Non-Combustible Construction
Hi,
I am considering building an off-grid cabin in a remote area in Northern California which is in a high risk area for severe wildfires. The priority for the structure would be first-and-foremost survivability in the event of direct hit with a major wildfire or fire-tornado (survivability of the structure and potentially an occupant). As such all decisions on structural and cladding materials would be made to support this priority.
I read the story at:
(although the original reference link seems broken)
https://www.greenbuildingadvisor.com/question/exterior-materials-for-unstoppable-wildfire-tornadoes
The discussion in that story largely centers on selection of fire resistant exterior cladding and roofing materials, but with the assumption of a wood framed structure, and the OP in that thread stated he did not want a ‘concrete bunker’.
I have somewhat different preference… I am fine with a ‘concrete bunker’ and am looking for building systems that can provide for a fully non-combustible building structure/envelope including the roof structure. Concrete, Rebar, Rockwool seem like good materials for this. Though, like the OP in that story, moving to Vermont is not a practical option for me.
My thinking is to use something like cast-in-place Thermomass with rockwool core for the walls, and pre-cast panels of similar composition for the roof. This would provide for building structure with a fully non-combustible composition. I would plan on adding rolling metal shutters to the windows and potentially an underground non-windowed cellar for shelter just in case I were unexpectedly trapped by a fast-moving fire and unable to evacuate.
Although I love ICF, I do not like ICF for wildfire areas because I think the outer layer of styrofoam would melt in the event of a serious wildfire, and most ICF uses traditional wood truss roof anyways. So I am really looking for a completely non-combustible building assembly which would have a high probability of surviving a severe fire with only minor damage. Insulation should be either non-combustible (like rockwool) or fully encapsulated in thick concrete so it can not burn/melt. I don’t want to rely on venting details to hopefully block embers from getting into the roof structure… I want a completely unburnable roof structure.
I have not found any examples of roof structures using pre-cast thermomass panels, or any fully non-combustible roof assemblies.
If you were going to build a cabin in wildfire territory, you only cared about ensuring survivability of structure and occupant in the event of an extreme wildfire, what building system would you use? Are there any fully non-combustible building systems that could give high-confidence survival in an extreme wildfire event?
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Replies
I try to convince clients to use the lowest-carbon option that meets their needs, but living in a wildfire zone limits your options. I would consider an insulating concrete block such as this: https://www.greenbuildingadvisor.com/article/maine-firm-develops-a-new-type-of-insulated-block. Or hemp-crete, with metal siding.
For your roof, I would consider framing with wood or metal trusses, then insulate above the roof with mineral wool, steel battens for venting and a metal roof. In some places venting is not allowed for fire reasons but if the assembly is non-combustible it might be allowed.
Here's a company that has a similar design for insulated block and has producers in Red Flag's region.
https://www.concreteproductsgroup.com/products/energy-efficient-wall-systems
With a layer of mineral wool on the outside in addition, one would hope that the interior could never get to the point of melting the interior foam, but even if it did melt, the minimal thermal bridging plus the air space should all help.
In commercial buildings, steel structural elements get coated with insulation to improve their ability to maintain structural integrity in a fire. Ideally the region of steel roof trusses in a non-combustible building would never get to the point where that was required, but it's something to think about.
Another material to consider using as part of the structure is fiberglass skinned gypsum board sheathing. I don't know if there's a version of that rated for use as roof decking, but that could be something to look into.
You might want to look at autoclaved aerated concrete (AAC). Sourcing the material might be a challenge (or not), but it would be more fire resistant than traditional construction. For more info, see https://www.aerconaac.com/fire-resistance.html#firegeneral.
"Off-grid" and "remote" are synonyms for "difficult to access." Have you considered how you are going to move all of that concrete, precast or not, onto the building site? Something smaller and modular, like Aercon or AAC, would be easier to move in small loads if necessary, with on-site grouting and reinforcement. I see that Aercon also makes reinforced panelized roof/floor slabs that might be usable for your roof, though again, size and transportation issues may govern.
Look up "non-com construction" ("non-com" is short for "noncombustible"). This is fairly common in the commerical world, and VERY common in critical facilities like telecom buildings. A lot of precast concrete gets used, you can even get entirely prefabricated small buildings (commonly known as "telecom huts" or "equipment huts"). The small buildings you see at the base of cellphone towers are usually one-piece precast buildings of this type.
If you want a fireproof building, you basically need concrete and steel. Block will be easier to work with in a remote location since it can be brought in in small batches. Precast structures usually need at least a crane for placement, and often special trucks to bring them in too.
For roofs and floors, post tensioned concrete can be used:
https://iibec.org/roof-decks-a-to-z-post-tensioned-concrete/
or the more common floor pan with a slab poured on top, which is how nearly all high rise buildings are constructed. You get a very solid structure this way.
Bill
In theory, one could use SCIP panels (for walls and roof) made with mineral wool instead of EPS foam. Ie, wire mesh with sprayed on concrete. Probably with steel/mineral wool shutters to protect the windows.
One could also build underground. Or +1 on metal studs/trusses, mineral wool and steel siding/roofing.
> survivability of ... occupant
Breathable air might be an issue.
have you considered MGO sips (sips with magnesium oxide) These are extremely fire resistant and have all the advantages of sips otherwise. Have you come up with any ideas for building a fire-resistant deck and outside stairs?
Thanks for these replies. By 'remote' I mean 40 minutes by dirt road from the town of Cloverdale which has a ready mix plant. The dirt roads are in good enough shape to get a flatbed or concrete truck up in the summer time, but block would probably be easier to manage.
I like the AAC and comfort block, but this is also a seismic area (yay Northern California! :-).. Can AAC and comfort block be used in seismic areas?
Are there any AAC roof panels that have additional insulation incorporated? Or would you start with the panel and lay mineral wool on top and a metal roof? I will look up Non-com construction.
Thanks
It's been a while since I investigated AAC, but it's my recollection that it is used a lot in Mexico and Central and South America. So I think it should work in seismic areas. Roof and floor panels are available. The units has some R value, but you would need to add insulation to get better performance. You'd probably need a small crane to lift the roof and floor panels, but there may be a workaround I'm not aware of.
I did look into MGO SIPS, but kind of got scared off from them after reading about corrosion issues. For outside stairs and decks, I would think concrete. I wouldn't want to have a wood deck or stairs.
SCIP would be a potential way to go if it could be done with Rockwool panels. Has anyone heard of SCIP with Rockwool?
Breathable air would definitely be a problem... im not really planning on being inside it in a fire, but since a long dirt road through wildfire country could be a bad choice once a fire is nearby, (just watch the you tube videos of the Paradise evacuation) I would plan on an underground basement/bunker with no windows and closable vents. 1000 cubic feet per person with a fire rated door separating the basement from the house should provide sufficient air for to wait out a fire as long as the basement can be reasonably sealed.
Maybe I should ask a different question since I am a homeowner/Landowner and not a builder: Does anyone know of a design-build firm in northern California that has done a project like I am describing?
There is this example, but it doesn't address the roof. Could be cast on site tilt-up.
https://www.rockwool.com/why-stone-wool/case-studies/precast-case-study/