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Insulating a Cathedral Ceiling

paulm8's picture

I am finishing off a room above the garage at my house in Syracuse NY and have a ceiling insulation question.  When the structure was built some time ago, we had long lengths of 1/2" drywall boomed into the space.  After talking with different people I'm hearing that the drywall spanned across the 2" O.C. spacing of the 2"X12" rafters might sag over time.  My plan is to install 3/4" furing strips across the bottom edge of the rafters 16" O.C. to create a shorter span for the drywall.  I also am thinking this  will prevent thermal bridging through the rafters to the roof deck. I have already installed insulation baffles on the underside of the roof deck, sealed them to the adjacent rafters with a fine bead of sprayfoam, and installed and sealed ridged blocking from the bottom edge of the baffles to the top plateof the wall to prevent any air from contacting the edge of the fiberglass batts.  I have not purchased the fiberglass insulation yet and I am second guessing myself on how to proceed. My original thought was installing kraft faced insulation in the rafter bays, then install the furring strips, then install the drywall.  Now  I am wondering if i need to be concerned about the 3/4" thick air space that I am creating between the drywall and the insulation with furring strips.  Should I  fill that gap? If I should be filling the gap with sheets of foam, should I be using unfaced insulation in the rafter bays to avoid trapping moisture?  I do not understand enough about moisture to know if I would be creating a problem by having the 3/4" gap.  One thing i have taken great care to do is airseal all of the baffles so that the air entering at the vented soffits will move only through the space between the baffles and the roof deck to the ridge vent.   Any expert advise on whether or not  i should be doing something with the 3/4" gap created by furring strips is greatly appreciated.

Furring (post #216462, reply #1 of 6)

Someone else can better advise you about insulation, someone with worse winters than So Cal. But, go to a real drywall supply, not a big box store, and buy furring channel. It's probably cheaper than 1x and won't ever split. Use fine thread drywall screws like you'd use for metal framing. Even though it's metal, thermal bridging should be minimal because there's very little contact.

 

Mike

Insulating a Cathedral Ceiling (post #216462, reply #3 of 6)

Thank you Mike.  I already have the wood furring there, but if i end up having to put foam in i'm betting the furring channel is better suited for the precut size of the foam sheets. I also like the benefits of not splitting and less contact area that you brought up! Thanks again!

If the rafter spacing is (post #216462, reply #2 of 6)

If the rafter spacing is really 2" O.C. I don't think you'll have a problem.  But if it's 2' O.C. then regular 1/2" drywall will definitely sag.  While you're going to the trouble of installing furring you might want to go with 12" O.C. vs 16" -- will reduce the chance of sagging significantly.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Hi there, I am in (post #216462, reply #4 of 6)

Hi there, I am in Massachusetts and probably same climate zone as you.  We never install fiberglass batts/vented roof assembly unless we absolutely have to.  We prefer an unvented cathedral ceiling with 3 or 4" of closed cell followed by open cell to fill the cavity.  That is not to say that a vented cathedral with fiberglass can't work, it certainly can.  I would recommend unfaced batts to fill the cavity, 3/4" foam between the strapping and air sealed to the strapping with tape.  The tape should be 4" wide or so, such that it will cover the 1x3 and each edge of the adjacent foam by 3/4" or so.  That additional R-5 is huge, but more importantly is the foam allows for the flat plane that is much easier to airseal.  Also of upmost importance is to have as few penetrations in this assembly as possible.  Traditional recessed cans are definitely out.  You should use surface mount fixtures or extremely shallow recessed LED puck lights like those made by Lithonia or Lotus.  Better yet, use wall sconces and keep that ceiling free of any holes.  Good luck

Insulating Cathedral Ceiling (post #216462, reply #5 of 6)

Hello.  

This advise helps more than you know. Using foam board between the fiurring strips feels right to me but I was really confused on whether or not to use craft faced insulation. Thank you for clearing that up  I hear what you’re saying about penetrations. I will not have any. I really appreciate you taking the time to reply.

Thanks a million!

Sure thing Paul, Happy I (post #216462, reply #6 of 6)

Sure thing Paul, Happy I could help.