Unvented cathedral ceiling with all mineral wool above and below.
I have read a few things about unvented roof assemblies, but am a little unclear on two possibly risky ways to “be creative.”
Main problem: Distaste for foam, long screws through osb, etc, but want to upgrade my R19 fiberglass vented cathedral roof assembly with min. R38 and preferably more. Distaste for MW industry impotence at providing availability of rigid “board” MW products.” Practically, this will mean R30 Mineral wool in the cavity and some thickness of insulation above the deck at the right proportion but probably more than the min. R20 in my 5A climate.
Two sketchy ideas that might be just fine.
a. To heck with finding the Comfortboard 110 or Rainbarrier HD. Menards will get me some nice MW batts. Can I just make 2×4/foam sandwich nailers and use batts over the roof deck and then apply a top deck over it all? I saw a post about his somewhere, but lost track of it.
b. Is there a problem with the permeable insul both above and below if I use Ice and Water shield across the whole lower-most roof deck?
c. If R30 in the rafters, what for above regardless of the other questions? More than R20 I assume. Climate 5A. R24 would be 44% of the total assembly nominal r value.
thanks
Shawn
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I don't have a clear picture on what's meant by "...2×4/foam sandwich nailers and use batts over the roof deck...".
In reality 40% of the total-R above the roof deck is sufficient for dew point control at the deck in zone 5, even though the IRC code prescriptive would imply a minimum of 41%. With R30 in the rafters, R20 above, you'd be fine, R24 above would give it a bit more margin but isn't absolutely necessary.
The vapor permeance of the above-deck insulation doesn't matter- it's about the R-value.
The vapor permeance of the WRB above the structural roof deck doesn't MUCH matter either, but going with something extremely low-perm like Grace Ice & Water Shield would reduce rather than enhance the overall drying capacity, since it's even lower perm than a #30 felt + asphalt shingle layup. A plywood or OSB structural roof deck has a vapor permeance of a bit less than 1 perm (give or take, depending on it's moisture content), a Class-II vapor retarder, and that deck is the moisture susceptible layer you're trying to protect. Since it's a Class-II vapor retarder it will also protect any new nailer-deck at the top of the stackup from wintertime interior moisture drives. Using something with with low but variable permenace such as #30 felt is a bit more resilient, since moisture that finds it's way into the space between the structural roof deck and the nailer deck could still dry toward the interior (albeit slowly.)
Shawn,
Dana gave you a good answer. You can't have an unvented roof assembly with mineral wool between your rafters unless you can figure out a way to add enough R-value above the roof sheathing to keep your roof sheathing safe from moisture accumulation. At least 40% of the total R-value of the roof assembly needs to be on the exterior side of the roof sheathing.
For a sloped roof, using mineral wool for this purpose is an outside-the-envelope approach that most roofers will be leery of implementing. If you choose to do it, you'll be accepting the risks associated with being an early adopter, and you'll definitely need the densest mineral wool you can find -- something approaching the density of a stiff board -- and you'll need to sandwich this mineral wool between the existing roof sheathing and a new layer of plywood or OSB sheathing.
Ordinary mineral wool batts won't work -- they are far too squishy.
Thanks so much Dana! My 2x4/foam sandwich nailer thing is attempting to avoid the poor availability of rigid boards of MW without creating a significant thermal bridge. I saw someone else mention it in a thread, but can only fine this mention in comment 1 of another: https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing.
If i were going for 6" of mineral wool batts (R24 at Menards with only 2 weeks lead time). I feel like I could make a 6" tall nailer for over the existing rafters/deck and under the final top deck. and that would avoid the fatal flaw of the batts for this application. 2 count 2x4's on the flat with 3 count 1" x 3.5" foam strips in between: foam, 2x4, foam, foam, 2x4. I guess parallel and right over the existing rafters.
I liked the fully adhered membrane that I see in spec'd in several detail images and articles, but hear what you are saying about the 30lb felt. I guess I just detail the air barrier some other way, like at the drywall level? Membrain?
I can accept Polyiso as my best option, but the prices of mineral wool are almost reasonable now it seems, so that's where the temptation comes. For example, for my 2x4 wall cavity, the MW in only $.56/ft versus R15 fiberglass batts at $.33/foot. Rainbarrier is around $.86/foot at the 2" thickness for outside the walls.
Cheers
Shawn,
There are a couple of problems with your suggested approach.
1. If you want to keep the lower level of sheathing warm enough to prevent moisture problems, you need continuous insulation above the roof sheathing, not insulation installed in strips between rafters.
2. Most building code officials will interpret this assembly as a 100% fluffy insulation (air-permeable insulation) between framing members, without any vent channel above the fluffy insulation -- making this approach illegal.
I think it's time to stop being creative, and instead to adopt one of the standard ways to create an unvented insulated roof assembly.
Cathedral ceilings often have moisture problems. This is no place for experimentation or creativity. Follow a tried-and-true approach.