Cathedral Ceiling – Is this a viable plan for vented fiberglass? Or should I consider..
Hello everyone,
Sorry in advance for the long winded post but if anyone has the time for input, I would greatly appreciate. I have tried to do as much due diligence and read as many (mostly Martin’s) posts on the cathedral ceilings as possible.
Hopefully my terminology is close and makes sense. Not a pro here.
So here it is:
• Climate zone 5B, ¾ Detached, unfinished, single car garage that measures approx. 10ft by 20ft.
• Gable/shed style roof, which originally had an unvented ceiling with 2×6 rafters and a few 2×6 joists with electrical/lighting/garage door attachments.
• I had a square top roof vent installed when the shingles were replaced 2 years ago, so there is approximately 1 sq.ft of roof ventilation for the ~200 Sq.Ft. of ceiling space, in addition to soffit venting all around.
• The goal is to create a 4 season workshop space that can be heated with something like a 5000W 240V ceiling mounted heater.
• Walls are 2×4 16″ OC, which I am intending to insulate with R-14 fibreglass batts, vapor barrier etc.
The walls are the straight forward part, which brings me to the ceiling. I’ve gone full circle, so I am hoping to get some more experienced input.
I was originally hoping to install 2×6 joists and hang an attic ceiling with unconditioned space above. Installing and hanging the ~10′ joists felt like I was making this a bigger project than it needed to be given the goal.
There is no ridge vent, so I don’t believe a full on cathedral ceiling is possible/practical unless I hire a spray foam contractor. I was recently quoted $1500-1700 CAD + tax for 3in R18 foam. It’s tempting as the easy fix..but, pricey.
The alternative plan is to air seal (with tack tape) the proVent 14″x48″ baffles with tucktape, to create an air tight vent from the side soffits, 4ft up the roof sheathing.
The square top roof vent I had installed, is 2″ higher than the end of the baffles.
So the current thought is to install the air tight baffles, install fibreglass batts between the rafters for the length of the baffles (minus an inch or two), and vapor barrier it air tight. This would create a ~4′ long cathedral ceiling with an open air vent into the upper portion of the attic.
From where the insulation ends, install ‘joists’ spanning 3 ft between the rafters on both sides of the roof, to create a flat ceiling with unconditioned attic space above it. The new 3′ joists would be insulated with the same R-14 batts (possibly double stacked), air sealed with vapor barrier, and dry walled over.
The space above this new ceiling is between 1-2′, and the attic ventilation would be the square vent I had previously mentioned.
Before getting into the corner venting details/issues, is this plan asking for trouble?
The final alternative plan is the “flash and batt” method, 1″ of spray foam against the sheathing to serve as an air sealer, and install batts over the 1″spray foam.
Open to any other suggestions!
Thanks,
Kyle
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Replies
Kyle,
That sounds like a good plan to me. If you are worried about air-flow you could always add a couple of small gable vents too, but I'm not sure they are necessary.
If you really want a full cathedral ceiling, adding a ridge vent is pretty simple, and I'd go that way before resorting to spray foam.
Hi Malcolm,
Thanks for the input, I really appreciate it! The spray foam is such an expensive route.
To explore the flash and batt alternative, I did request a quote for 1inch of spray foam to see how that option would price out.
One correction, I mentioned it was a gable style roof but in fact the garage is a hip roof.
As you may expect because of the hip roof, the rafters in the corner create a bit of a complication for insulating asthe corner rafters are smaller areas that do not span the length of the roof.
I'm not totally confident in how to approach this detail. I am leaning toward installing an air tight rigid foam board in the rafter above the soffits to block the air front entering. If I did that, could I not insulate these "unvented" rafter spaces with fibreglass batts and vapor barrier?
Should I leave the corner rafters as is, uninsulated with the soffit vents open?
Appreciate any input on this..
Thanks,
Kyle
Kyle,
No, unfortunately if you go with an un-vented assembly on the hipped bays you can't use permeable (fluffy) insulation. Moisture will move through it and rot your roof sheathing.
The usual solutions are to insulate with rigid foam above the sheathing, strap the roof below the sheathing to allow lateral air movement, or use spray foam - none of which are particularly palatable at this stage.
For those small bays I'd suggest using a cut & cobble method. Close of the ventilation to those bays. Put in 1 1/2" of foam board against the underside of the sheathing sealed with canned foam at the perimeter, and fill below that with batts.
It might also be worth considering lowering the flat portion of your ceiling. The lower you go, the more bays you can treat conventionally and vent.
Thanks again for the input Malcolm,
You are correct that rigid foam above the sheathing is unpalatable at this point :)
I took your recommendation into consideration to lower the flat portion and it may help to reduce the number of corner bays down to 4 smaller sections (2 in each corner which is not connected to the house).
To make sure I am understanding correctly for the corner bays, I will want to:
• Install and seal a piece of rigid foam so that it blocks air from entering through the soffit and into the rafter bay?
• Install a piece of rigid foam to the underside of the sheathing in these non-vented bays.
• Install rigid foam on the 2x6 jack rafters.
• Spray foam the seams between the rigid foam pieces in each rafter bay.
• Install fluffy insulation in the sealed rigid foam cavity which has been created.
• Vapor barrier over the bays.
I'm currently still installing the air tight baffles which turns out to be quite the time consuming process when air-tightness is a high priority. I'll post pictures at some point.
In hindsight, I wish I had thought to have a ridge vent installed when my roof redone last summer lol.
So a bit of an update with a few pics:
I started to install a few of the 6' 2x4 joists for the raised attic ceiling. The ceiling will only be R-14 fibreglass batts between the joists.
Another concern has come up whether the gaps between the joists and the ridge is enough for air ventilation? The gap will be about 4" high, and about 24" wide, obviously in a triangular shape.
Do we think this will be adequate for air movement? It is a garage and I don't expect much moisture generation in the garage as it'll primarily be for storage and a workshop for woodworking.
I suspect it is not enough space for air flow but any feedback is very welcome.
I'm getting terribly close to just spray foaming the rafters and having it done with lol.
I suppose my question is: Can a small space like this (up to 4") above the attic floor insulation be enough space for air to ventilate around the rafters via the baffles, box roof ventetc?