Unvented roof, Zone 3B, foam-free
Hi Martin,
First off, I have read a wealth of articles on the subject of unvented roofs, by yourself and Joseph Lstiburek. I started with your “How to Build A Cathedral Ceiling” article and read pretty much everything related to this from you and Lstiburek. The conclusion being that for an unvented roof you either install closed-cell spray foam between the rafters, or rigid foam above the sheathing at a thickness dictated by your climate zone to keep the sheathing warm enough.
Then I read your article on “Building a Foam-Free House” (https://www.greenbuildingadvisor.com/blogs/dept/musings/building-foam-free-house) in which you discuss Unvented Cathedral Ceilings:
“Creating a foam-free unvented cathedral ceiling is challenging. The only approach that I can think of is to install a thick layer of semi-rigid mineral wool insulation above the roof sheathing, followed by another layer of roof sheathing. The mineral wool insulation would need to be thick enough to keep the lower layer of roof sheathing above the dew point during the winter.
While it’s fairly common to install semi-rigid mineral wool insulation above roof sheathing, the method is usually restricted to low-slope (flat) roofs. Using this method for sloped roofs would be considered experimental, so builders should consult an engineer before proceeding with this technique.”
Then I read some of this post: https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/39891/unvented-slanted-roof-radiant-cooling-and-no-polyurethane-4c and the reader seems to wanting to do something similar to what you theorized about in the foam-free house blog.
And lastly, I saw a bit on your article “Can Unvented Roof Assemblies Be Insulated With Fiberglass” and Joseph Lstiburek discusses proposing changes that will allow unvented roofs to use air-permeable insulation under certain conditions.
My questions are:
Climate Zone 3B Los Angeles Area
1. Your solution for a foam-free unvented cathedral ceiling on a sloped roof – adding the mineral wool above the sheathing followed by another layer of roof sheathing, a) this is to code? b) if so, what type of ‘engineer’ / expert would you suggest consulting regarding this approach?
2. If this is not to code, I take it you were simply brainstorming a solution. Would Joseph Lstiburek’s new proposal change this code for a mineral wool installation above the sheathing?
Thanks for you time,
Joe
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Replies
Joe,
The way I read the International Residential Code, you can install mineral wool above the roof sheathing to create an unvented insulated roof assembly if you want. The code describes the type of insulation that you would install above the roof sheathing as "rigid board or sheet insulation." I think that it can easily be argued that a mineral wool panel falls into this category; however, your local code official has the final say.
It's important to note that very few builders install mineral wool panels above roof sheathing except on flat roofs. If you do this on a sloping roof, you will be breaking new ground. You may discover (after a few years) that it wasn't such a good idea. I doubt if Roxul or Thermafiber support this type of installation.
Building scientist John Straube plans to experiment with this technique on a new carriage house that he is building in his yard in Ontario. He recognizes this approach as experimental. He will be using two layers of very dense mineral wool, followed by 2x4s installed 16 inches on center (with one 2x4 directly above each rafter, perpendicular to the eaves, installed on the flat), and then 1x4 purlins, and then metal roofing.
If you want, you can simply call up Owens Corning and tell them you want to use their new “ProPink High Performance Conditioned Attic System.” You are located in a climate zone (Zone 3B) where the company says that their system will work. Owens Corning told me that they will work with your local code official to help you obtain special code approval. You don't have to wait for the code to be changed if you go this route.
Finally, have you considered simply building a vented cathedral ceiling?
Joe,
We have done many foam free roofs in the USA and worked with building inspectors in many climate zones to get them approved. The principle is to use ProClima INTELLO (that we distribute at http://www.foursevenfive.com) on the inside of the rafters - this best in class smart vapor retarder will protect the sheathing from condensation in cold weather, while allows inward drying of any present moisture in warm weather. I have modelled a roof with dense fiberglass in LA in WUFI, and it work well - both with a bright and roof shingles - this does make a significant difference in certain climates.
Please see also our blogpost of this approach in the more challeging climate zone 5).
Martin,
John Straub experiment is common practice in Germany - especially for retrofits.
Remove roof - add (smart) vapor retarding temp roof membrane/airtight layer
Add dense mineral wool boards (rockwool) with SOLITEX pre-applied on it's top side - tape over laps - followed by a vented roof on battens (tile or metal). See image
Floris,
The main hurdles for U.S. builders who want to use more mineral wool insulation on the exterior of their building is that the dense panels that are needed are expensive and distribution is limited. It's hard to buy the insulation. Hopefully, that situation will change.
Martin,
As always, thank you for the thorough response.
Why is it that mineral wool above sheathing is typical only on flat roofs? Just curious.
The Owens Corning system sounds interesting but it's not a complete system for our roof. We have Cathedral Ceilings on the back half of the house and the attic space on the front side. From what I read, the Owens Corning system is only for Attic Spaces?
We have considered building a vented cathedral ceiling / attic but I have decided against it for a few reasons. For one thing, the current roof is 'unvented' (except 2 gable vents, 1 on each side of the house) and we haven't had any issues, so I would be concerned that going vented would possibly create problems - I thought I read something similar in one of the related Lstiburek articles or discussions. This isn't a big deal, obviously it's been done. But most importantly, we live in Southern CA with the threat of wildfires. I know they have special soffit vents to keep embers out but that's just one more thing that needs maintenance and I'd rather not deal with even the slightest possibility of embers getting into the roof. That would be a whole other amount of work as well - the current overhangs / soffit area is all stucco.
Any idea when and what exactly Lstiburek plans on suggesting regarding this topic?
Floris, thanks for the info. That sounds like an interesting approach. The tough part about option one with the ProClima is that we'd have to rip out the Cathedral ceilings in order to add the treatment and we'd want to use Mineral Wool instead of fiberglass (though I suppose Fiberglass is still preferable to foam). We're currently looking to go into the cathedral ceilings from above, replace all the canned lights with airtight IC rated cans, new insulation etc.
Regarding the German retrofit approach: this sounds similar to what I'm trying to accomplish, though you said it's a vented roof?
My current goal / hope has been to do the mineral wool insulation above and below the sheathing, followed by a batten system with metal shingles, on an unvented roof.
Joe
Joe,
Q. "Why is it that mineral wool above sheathing is typical only on flat roofs?"
A. I'm pretty sure that it has something to do with gravity, and worries about attachment of the roofing and top layer of sheathing through the insulation layer. Gravity complicates these concerns.
Q. "From what I read, the Owens Corning system is only for attic spaces?"
A. That is correct.
Q. "Any idea when and what exactly Lstiburek plans on suggesting regarding this topic?"
A. His proposal may already have been finalized, but I haven't seen it yet. It won't be considered for a vote by code authorities for several months. I'm pretty sure that his proposed approach will include the use of diffusion vents at the ridge -- that is, a type of housewrap-covered ridge vent that is airtight but vapor-permeable.