During my renos, I have completely removed the ceiling drywall (was actually two plus layers of ceiling in place) and we are going to be having cathedral ceiling (yes single story building). I had originally placed 6 inch batts of pink fiberglass insulation. I use wood fire heat and during the first winter I quickly encountered condensation above the vapor barrier (hadn't noticed it until spring). Removed the entire vapor barrier to allow the batts to air dry (was successful). Next heating season (ceiling still not installed as I wanted to observe the batts for potential of condensation.
Of course as I had no vapour barrier installed I very quickly experienced water droplets in a variety of places (immediate thought was damn I have roof leaks). Droplet rate was erratic and so I decided to pull back some batt insulation to have a look - I found ice forming on the interior of the roof sheathing (time for more research).
OK, so my R-value is no where near high enough and truth be known, I need something to stop radiant transfer to the sheathing - my resolve, spray foam. It has worked extremely well - perfect actually. The only issue I see is that as my rafters are 2X6, the insulation when sprayed in did not completely come out to the edge of the rafter. This leaves me with the situation of having approx 2 inches of insulation in a few spots (dips in spray foam) and of course shy of the 6 inches I was hoping to attain. I plan on installing hard board foam (with reflective coating facing in toward the living area) to increase the R-Value below the roof structure. It would get installed onto the face edge of the rafters and as a result, I will have cavities between the rigid foam and the spray foam.
Here is my first question - do I need to be concerned with the cavities (even if spray foam were all even at 5 or 6 in thick, there would be minimal but then I would not be concerned) as some of them will be 2 inches or a bit more.
My thoughts have been this - with a DIY Kit, add spray foam to the current spray foam and bring to the leading edge of the rafters and then add the rigid foam I spoke of - OR - use roxul insulation to cover the sprayfoam and bring to the leading edge of the rafter then add the rigid foam I spoke of. My biggest concern I have with approach two is am I just adding potential problems if I only use roxul insulation instead of sprayfoam to bring insulation out to the rafter edge (if I really need to). Pretty convoluted I know, but I have no doubt you guys will be able to follow me mode of operendum as well as my train of thought.
Thanks in Advance.
Roxul is Fine. It does not retain moisture so you'll have no concerns with the condensation or mold. I would cover ceiling with a moisture. Air Barrier Tyvek or Typar then drywall. No need for additional Insul, save your $
The problem it sounds like to me is your not giving any air space above the insulation yet you likely have roof vent or soffits unlike a wall allowing cold air/moisture to get in and form ice. Spray foam would seal everything off and in this scenario probably would work just fine so long as it does seal and stop the airflow being passed through the "Attic" vault.
P.S hire a proffesional for the foam, if you spray it in too heavy it wont cure and will leach chemical smells and generally only a couple inches at a time is ok. The other is there is different grades of foam and the kind in the DIY kits is low poundage whereas pros have 3-4 lb foam usually a blue or purple colour. The cost and coverage of those kits would end up being a lot and a proffessional also is likely cheaper.
Regarding the venting - as I am making cathedral ceiling I had sealed off all ventings for the attic (1 roof vent and entire soffits). With that no cold air had any chance of entry. The ice forming was when I had fiberglass insulation (moot now as I have gone to sprayfoam). As I already have commercial sprayfoam installed, all I am looking to do is reduce the cavities (some areas are 1 - 2 inches less filled than the majority of the area) in the roof truss area; do you still recommend not using the DIY foam kit just to top off the fill. What I actually have on hand is by DOW - Froth Pak High Density (3.4lbs) (had purchased for doing rim joist area)
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