Dense pack a cathedral ceiling?
rhl_
| Posted in General Questions on
Ok I’ve read this:
https://buildingscience.com/documents/insights/bsi-043-dont-be-dense
It suggests enough foam on the interior and you don’t need an exterior vapor barrier.
BUT what about if you first apply VISCON https://foursevenfive.com/visconn/ to the underside of your roof sheathing and then dense pack, with the intello. https://foursevenfive.com/intello-plus
The roof has Ludwici clay tile shingles on old tar paper attached to wood to the exterior. So no exterior insulation.
this is in climate zone 4a.
Is is there still moisture risk ?
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Ryan,
Here is a link to an article that directly addresses your question: "Smart Vapor Retarders for Walls and Roofs."
A recent code change (it's in the 2018 IRC) allows the use of "vapor diffusion ports" instead of ventilation channels in some types of roofs. However, this approach is only allowed in Climate Zones 1, 2, and 3. In your climate zone (Zone 4) that approach is both risky and illegal.
So, the short answer to your question is: No, you can't install dense-packed cellulose in an unvented roof assembly.
For information on safe ways to proceed, see this article: "How to Build an Insulated Cathedral Ceiling."
"The roof has Ludwici clay tile shingles on old tar paper attached to wood to the exterior. So no exterior insulation."
So, what type of "wood" is that old tar paper attached to? It may make a difference.
Old school tile, slate, and cedar shingle roofing was commonly installed on battens or skip-sheathing, with spaces between planks rather than continuous plywood or ship-lap/t & g planking. You can't really dense pack under skip sheathing since it would push through the spaces between the planks. But if you installed blowing mesh to the interior side of the skip sheathing it can work, and #30 roofing felt is high enough vapor permeance when humid to qualify as a vapor diffusion vent (greater than 20 perms). Even though that isn't currently enshrined in code for zone 4A, it's probably going to be fine under a tile roof, given that a tiled roof is inherently top-vented, and the total amount of diffusion vent area of skip sheathing would be several times that of a code-min diffusion vent for zone 3A.
If instead of skip sheathing tiles are installed on battens attached to plywood or any other full-coverage roof deck I'll second Martin's comments. Ludowici's installation manual doesn't show skip sheathing OR battens, though there is nothing to preclude it from being installed on skip sheathing, but in an older house it might still have skip sheathing. See pp 35-58:
https://www.ludowici.com/wp-content/uploads/LRT-436-Ludowici-Interlocking-Tile-Installation-Manual-0617.pdf
Ok I’m heavily leaning against this after these comments, but I will point out my plan is _not_ exactly discussed in these articles. My plan is to put the viscon product between the insulation and the sheathing. I do see that in 4A you can use open cell foam to sufficient thickness on the interior. This seems similar in that it’s a vapor retarder to the interior. So i think so thought on this is warranted.
Also I plan to insulate and air seal the floors and use intello + dense pack on the attic floor.
I’m not sure what it is. It’s not skip sheathing. But I can see the tar paper in some spots.
Ryan,
Viscon is a liquid-applied air barrier. Its use on the underside of the roof sheathing makes no sense. It doesn't provide any R-value, unlike spray foam, so this product doesn't allow you to assume that it will behave like spray foam from either a building science perspective or a code perspective.
The fact is that you can't do what you propose. Filling rafter bays with cellulose is risky because the roof sheathing can accumulate moisture, eventually leading to rot. Installing a vapor-permeable air barrier (a liquid product) to the underside of your roof sheathing won't prevent this type of disaster.
Understood. I think I will modify my plans to not include the dense pack cellulose on the rafters.
However, I’m not sure why you say using viscon on the underside of the sheathing makes no sense. It’s still an air barrier and it’s still a vapor retarder so using this on the underside of the sheathing should definitely help make the attic space more airtight.
My specific plan is to use this on the sheathing with intello on the other side of the joists, as well as using intello on an appropriately air sealed floor above the joists.
Then the plan was to dense pack the 2x6 floors, and use as much loose fill around as I can.
Much of my attic space is not something you can walk on, so I was going to use loose fill insulation, to get a high R-value.
The 475 people were encouraging me to dense pack the rafters which I was skeptical enough but they kept insisting. I won’t do that after this conversation.
It almost never makes sense to insulate at the roof deck unless you are converting the attic to conditioned space, either living space, or to bring HVAC equipment installed in the attic fully inside the pressure & insulation boundary of the house. If it's just going to be an empty attic, insulating at the attic floor is cheaper, and less surface area, which is less lossy, and safer for the roof.
The all-important first step to insulating at the attic floor is to air seal between the attic & conditioned space rooms below.
I understand as I said I’m going to nix the plans to insulate the attic ceiling but adding more air barriers above the floor clearly can’t hurt, right ? I can’t see what’s wrong with making the attic ceiling more airtight.
Ryan,
If you want to create an air barrier at the sheathing layer, you can. The usual way is to tape the OSB or plywood seams. One example of this approach is Zip sheathing with Zip System tape.
Once the sheathing is installed, trying to create an air barrier from the interior with a liquid product is quite difficult, because the sheathing seams are on the rafters, and the sheathing seams aren't accessible from the interior. I suppose you could use one of the sprayable caulks, but nobody does that, because it's a lot more work than applying tape at the sheathing seams from the exterior side.
For more information on sprayable caulks, see "Air Sealing With Sprayable Caulk."
Sure I understand what your saying, but spraying this stuff on isn’t any harder than spraying foam — and there are plenty of air leaks not against the rafters where these old pieces of wood sheathing have small gaps.
Respectfully though, picking up my very expensive old tile without breaking it and doing something on the exterior is clearly more work than going into the attic with an airless sprayer.
I’m not expecting to make the house so perfectly airtight, but we can try to make it better, which is why I was asking if air sealing the attic ceiling could do any harm.
Has anyone else done this successfully. Applying a Visconn layer in a rafter bay for complete airsealing and vapor barrier in a cathedral ceiling retrofit with plan to dense pack?
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