Re-visiting Cathedral Ceilings
krom
| Posted in General Questions on
I’ve searched and read lots of info on here, including Martins original write up from years ago.
Going to be building in Upstate NY, CZ 6 12:12 pitch
Has anything changed recently, new improved methods?
Does dense pack cellulose work in the real world?
The insulation companies I’ve spoken to either won’t do it, or insist on signing a release stating that they aren’t responsible.
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Martin revisited cathedral ceilings two years ago: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work. I usually use cellulose for cathedral ceilings and it works great, as long as it's designed and installed properly.
What are your insulation companies saying will happen? With cellulose, the roof needs to be vented, and dense-packing a large cavity is more challenging and requires different techniques than insulating a more conventional wall cavity.
Thanks for replying so quicky, and the updated arcticle. I know this questions gets asked a lot, but missed that one.
They aren't comfortable with getting the install correct, settling, and condensation.
My guess is they are so busy with easy jobs, that its just not worth the effort.
From what I've heard from industry pros, and seen myself, it takes extra effort to get deep installations dense enough. Doing a second pass with a smaller hose than for the initial pass is definitely needed. My friend Jon Riley wrote a good piece on what to look for: https://www.finehomebuilding.com/project-guides/insulation/dense-pack-cellulose-insulation-done-right. From what I've heard, finding good installers is probably the biggest hindrance to cellulose installation.
If the question is whether you can have a dense pack unvented cathedral ceiling in zone 6, the answer is a definate no. Unvented roofs need either closed cell spray foam or exterior insulation for condensation control, you can read about it here:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
For a vented roof, you can definitely dense pack but it takes a lot of material thus weight to get up to R49, which can be a challenge. I think the better option for a vented 12:12 roof is batts.
Another option with a 12:12 roof is to frame in a mini attic near the ridge in which case you can loose fill the roof which is much less weight. This still require full length vent baffles in each rafter bay.
Though I read that I would be fine with a "double decked" roof. air sealed sheathing followed by spacers over the rafters, then another layer of sheet goods for attaching the actual roof to.
I don't feel comfortable with an unvented roof, just don't think that in the real world its possible to get perfect installation of any building material. IMO even a tiny leak will destroy an unvented roof long before its ready to be re-shingled as maintence.
Unvented, dense-packed roofs can work, but they are much riskier than providing ventilation. Over-venting or top-venting as you describe is a good approach. I've done that either using Zip sheathing or Pro Clima Mento in a Sarking membrane system. The more vapor-open the better, so Zip or other OSB is not ideal but as long as moisture movement from the interior is controlled it should be fine.
krom,
"Double decking" a roof only helps if the upper layer is detailed as a vent space - that is with openings at the eaves and ridge, just as you would if that vent space were below the sheathing.
That was the plan, full eve and ridge venting, on a simple roof with no dormers, or corners
krom,
I would consider Michael's suggestion of using a Sarking membrane, rather than an additional layer of sheathing. It would be cheaper, but also the disadvantage of venting above the sheathing as opposed to below is that any moisture that does accumulate in the roof has to diffuse through that lower layer of sheathing before it can be expelled though the vent channel.
What does the insulation contractor want to do?
I asked about dense pack, was told they recommend spray foam, but would do batts if there was enough joist depth to get to code (didn't even get to the point that I wanted to do R60)