, a foot or so open gap (no chutes) at the ridge vent that you can get your head to or look into the chutes with a worklight or strong flashlight and mirror to detect any wetness or mildew/mold. However, you do need to be sure the airspace will not be taken up by the batts if they expand a bit or move around a bot in strong wind - another reason to staple the kraft paper.

I do have one more question for clarification - how did you fir your 2x6 rafters with 2x4's to get 2x10 (actually 9") depth - how did you fasten the 2x4's on edge to the rafters ? Presumably done when you had the roof done, but I am a little concerned how they were fastened - what gives them lateral stability against lateral wind and snow loading as well as the horizontal component of the roof weight (assuming the firring was done on the tops of the rafters). If done on the bottom, then at the eave areas you are actually still 2x6 thickness, so your insulation would have to taper down in thickness in that area. And of course, if the firring out was actually with flat 2x4's nailed down the length of the rafters then your bay thickness is actually 7", not 9 ". If nailed crossways to the rafters, then unless each crossing point has proper metal purlin supports you might not be up to code to carry the load. And of course, you have to be sure the firring was fastened well enough to hold the ceiling load. Of course, if this was designed by a structural engineer or architect that should all have been taken into account already.

I am not saying you have a major problom, but as you apparently realize you do have a possible insulation thickness issue here, plus the obvious airflow continuity and adequacy issue. And the question, which might just be my not visualiing it, of the firring out.

I am totally lost on what the LVL does - because the rafters presumably run ridge to outside wall (and beyond as overhang rafter tails) same as originally, right ? - and thecathedral ceiling will be fastened directly to the rafters, right ?

Do you have a few photos you could share of the ceiling and especially the attic framing over it - might answer my questions. You can post them using the leftmost yellow icon right above the My Answer box that comes up when you click on the Answer This Question link right below your question.


This air gap/insulation subject was covered a few months back in quite a bit of detail, so I am going to refer you to those questions at the links below - a couple of which had a LOT of discussion - then come back for more detail or side questions if you desire -

Answered 4 years ago by LCD

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