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Community and Q&A

Rainscreen with exterior rockwool in the WUI (Colorado)

lastlightcon | Posted in General Questions on

Hey folks, 

Long time lurker finally going for my first thread. Thanks in advance to everyone. 

I’m building a 1900sqft house in central Colorado mountains in climate zone 6B around 8600ft in elevation. Our county recently adopted the 2021 IWUIC.

The build is a 2×4 double-stud 16″oc and a 5″ cavity between walls. I’m going dense packed cellulose, triple pane windows set in the middle of the assembly with Siga Majrex on the inside and Siga Majvest on the outside for a vapor open assembly.

My current exterior plan is to use fire-retardant plywood, then Majvest, then a rain-screen (furring strips cut from the fire-retardant or metal hats) plywood, then Chemco treated wood siding, natural thin stone veneer, or metal sidings. However, while this gets me multiple levels of class 1 ignition resistance, I don’t believe it’s actually a true fire rated wall. 

So what, could I do to make this wall more wildfire resistant? 

As a note, I’m well aware of all the other important factors that go into hardening a home. For the sake of brevity, let’s assuming I’m nailing all those and just focus on just the exterior wall assembly.

Here are the rabbit holes I started chasing and where I’d love some opinions:
1. Seems like a rain screen around the 3/8″ – 1/2″ gap with ember screens would probably mitigation the risk of fire getting into the channel. I’ve read most of the big(ger) threads on here about it, but curious what folks *actually* did and how it’s working. 

2. Exterior mineral wool seemed like an obvious place to look, but Rockwool only has a few UL listed assemblies, none of which are double stud. The closest application has a 1.5″ minimum thick exterior coating…but after calling rockwool it was unclear if exterior rockwool would do much since it’s not actually fire rated. Additionally: cost and carbon…

3. In doing a lot of google research, a common refrain is that while metal is non-combustible, it is efficient at transferring heat into the wall. Nucor makes “ThermalSafe” which is basically mineral wool between metal…so do I end back at doing rabbit hole #2 and suck up the cost of exterior mineral wool?

Here’s an article that has a similar exterior assembly – see figure 2

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  1. Ryan_SLC | | #1

    Seems to me, homes burn down because of the vegetation/fuel around them not their construction.

    You're chasing Rockwool that is thermal barrier rated, not wild fire tested.

    1. freyr_design | | #11

      This is not true, it is wildfire tested as well as fire rated with a multitude of assemblies. Also to my understanding it is ember storms that are one of the main reasons structures burn down. Obviously vegetation right next to house is bad idea, but it is not the only, or even main thing, that burns down houses in wildfires

      Here are their certs:

      1. lastlightcon | | #12

        freyr_design is correct, most home ignitions in wildfires happen from ember storms which have been documented in Colorado as more than 2 miles ahead of the actual fire. Because blowing embers is the likely form of ignition, penetrations are usually the culprit.

  2. freyr_design | | #2

    Rockwool is considered non combustible and at 1” thick satisfies wui requirements for roofing (I believe wall as well). Read through the actual code as mineral wool is mentioned as acceptable in multiple places, it does not need a UL cert. from a practical point of view rockwool will not burn, take a look at some of the testing done with it in comparison to foams.

    Also if you are trying to decide on exterior materials, take a look at the CA state firemashal list of approved products. They have to satisfy fairly strict guidelines to achieve that listing.

    Last, if I had exterior rockwool I would not really worry about the rainscreen gap as there is a noncombustible material behind siding. If not, I would probably install a continuous fire rated vent on bottom, Vulcan vent makes one that works.

    If it were me, I would spend the extra on exterior rockwool, in the long term I think it is cheap insurance.

    Edit: lastly, I would personally just use standard cedar for siding, it is approved for use on walls by CA state firemarshall and it doesn’t have any of the chemicals, which I’m not sure what they use, but I bet are nasty.

    1. lastlightcon | | #3

      Thanks, appreciate the reply. I've read through the 2021 IWUIC and don't see it calling out mineral wool. It does bring up mineral surface cap sheets, but that's all I'm seeing. Are you talking about in the IRC?

      Also, I don't think Vulcan makes a rain screen vent. The only one I've found is Tenmat's FF102/50, however it doesn't have a bug screen...though it's easy enough to make one.

      Do you recall where you saw that 1" rockwool satisfies WUI requirements?

      It's a good point about cedar, but I'm concerned about the sustainability of cedar harvesting. Plus, in as dry an environment as we are in CO and the price point, it's a tough sell. Check out Chemco's Firesafe pressure treatment system. It's really cool and doesn't have PFAS in it, unlike Flamestop and other topical treatments. Plus, because it's pressure impregnated there's no reapplication after cuts or every few years. Montana Timber Products, reSawn, Hewn, Metaverde, and a few other brands are using it with really nice results.

      1. freyr_design | | #6

        I will have to look later for the code snippet, I have saved in upcode and it is under construct. I know it is in CA WUI and CA adopts almost all international code so I assume it is in IWUI.

        Edit: oh also that piece is deep in Vulcan library, call them and they will tell you product number.

      2. freyr_design | | #10

        R337.5.2 Roof Coverings

        Where the roofing profile has an airspace under the roof covering, installed over a combustible deck, a 72 lb. (32.7kg) cap sheet complying with ASTM D3909 Standard Specification for "Asphalt Rolled Roofing (Glass Felt) Surfaced with Mineral Granules," shall be installed over the roof deck. Bird stops shall be used at the eaves when the profile fits, to prevent debris at the eave. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers.

        Exception: Cap sheet is not required when no less than 1 inch of mineral wool board or other noncombustible material is located between the roofing material and wood framing or deck.

        Alternately, a Class A fire rated roof underlayment, tested in accordance with ASTM E108, shall be permitted to be used. If the sheathing consists of exterior fireretardant-treated wood, the underlayment shall not be required to comply with a Class A classification. Bird stops shall be used at the eaves when the profile fits, to prevent debris at the eave. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers.

        Also here are a few of their 1 hr wall assemblies:

        Also, I realize you are dealing with CO but here is the CA listed product for exterior, many of them may also apply to your wui jurisdiction.

        Edit: lastly hopefully attachment works but this is the Vulcan vent to use with rainscreen, you will need to use 5/4 for furring as min thickness is 1” due to honeycomb structure.

  3. Expert Member
    Akos | | #4

    I'm not in a WUI area, just me thinking out loud.

    Double studs walls generally benefit from permeable sheathing. One of the more permeable products is exterior gypsum sheathing. I've used it for fire rated exterior assemblies, pretty easy to to work with. About the only thing is that it is much heavier than equivalent osb/cdx so tipping up longer wall sections could be a challenge. There are companies that offer it coated in a WRB similar to ZIP, harder to source but very quick install.

    Gypsum sheathing won't work on a roof though but you can get similar class A underlayment to protect a regular roof deck. Vented roofs are doable with proper protected vent, I would look at an unvented assembly though. This would mean either spray foam on the inside or exterior polyiso.

    If a fire is strong enough to burn through the metal and the gypsum, your windows and doors are long gone, your place is pretty much toast anyways.

    This avoids any of the complications of exterior MW, doesn't add all that much cost and keeps it close to a standard build.

    1. freyr_design | | #5

      How is lateral resistance achieved? Most of the apartment buildings I’ve seen using exterior gyp like dens glass usually achieve this in other ways than sheathing, like mass timber or steel. Though Simpson does have their strongerod ats system.

      1. Expert Member
        Akos | | #7

        Our code allows for gypsum sheathing only for bracing walls, I would assume others would as well.

        Not a scientific answer, but walls built with it feel very solid.

  4. Ryan_SLC | | #8

    You have ProRox which is the industrial application Rockwool. I spoke to Rockwool and they do not have ProRox rated for any home application. You have Comfortboard 80. All requirements are online for it.

    If Comfortboard 80 at 1.5" is UL listed as both an interior ignition barrier and and interior thermal barrier, I don't know if I would chase 1".

    But, regardless, if the fear is flame wicking up your rain screen, you have a vegetation/fuel issue. If flame is that close in or forest fire, it seems more than unlikely there would be a fire team trying to save your house.

  5. Ryan_SLC | | #9

    Is there a source for case studies that control for wealth variables for homes destroyed/saved in wild fires?

    I mean to say, I'd be a little skeptical of sweeping claims that a fire proof material house survives better always because the factor there is $. A home that is built with many expensive fire proofing materials is probably an owner that also spends money on professional fuel abatement/design on the "home ignition zone."

    So, is there any truly scientific studies that show seeking out lots of fire material parts, which are not inexpensive, have ANY RIO in real world wild fire situations compared to like equal homes in the area that didn't actively spend a grundle on individual components?

  6. rkymtnoffgrd | | #13

    I gone down many rabbit holes on building a house in a high fire zone... Here is the benefit of my research and my assembly: SW Colorado at 10,020ft, zone 7, this build is a good bang for the buck for fire resistance, insulation, air tightness and attempting a relatively straight forward construction.
    1) 2x6 @ 24OC (modified) advanced framing: Keep jack studs at all doors, jack studs on windows on load bearing walls only...eliminate jacks everywhere else, no cripples anywhere. Engineer windows to align to 24"OC so no additional stud/jacks are created... Keep double top plates. The key is to reduce lumber, thermal bridging, and keep strength.
    2) taped zip sheeting, taped/sealed over topplates, then taped/sealed to interior subtruss osb, taped/sealed to subslab air/vapor barrier, and then finished, taped and painted drywall, HRV, resulting in .95 ACH50 even with a wood burning stove installed. Electric Stove and Bath Vent routed to ERV, no roof penetrations, heatpump wash dry, no dryer vent.. Single side vented plumbing stack. Side Vented accessible wood stove penetration for keen sealing and detailing.
    3) 2" continuous exterior mineral wool
    4) 3/4" rain screen, with spark arrester venting at bottom, no top vent. (too much vent adds to fire hazard)
    5) 4" prefinished lap concrete siding, (4" exposure provides sufficient air vent for drying vapor open mineral wool even with unvented 3/4" rain screens.) With this minimal exposure bottom vents are unnecessary. If I did it over... I would keep the rain screen unvented. 4" is important as it increases strength of concrete siding, wall wind resistance and sear strength.
    6) 24Ga metal roof with vented truss assembly, 22" blown cellulose, spark arrestors over all lower concrete soffit vents.
    7) Attention to fire-wise basics (look up fire-wise communities)

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