R30 air impermeable non-vented roof insulation without petrochemicals?
For my wildfire rebuild in Los Angeles County, we are putting in an unvented cathedral roof with clay tiles. It has to be unvented for fire safety. This is requiring insulation with air impermeability that also achieves R-30. We are trying to minimize embodied carbon and toxic chemicals, and all conventional options are petro-chemical based, which we hope to avoid. One suggestion was Thermacork, but it is extremely expensive to achieve the R-value requirement (at 8 inches thick), and the lab data is not clear that it would meet the impermeability requirements – I am waiting to hear back from the company. Any suggestions for solutions are welcome!
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Put up strapping to accommodate the thickness you will need, and then blow in fluffy stuff: either cellulose, fiberglass, or wood fiber to reach your R-value. Then cover up the strapping with a smart barrier like CertainTeed MemBrain or Intello Plus, both are < 1.0 perms when dry so they should count as Class II vapor barrier. You can possibly even use the membrane as your netting to support the blown-in insulation, but if it's going to be really heavy you will still need strapping across the rafters to support it.
Thank you, Matthew25. I apologize - my original question had an error that I have now corrected. The insulation needs to be air impermeable with no vapor barrier.
I can't think of any material other than cork that has a chance at meeting all of your requirements, and I'm not sure I would even trust cork in that kind of assembly. Which of your requirements are you most willing to compromise on?
We can't compromise on vapor permeability, air impermeability and R-30. Those are in code. We would prefer to not use petrochemicals - or at least to have as low embodied carbon as possible. We are willing to pay more for it. I am not getting a straight answer yet regarding whether cork meets the air impermeability requirement (although I know an architect in Nor Cal who has done it a few times, but our building department is tougher down here). Why would you not trust cork, curious?
Clay tiles are vapor open so as long as you use a vapor open underlayment under them, you can use regular batts.
If this can't work, you can look at a vapor diffusion port at the ridge. The diffusion port can be made with gypsum sheathing which is not combustible. Again, the roof can be insulated with regular batts without any vapor barriers.
Thank you! Air impermeability is the other requirement. Batts are typically air permeable, unless there is something I don't know, in which case, school me please. :-)
Air impermeable, or vapor impermeable? And do you have a link to a code reference so we can take a look and make sure you have these terms correct?
Air IMpermeable and vapor permeable. I am 100% sure this is correct per approved building plans and an explicit conversation today with my architect.
dianemm,
I'm pretty sure the requirement is for an air-barrier in the roof assembly, not that the insulation needs to be air-impermeable. That can be achieved in a variety of ways on the interior.
Thank you, Malcolm! I think this is the relevant code: https://up.codes/s/unvented-attic-assemblies. (I am in Climate Zone 6). LA County also often refers to LA City, whose code is here: https://up.codes/viewer/los_angeles/ca-building-code-2022/chapter/12/interior-environment#12. As you will see, the question of air impermeability is a bit nuanced. I can't tell you, everyone, how grateful I am for your interest and weighing in.
You are not in climate zone 6 per irc, what you are citing is a CA specific zoning chart when you say 6. You are either in 3 or 2. Also your confused on your roof construction requirements, or you architect is.
We are in ICC Climate Zone 6. LA County follows ICC. Also mentioned that because it is referenced in the code I posted. While I am 100% sure that what is in our plans that I copied and attached in this thread meets local requirements, I am not 100% sure that this is the only we can go to meet our local code requirements. If you have a suggestion of what we can do to meet code and be healthy, fire safe, and non- (or less) petroleum based, I certainly welcome it!
You are in IECC Climate Zone 3B. Section 1202.3.4 does not apply to you. Problem solved.
Thanks.
Here is the text in our plans - see attachment. I am trying to find an alternative to the spray foam specified.
That is only one, very limited way, to build an unvented cathedral ceiling. There are a number of other code approved methods you could use.
Would you care to offer examples?
You are in IEEC zone 3, so I said earlier you can do batts only as long as the roof has enough permeability. Here is some more details about it:
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
The batts can be any fluffy you would like. Fiberglass, mineral wool, cellulose, they will all work just fine.
No need for any spray foam.
Photograph 6: Fireproof Diffusion Port Layer shows the diffusion port I mentioned earlier that will work in a fire zone.
As akos mentioned a vapor diffusion port would work in your area. Alternatively you could use exterior insulation, rockwool is ideal in fire country but it is in low supply right now (you can find it out of Canada right now), but other insulations also work, ie cork, wood fiber, etc.
I realize it’s confusing and it is 100% CA fault for naming their climate zones that way but you are indeed in zone 3, so when you read anything in code that refers to climate zone use that. The only time that zone 6 has any bearing is in reference to CA T24, and not in reference to any adopted irc codes. You can basically forget zone 6 completely.
If you need more resources, there are a lot of articles discussing unvented cathedral ceilings and insulation ratios on this site and on building science corporation.
Or, alternatively, you could use a vented assembly and just use a fire rated vent screen, like Vulcan vent.
Thank you, freyr_design and Akos so much for the idea (and for setting me straight on ICC climate zones). I am going to take the information a vapor diffusion port with wildfire specs back to my architect. Anyone have ideas about more sustainable alternatives to gypsum that would work in this configuration? In case it is relevant, we are planning on using lime plaster on the walls (straight on top of wood fiber batt and/or board insulation, availability permitting) and ceiling, and non-structural wood beams on the ceiling as well. You all are the best. Thanks for your patience and help!
I think you are well past digging the weeds if you are worried about the greenness of a sheet or two of gypsum sheathing. Never mind that it is not even inside your house.
Important part with getting good efficiency and durability with these is a solid air barrier. This generally means taped roof sheathing seams before the roofing underlayment goes on, make sure this is part of the install.
Thanks, Akos. My other half is German, comes from an old school building trade family, is passionate about healthy building and cradle to cradle materials, and loathes drywall. :-)
So don't get me wrong, lots of people want to eliminate petrochemicals from their lives, I get it, valid idea.
To put a number to it, a 3000 square foot roof with 5 inches of iso foam[R30] amounts to about 370 gallons of oil[per Martin 10 years ago if my math is right]
About how much the average 'merican burns in their car every 6 months
Since I live in the Northeast and we[and myself specifically] heat with oil, I have perhaps a better correlation of oil in the abstract, and in the specific, with a big tank outside and all.
When I am putting 100 gallons of 'oil' on my roof, it is preventing me from burning 100 gallons of oil, every year, forever. Or at least, realistically, 2 roofs. 50 years if I am somewhat lucky.
So 100 to save 5000, I'll take that deal.
Pretending of course that any form of insulation is zero petroleum.
I believe the stupidest thing you can do with a gallon of oil is burn it.
So, while we all have a kind of visceral reaction that petroleum products are 'bad' it is significantly less bad to put them to use in preventing the burning of petroleum products for a long period of time.
California at present burns natural gas for 42 percent of its electricity. So no matter how you heat[or cool more specifically] it still means petrochemicals
Just putting that out there so you don't feel like there is a lose side of this decision making process
I hear you. Lifecycle emissions are important to bear in mind. That said, we would like to push the envelope as much as we can on letting the dinosaurs rest in peace.
I should also note in response to what we are willing to give up is that even more important than carbon emissions to us is health (non-toxicity) and fire resistance. As much as we profoundly wish to be climate protective, our health and safety are even more immediate priorities. We ideally would like to have all of the above.
dianemm,
Add to those the threat of a bad cure. Rarely a week goes by that we don't have a thread in the Q&A about an irremediable spray foam problem.
Yes, ugh. Seems perhaps rigid foam gets less complaints? But I would still really like to find another route. Any suggestions welcome!
Never heard a problem with sheet foam
I wish someone would answer what goes on with bad spray foam, I always assume bad mix, but it is a bit puzzling.
cost no object-foamglas T3.
Thank you!! Really interesting. Looks like it is made just of glass and soda lime (not to be confused with lime soda in a glass - ouch, sorry, long day). Could not at first glance find anyone who sells it except Passivhaus in the UK, which seems to import it. I will call the mfr. If you know of a US distributor, I welcome the information. This could make life easier by being plug and play into our existing plans.