Rainscreens and Fire Resistance
mikkelsen
| Posted in General Questions on
I consider rain screen details on exterior walls a mark of quality/intelligent building…but I had a thought that came up as I was considering what this wall would do in the case of being exposed to a fire, with the source being outside like in the case of a forest fire.
While the home may be sided with non-flammable siding, there is a nice air gap and plenty of oxygen flow behind this siding with the flammable plywood behind it.
Could this design to quickly dry your home, also quickly compromise it in a forest fire?
Thoughts?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Eric, Here is a link to an earlier discussion on the subject:
https://www.greenbuildingadvisor.com/community/forum/general-questions/19325/rainscreens-wildfire-hazard-and-other-unintended-consequence
wow, at first i wanted to say thanks, but after seeing what that link turned into, i'm not sure!
here is a different idea/question: What are other's thoughts on using a bit of fiberglass insulation along the bottom and top of the rain screen? my thinking is thus: fiberglass is not combustible yet it allows air-flow. will it stop flames and embers? if so then it seams like it might be a better product then using a "bug screen" at top and bottom of the rain screen. thoughts? how much would it take to stop a flame?
I don't think fiberglass would be a good choice. It doesn't do well when it gets wet, as it may well at the bottom the cavity. My first though would be Cor-A-Vent but I don't know how fire resistant it is. What about something along the lines of stainless steel wool?
would fiberglass dry slow? good questions...this is what I was hoping to find...solutions to an interesting problem...unlike the other discussion which focused more on what a rain screen is and whether or not it is needed. i'll look into how fire resistant Cor-A-Vent is.
Cor-A-Vent is only "heat resistant" which i think means it won't melt in the sun.
Rock wool has a much higher melting point than fiberglass, and wicks less moisture. Using rigid rock wool panels with a minimal rainscreen gap (2-3mm) as a capillary break would be fairly moisture-purging and fire-resistant. (It would be nice to see test data on that sort of assembly first though.) Using strips of lower-density rock wool in place of Cor-a-Vent would prevent the ember penetration issue, though it will slow the drying convection currents.
That idea sounds promising...seems like there would be more testing with these ideas already out there. How do rigid rock wool panels compare structurally to plywood?
Mineral fibres can act as a wick, as a catalyst. Turning a small thermal load into an intensive heat. Similar to a gas lamp/light.
Contact a civil engineer about chimney effects, about fire safety.
Here a European experience:
http://www.aftonbladet.se/webbtv/nyheter/inrikes/article14468019.ab
http://www.dn.se/nyheter/sverige/kraftig-brand-i-mjolby
good input, thank you. so is there truly no safe rain screen developed yet?
Well, there is.
But not for the design you have in mind:
"While the home may be sided with non-flammable siding, there is a nice air gap and plenty of oxygen flow behind this siding with the flammable plywood behind it. "
The problem with rain screens is the gap, the 'chimney' and the effect it has during a fire.
No one would build a chimney from non-fire proof materials.
The plywood in your design would feed the fire. No matter how safe the actual screening is, such a wall is a fire risk. Timber burns.
The big disadvantage during a fire (behind a screen) is that it is difficult to tackle, the exact point of combustion hard to determine and the rain screen doing what it is designed for: keeping water away.
I would think that a good compromise would be, as Dana suggests, a minimal rainscreen gap, enough for venting/drying purposes, but not enough to encourage rapid inflow of hot combustion products. Metal mesh covering for the openings, perhaps wrapped around something like Coravent, would act as a flame arrestor. Protection against combustion due to a multitude of small glowing embers blown up against the house is one thing. Protection against envelopment of the house by severe high temperature forest fire like conditions is another.
Hein, what rain screen design do you prefer?
" Hein, what rain screen design do you prefer? "
One which fullfills it's purpose.
...and does not increase fire hazard risk? do you have solutions? anyone else?
Edited for typos:
This is an old thread, but if I'm searching for this information, I imagine others are too.
You want a drying, but not fire-promoting rainscreen over plywood?
I'm no engineer or fire-safety expert, but how about this (inside to out)? Plywood, high-density rock wool exterior insulation panels (use two layers with offset seams if you want extra protection), vented aluminum hat channel to provide your air gap (instead of flammable 3-d plastic mats), then non-flammable siding. The plywood is well isolated from any fire and you haven't added a lot of flammable material in thin stands inside your wall.
You'd still probably want a bug screen top and botton to keep out bugs even if you weren't hoping for it to serve as a flame arrestor and keep out sparks. You'd also want to ensure that your channel was deep enough to stick past the outer surface of the insulation even once the insulation under the channel was somewhat compressed, but those higher density panels don't compress much.
The structure would also likely be pretty sound resistant. You could up that by using resilient channel, but you'd need to be sure it was strong enough to support the siding, wind loading, etc.
None of tbis is tested and only a structural engineer (or maybe the nice folks at Roxul) could tell you if there's a structural problem with the assembly. Probably nobody can verify its performance in a fire without testing.
And if you want a house reistant to wildfires, you shouod really get rid of any vented roof.
I imagine what Hein had in mind is something closer to commercial building techniques to get rid of the flammable sheathing, like cross-braced steel studs, DensGlasss fiberglass-mat covered gypsum, aluminum hat channel, then some sort of sheathing. The are also commercial products for hanging heavy studying panels well away from the framed wall. They use nothing very flammable, aren't cheap, and are not coming to run-of-the-mill SFRs any time soon.
Of course an obvious solution to helping prevent a wildfire from getting close enough to your building that it shoots up the wall would be to remove any flammable plants from around the building.
The worry seems to be that the fire will enter into the rainscreen gap from below. Why not add a piece of "L" channel to the bottom of the rainscreen? A "U" channel will be attached to the plywood sheathing at the bottom of the wall and then your rockwool panels will sit inside it. Attach your metal hat channels to the surface of the rockwool panel. Put your metal ventilated insect screen across the bottom of the rainscreen gap. Then add the "L" channel over the insect screen so that the bottom of it hangs down a bit lower (maybe 3/4") than the insect screen (the bottom leg of the "L" facing in towards the building).
Air can still get behind the "L" channel and into the rainscreen gap but fire has this extra "block" to get past. Yes, if it's a roaring wildfire it will get sucked up past the "L" channel but it should help to prevent embers from getting in.
One thing not mentioned is what happens to the fire when it roars up the exterior wall surface and hits the roof overhang? The soffit and fascia material better be fire proof (or resistant) and if there are any vent openings into the attic or under a cold roof they should be sized to prevent embers from entering.
https://www.vulcanvents.com/
In addition to the suggestions above, you could close the rainscreen gap at the top of the rainscreen. That would minimize the chimney effect inside the rainscreen. Water still escapes through the bottom opening of the rainscreen gap. (Yes not as effective at evaporating water vapor inside the rainscreen, by blocking that chimney effect and reducing the area open to outside air.)
In most cases, you probably know a wildfire is in the area, and could take steps to seal the lower rainscreen opening temporarily if you believe fire will burn all the way to the structure.
The point of my suggestion is to eliminate flammable materials from the chimney Mr. Hein was discussing.
You won't get a fire burning in that chimney with non-flammable material (stucco, Hardie Board, etc.) for siding, non-flammable rock wool insulation protecting the plywood, and non-flammable aluminum channel maintaining the air gap. Penetration of some flame or sparks through the insect screen becomes almost irrelevant.
Some years after this discussion, and as I recall occasioned by the fire At Grenfell Towers in London, Joe Lstburek of BSC chimed in on rain-screen cavities. He suggested they were too narrow to sustain flames.
If the cavity was only open at the bottom (as the vast majority of well-functioning rain-screens are), I wonder whether the non-flammable material needs to extend much beyond say, a foot from the bottom? This would protect against the few embers that are swirling at the base of the building that make it into the cavity. Perhaps the solution is as simple as running some flashing up the WRB at the bottom?
The wildland attic vent provisions intend to keep embers out so they won't start a fire in the attic. One of the options I saw discussed ( not sure it was approved though) was using a very fine mesh screen. In municipal water tank vents they use a #24 mesh to keep bugs out, it is an opening size of 0.034" to 0.028" so quite small. Perhaps this is small enough to keep embers out while letting air in to dry the assembly. It is available in aluminum & stainless at McMaster
The County of San Diego has wildland compliant wall assemblies, none are rain screen. I think it would take a lot of work to get one approved in San Diego.
I guess another question about approval in wildfire areas is whether building really benefit from them in most of the places where wildfire risk is highest? Not being allowed to include rain-screens in BC would cause a lot more problems that in San Diego.
What about something like this: Gold Bond® eXP® Fire-Shield® Sheathing
They describe it as fire and mold resistant and designed to go under cladding.
https://www.goldbondbuilding.com/products/sheathing/exp-fire-shield-sheathing
MetlMann,
faced gypsum board sheathing is fire resistant, but very seldomly used in residential construction.
Yeah I thought it looked like a commercial type product. It does seem like it would have a place in the rain screen universe. Too expensive for most residential? Builders unfamiliar with it? Difficult to install? Just curious as to why it's not used.
I'm planning a transformational remodel of my early 90s, mass produced, builder's home. The big considerations for me are increasing energy efficiency with out-sulation and adding fire resistance with metal siding. We had a wildfire in 2011 that destroyed several houses right around me, but fire fighters managed to get into the neighborhood just as the fire was coming up our backyard. If I'm going to spend money remodeling, fire resistance must be part of the plan. Austin is considered a humid climate and we certainly have torrential rains, powerful winds and wild temperature swings so it seemed logical that a rain screen paired with metal siding would be something to investigate.
MetlMann,
Unlike small scale residential buildings, large commercial ones usually rely almost entirely on their framing members for their structural integrity. The common residential sheathing - OSB, Plywood - provide high shear resistance, and can be used as backing for furring, cladding and trim. If you are going to substitute a gypsum-based product you will need to figure out how to make up for the loss of those attributes, especially in a high wind area.
Regarding shear and gypsum sheathing, there's a product for that! https://sureboard.com/
It's basically gypsum board with a sheet of metal laminated to it (which makes it a vapor barrier, so keep that in mind). It provides lateral strength for the walls and you can fasten things to it with screws.
I've seen engineers specify it on commercial projects I've worked on, and I've specified it on an interior shear wall in a residential project where it solved an alignment issue that arose during construction (1 sheet of sureboard is thinner than 1 sheet of gyp + 1 sheet of osb).
I assume it's more expensive and harder to work with than osb and plywood.
Good points.
Attached is a photo of mineral wool board insulation. It is used as exterior insulation. A continuous flame is being applied to this sample, and it fails to ignite...They run this display all day long at conferences to advertise their product, and demonstrate that it is fireproof.
https://www.rockwool.com/north-america/products-and-applications/products/
Note in the web page photo, you see the product installed, with vertical battens that create the rainscreen behind cladding (cladding not yet installed in this photo).
Rockwool is my first choice. It seems to be versatile, so whatever system we come up with, it should have a place. The house is one of those abominations with "three sides brick" - full two-story brick in front, half brick on sides, composite siding everywhere else. That presents a challenge, but I'm open to anything that improves energy efficiency - including covering or removing the brick.
I am doing an exterior wall with LP Flame Block over the top R-8 Zip sheeting. No rain screen. Cement siding. https://www.huberwood.com/uploads/documents/technical/UL-U364-Fire-Rated-Assembly-using-ZIP-System-R-sheathing-UL-Assembly-Wall-ZIP-System.pdf
BryanBend,
For wildfires it sounds like a good wall. Depending on what climate you are in it may be quite risky from a moisture accumulation perspective. Both the sheathing on Zip-R, and the OSB based LP Flame Block are vulnerable to moisture damage as they are located where they can not take advantage of any warming to help drying. Coupled with a reservoir cladding like cement siding which draws moisture back into the wall, and no capillary break or drying path to the exterior through a rain-screen, that can be a recipe for problems.
Thanks: Northern Idaho: what do recommend? Maybe outside insulation rock wool or adding a rain screen?
Cold climate zone 6 and fire zone.
Thanks Bryan
I believe you can make the rain screen gap small enough (3/8"?) that it doesn't appreciably affect fire resistance, while retaining most of the benefit of the rainscreen (capillary break, drainage).
Lstiburek talked about this somewhere on youtube but I sadly cannot remember which lecture. It was a longer video, at least 40 minutes. There are only a handful of them from him this long, and all of them are good.
I think I’ve seen that video of Lstiburek, but for an easier-to-scan version, there’s this post of his: https://buildingscience.com/documents/building-science-insights/bsi-129-wildfire.
> keep the vent space ¾ inch or less to limit fire propagation. … The main objective to limiting fire spread is to slow air (oxygen) from feeding a fire. Where gaps are ¾ inch or less the fire risk is minimal due to boundary layer friction of the two surfaces.
My notes from this video from Delta — https://m.youtube.com/watch?v=B2i_sLVxQqg — say that a 1/8” to 1/4” gap will drain water without allowing a lot of air movement, but I’m not sure where in the video you’d find that.
Bryan,
With cement siding on one side and the LP Flame Block on the other, I don't think a thin rain-screen cavity such as Andy Frog suggested is a fire threat - especially if the bottom is protected by screening for embers. I wouldn't vent the top of the gap to prevent air-movement.
Hi Malcolm: Thank you for the information: I said exterior wall with LP Flame Block over the top R-8 Zip sheeting: I meant to say: Exterior wall with Zip R-6 over LP Flame Block; that what the https://www.huberwood.com/uploads/documents/technical/UL-U364-Fire-Rated-Assembly-using-ZIP-System-R-sheathing-UL-Assembly-Wall-ZIP-System.pdf
Huber Recommends Zip R over the Lp Flame Block!
I think that doesn't make sense for exterior fire.
Zip R use foam board under the osb and tape that is not flame resistant.
I might use the LP flame Block over the Zip R 6.
I found a fire block FF102/50 at http://www.Tenmatusa.com 1/4 in thick that will for my rain screen.
Now I am searching for 1x2 fire treated fir or pine for the battens.
Any comments would appreciated! Thanks Bryan
I raised a similar question last year and you may find the comments helpful.
https://www.greenbuildingadvisor.com/question/fire-and-external-wall-insulation-your-thoughts
I was designing a wall section with 2" of exterior EPS insulation, and 3/4" furring for the rain screen. I needed something at least 3/4" in order for siding to have enough material to hold nails and not have to nail all the way through to the studs behind 2" of EPS.
I received assurances that there was no important wildfire threat. But I had no real fire testing data to support that. So I ended up moving to a different solution, partly in response to the fire issue but also out of a desire to move away from using petro-chemical based insulations. So now I am not doing continuous exterior insulation, which means I can go down to 3/8" furring attached directly over the WRB, to the studs. Then my siding can easily nail through to the studs.
To compensate for the lost insulation I am doing 7" thick stud cavities and dense packed cellulose.
Even without testing I'm confident that a 3/8" gap with Cor-a-Vent screens will block the passage of embers into the gap. As an additional assurance, and to reduce moisture issues as well, I require that there be a 2' wide band of gravel around the perimeter of the house with no plantings or irrigation within that 2' zone.
I've attached a partial detail to clarify.
edit - I'm adding another thought. Intumescent paint is a bit pricey but one could paint the back faces of the lowest 2 courses of siding, or the lowest 1' of the WRB with intumescent paint. Then if flames do manage to find their way into the 3/8" gap that would set off the paint. And it wouldn't take much paint to just do the lowest parts.
Wildfire vs rainscreen gap vs continuous exterior insulation
This is the kind of thing that happens so often with building code development. Individual problems are viewed and solved independently, not always with consideration as to their impacts on other aspects of the structure.
When I was leading multi-disciplinary engineering teams I so often had the conversation, "Great you've solved the HVAC (or structural or whatever) problem. But you've now created a new problem for the architects. That's not progress."