Insulating Cathedral Ceiling

[mxw087]

Hey Guys, I checked forum titles and didn't find anything related to this so I'm sorry if it has already been discussed! Also not sure if insulation is normally discussed on this site but I'm desperate and just want to finish this project. I have an insulation question regarding a cathedral ceiling.

I bought this house about two years ago and at some point someone added a side addition. It is a gable roof. The addition runs the length of one side of the house. It is not currently heated but I plan to heat eventually. I'd like to keep the height of the vaulted ceiling and put in proper insulation. Every forum, blog and post I've read on the internet has mixed views on how to go about doing this. I have read hours of articles and it's still not clear. I'd like to do it myself and avoid spray foam. I am in Ontario, Canada. Climate zone 5, I believe. So my goal is R 38 but I may have to settle for 34. The whole house has vented soffit.

So far I have just installed Moore vent channels (baffles) from the soffit to the top and drilled holes to allow air flow into the main attic of the house. I then plan on putting roxul batts below the channels (R14), then finally two layers of 2" rigid foam (R20) attached right to the rafters. And then drywall.

Only thing I'm concerned about and can't find a clear answer online is installing the roxul under the channels. It's mineral wool so moisture resistant, but just want to make sure to avoid any problems!

I appreciate any input! Thanks!

 

[Dana]

If the channels are vented from soffit-to-ridge, your plan should work just fine. If it's blocked by other framing along the path, it's potentially a problem. The foam will block moisture drives from the interior, and the rock wool will dry just fine into the channel. You'll need more free vent area at the soffits than the random collection of 1/4" holes in the picture though. Take a 1" hole saw or auger bit and give yourself 5-6 inch diameter holes (4-5 square inches of vent area) per bay.

If the foam is XPS (pink, blue green, sometimes grey) it's going to lose performance over time, as it's climate damaging HFC blowing agents leak out. Foil-faced polyisocycurate would be a greener choice (blown with relatively benigh pentane, at less than 1% of the global warming potential of HFC134a, the primary component of the blowing agents used in all North American XPS manufacturing) and you can get to R20 with just 3", and you'd be at R25 or more at 4". It's also easier to air-seal the seams (with foil tapes) than XPS.

Cap nail the first layer in place, tape the seams, can-foam the edge, then use dabs of foam board construction adhesives and a few longer cap nails to install the second layer. Through-screwing 2x4 furring to the rafters 16" o.c. gives you something decent onto which the ceiling gypsum can be mounted. The additional 3/4" space with a foil-facer (which is also a vapor barrier) adds another ~R1 to the whole-assembly R-value.

If there are obstructions between the soffit & ridge blocking convective flow you may have to take an unvented approach, which is more complicated and usually more expensive.

More here.

 

[Sponsor]

 

[mxw087]

Thank you so much for this information! So I drilled more holes at 1" and then filled with roxul. I'm working by myself so I put up 3/4" strapping simply to hold the roxul in place. It was needed because of the odd spacing of the rafters who ever originally built it at 36".
My original plan was to come somewhere near R34, but the information I was looking at was pertaining to a ceiling with an attic. Because this ceiling won't have an attic above, I believe code is 31 in my area.
My plan has since changed and could you tell me if this will work? I plan on stapling some thin strips of plastic (vapour barrier material) or even string to hold the roxul up and remove the 3/4" strapping. This way I can attach a 2" layer of rigid foam right to the rafters, then maybe use 3.5" drywall screws and attach through the rigid to the rafters? Should give me R 24 but there will eventually be a small gas fireplace that only heats this one room.

 

[Dana]

Installing string or even landscaping fabric to hold the batts in place as you remove the furring is fine. You don't need (or particularly want) a vapour barrier material underneath the rock wool- the foam board provides that function. (Even 4" of unfaced Type-II EPS would meet Canadian code specifications for vapour barrier", but foil faced polyiso would be 10x as tight.)

Drywall screws won't work for foam- the heads are too small. That's why cap nails for the first layer, and longer cap nails (temporarily) before the 1x4 furring 16" o.c. held in place with 6" (minimum) pancake head timber screws through screwed to the rafters for the second layer. The screws are expensive (~USD $135 for a bucket of 250 screws at some internet stores) but they won't strip, and they won't split 1x4 furring carrying the weight of the drywall even in an earthquake. With drywall screws it's guaranteed to split over time, and it would all come tumbling onto your head in an earthquake.

If it's only one layer of 2" foam it can be cap-nailed in place prior to the furring, and 4.5" pancake head timber screws will work fine. In my area you can get those in a box of 50 for ~USD$35 from the big orange box store.

 

[mxw087]

Thank you Dana for taking the time to answer this! I'm probably making it more complicated than it needs to be. I do have one more question if you dont mind.
I do have some leftover landscape fabric I will use to hold the roxul in place, great suggestion! Then I'll cap nail one layer of 2" rigid directly to the rafters with taped seams and spray foam the edges. My question is, is the furring necessary if I'm only using one layer of 2" foam? Could I attach the drywall directly against the foam using the pancake head screws that you suggested?

Or is the furring just to get back to 16" oc to hold the weight of the drywall?

I'm from southern Ontario so earthquakes never happen but I definitely wouldn't want it coming down. Again, thanks for your help! Much appreciated.

 

[Dana]

Pancake head screws would't leave the heads flush with gyprock, making for a real cosmetic issue, and gypsum is nowhere near as structural as 1x planks so the fastener spacing would have to be tighter.

With 2" foam it might be do-able with 3.5" - 4" bugle head drywall screws 12" o.c. (or tighter), but I've never seen a spec for that.

 

[Reach4]

With 2" foam it might be do-able with 3.5" - 4" bugle head drywall screws 12" o.c. (or tighter), but I've never seen a spec for that.

Washers are cheap and easy.

 

[Dana]

Washers will cause the heads to stick out too. It's easier to just go with a tight spacing.