Attic and roof stack
Does this seem like it will work? Austin TX, Climate Zone 2A.
From inside to outside. Air barrier at the interior ceilings.
Drywall (or lath and plaster ceilings)
Blown-in Cellulose (~16 inches, but less over outside walls, as no raised heels)
Mostly solid wood board sheathing (from 1950s), one area of low slope plywood (~1.5 in 12).
Solitex Mento as roof WRB (or other vapor open WRB)
Counter-battens (1x material criss crossed)
Metal roofing
The attic is vented through gable vents but there are no ridge vents (there are two solar power fans but I would remove them). I do not have vent channels under the sheathing and our attic makes it very awkward to install them (lots of over-roofing with multiple extensions). My concern is that the blown-in insulation will contact the sheathing in the eaves (possibly 2 feet in as well, depending on how deep we get it). So the intention is to give really good drying potential above the sheathing. From what I’ve read this should work fine for the solid wood boards, possibly less so for the low slope plywood? We’d strip the current asphalt tiles and tar paper on the roof.
We considered insulating at the roofline (foam or rockwool comfortboard) but it’s rarely done here, the gable to wall details were complicated, and the cost very high. So I’m hoping to have a roofing contractor do the WRB, battens, and metal roofing install and handle the insulation myself.
Another possibility is Solitex UM (with integrated vent mesh) rather than the counter-battens?
So the main question here is: if blown-in cellulose is against the underside of the sheathing can venting above the sheathing make things reasonable? Or is the fact that warm, humid air inevitable leaking up from the house condenses on the underside of the cold sheathing in winter, wetting the cellulose, just too problematic?
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Replies
James,
We need more information. I'm not sure whether you are talking about a cathedral ceiling, or an attic with insulation on the floor of the attic, or a combination of both.
I'm also unclear on whether this is a steep roof, a low-slope roof, or a combination of both.
Please clarify these issues so we can provide the best advice.
James,
Briefly, installing cellulose in rafter bays without providing a ventilation channel between the top of the cellulose layer and the underside of the roof sheathing is a code violation, as far as I know. So you won't be able to proceed with your proposed approach unless your local code enforcement official agrees.
You mention that you will use Solitex Mento as a "roof WRB." I understand what you are driving at, but you should be aware that there is no requirement for a WRB (water-resistive barrier) for roofs. The requirement is for roofing underlayment -- which is similar, but it's not the same as a WRB.
I don't think that packing the rafter bays under plywood roof sheathing with cellulose is a good idea. In a cold climate, this assembly would be at risk of moisture accumulation. In a warmer climate (like Austin), you may be able to get away with it -- but you are entering the realm of risky assemblies that require WUFI analysis, and I'm wary of this type of assembly.
For example, in some climates, the type of assembly you are talking about can work if it faces south, but can fail if it faces north. Or it can work if it is unshaded, but it can fail if the roof is shaded by a large tree.
Thanks Martin. Right, roof underlayment rather than WRB.
Additional details: Talking about insulating the attic floor, not the rafter bays. And loose-fill cellulose rather than dense-pack. The roof has many gables, but also quite a few fairly inaccessible sloping sections that end on walls. Most of the roof has medium slope, but there is one low slope section.
This is really just me considering blowing more cellulose into the roof. The concern is that as I blow towards the sloped section the lack of baffles means that the insulation will pile against the sheathing. I'm confused about the role that baffles play when there aren't any soffit or ridge vents anyway.
Thanks,
James
James,
This is a judgment call. But in your climate zone, the situation you describe is a low-risk situation, not a high-risk situation.
According to code requirements, you can't blow cellulose near the eaves in a way that blocks the soffit vents. That said, it happens.
Your plan to use a vapor-permeable roof underlayment is wise. I think everything will be OK.
Thanks Martin!