Non-toxic, fire resistant roof insulation for unvented roof
I am following up on a thread started here a couple years ago by someone else:https://www.greenbuildingadvisor.com/question/unvented-attic-insulation-options
I am building a home with an unvented mono-pitch roof in climate zone 6 in So California. We are in a high wildfire zone (this is actually our fire rebuild), and a vented roof is not an option, due to the fire risk. The roof insulation needs per code to achieve R-30 with low permeability (Class II). We will be putting in a clay tile roof, which satisfies the fire code, although we would prefer the insulation layer to also achieve Class A for fire (not required, just preferred by us the owners). We are also trying to build with low embodied carbon and as non toxic as possible. So far, the only thing close I have found to an alternative to spray or rigid foam (which is obviously petro-based, so we would really like to avoid it) is multiple layers of cork (e.g. Thermacork), which has a Class B fire rating. It is also shipped from overseas (and expensive), which is not ideal, but we can live with that if need be. Maybe there is a layer we can put on the outside of it to improve its fire rating. Anyway, if anyone knows of a) solutions other than cork that can fulfill all our goals, or b) a layer we can put over cork to improve its fire rating without messing us up on code, that would be much appreciated.
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Replies
dianemm,
Can you clarify your climate zone based on the map in this link: https://www.greenbuildingadvisor.com/article/climate-zone-map-including-canada
There may be solutions which are available to you based on being in zones 1, 2 or 3.
yes i think you are using the CA zone map which is not what everyone else uses (IBC). I would suggest vapor diffusion port if it is zone 1-3 as malcom alluded to. Otherwise you could use rigid rockwool exterior insulation which is good for fire resistance. Also if you are in WUI zone class a is required.
freyr_design,
That's what I was leading towards too. You can use a vapour-diffusion port at the ridge and avoid any roof top insulation or venting. https://www.greenbuildingadvisor.com/article/vapor-diffusion-ports
When I first read through your question I was under the impression that you were looking for a fire resistant insulation for the inside of the unvented roof assembly, in which case I can't think of anything better that Roxul Comfortbatt.
However, with your mention of the cork, you seem to be wanting an exterior insulation. I would have to ask why you think you need it? I would rather cover the entire exterior surface of the roof sheathing with a high heat underlayment like Henry Blueskin PT200HT or something similar.
https://henry.com/commercial/roof-underlayments/blueskin-pe200ht-high-temp-roof-underlayment
You don't say what your roof pitch is, but a clay tile roof is very heavy. Have you considered a double locked standing seam metal roof, or perhaps even a rubber tile roof? I had never personally thought that a rubber roof tile could be fire resistant, but apparently they are, and a lot lower cost than clay.
http://www.allproroofing.ca/roofing/rubber/
You might also consider using high heat, fire resistant roof sheathing like LP Flameblock or similar.
https://lpcorp.com/products/panels-sheathing/fire-rated-osb-sheathing
And finally, of course, all your exterior fascia and trim should be cementitious, from a company like James Hardie.
https://www.jameshardie.ca/products/hardietrim-boards
Look at TimberHP wood fiber insulation, should meet your fire resistance requirements
https://www.timberhp.com/products/timberboard
Great thread everyone. I am sorry for the duplication with the thread started today s=eparately. I had never seen these responses - was waiting for email updates, per the indication on this website, LOL, and got busy and distracted. I am getting a lot of great info from you all. Very helpful!