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Community and Q&A

Rainscreen performance during wildfires

Taylor Webb | Posted in General Questions on

I have started to work with some of the homeowners who were affected by the recent Colorado wild fires. To that end, I am putting together a presentation to begin to explain to local homeowners how to rebuild in order to best resist future fires. The presentation will highlight how many sustainable construction techniques for improved energy performance and durability also have benefits for wild fire resistance.

One strategy that I have not yet resolved is whether or not a rainscreen would be beneficial or detrimental (or neutral) to the overall system from the standpoint of fire resistance. The system that we are considering would be a cement board rainscreen over continuous mineral fiber board insulation over structure (preferably CMU though it will most likely be 2x stud). Is there any danger with regard to a chimney effect being created in the air space between the rainscreen and insulation? It seems that special concern would need to be paid to the furring strips; wood is clearly a no-no, steel seems to be preferable since it is non-combustable though I have some concerns about the steel losing strength under extreme heat (special attention would have to be paid to fasteners as well). Would the rainscreen provide any benefits by creating a buffer between itself and the insulation?

Has anyone come across any information on this? Tests, data or precedence? There is another thread on GBA that started to discuss the issue:

https://www.greenbuildingadvisor.com/community/forum/general-questions/19325/rainscreens-wildfire-hazard-and-other-unintended-consequence

though it provides little specific information and quickly (d)evolves into a discussion on moisture issues.

Any information or thoughts would be appreciated.

Thanks!

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Replies

  1. Lucas Durand - 7A | | #1

    Taylor,
    I like your questions - resilience to wildfires is something I have thought about a fair bit.

    I think Riversong is right that a rainscreen adds unecessary complexity in some climates,
    With my own house, I chose to go with no rainscreen - because of climate, but also because of concerns about "chimney effect" in combination with a combustible cladding.

    I think you're right on with the mineral wool sheathing and cement board.

    This is just speculation:
    If the entire cladding assembly is non-combustible (cement board over steel furring) and is installed over a mineral wool sheathing, I imagine the "chimney effect" (convection) would exist but would not be as consequential as if the wall were otherwise built (combustible materials).

    I think it could be possible that the cement board, when back-vented, would act as a sacrificial "buffer" to what's behind...
    I wouldn't worry about steel furring and screws losing their strength when heated...
    If the fire is raging fiercly enough to weaken the steel to the point that the cement board falls off, I'd say you'd be taking your chances with any approach you choose.

  2. John Brooks | | #2

    nevermind.......off topic.......

  3. Malcolm Taylor | | #3

    One consideration would be whether the cavity was vented at the top as well as the bottom. Most jurisdictions already do not allow the top of the rain screen to terminate in the roof space, but i would imagine that in areas prone to wildfires, the benefit of having any venting at the top would be outweighed by the risk.

  4. User avatar
    Albert Rooks | | #4

    Taylor,

    Great stuff.

    Perhaps I'm oblivious, but in the assembly that you propose, what will support the fire to burn?? I see no fuel in the cavity. What am I missing?

    Btw... The building insulation products like Roxul comfort board have a higher binder content since they arn't intended for a high temp application. The industrial products like Roxul RHT 60 or 80 have less binder in them. They are easier to compress, but won't smoke off the binder. The binder has urea formaldehyde in it so less might be better...

    If the only wood in the cavity is the furring strip, I see no issue with using it. Is it not possible to get fire resistant 1x4's?

  5. Kevin Dickson, MSME | | #5

    Hi Taylor,

    This past weekend I had the same discussion with a builder friend at his house in the forest above Boulder.

    Reviewing videos of the burning houses would be the most helpful.

    The cement board is a great choice. No doubt it has a fire rating that you can compare to other choices. It wouldn't cost too much to mock up a couple of different 4x8 walls and hit them with a blowtorch, measure some temperatures and see what happens.

    In Boulder County where he is, he explained an ambivalent requirement up there - no soffit vents are allowed, but ridge vents are required. It turns out that this lack of airflow won't ruin a roof in Colorado like it would in a more humid environment. And it helps keep fire out of the attic.

    Windows are the weak point. Once they shatter on two sides, then there is a roaring fire-laden wind coursing through the house. Any chimney effect behind a rain screen is negligible compared to that.

    I think the code officials will be making some recommendations after they look at the information that they are collecting now. Like prohibiting vented attics altogether.

  6. Robert Hronek | | #6

    I dont think a rain screen wall is needed in CO. There is also a paper by BSC named Mind The Gap. In it they discuss how very small gaps can provide the necessary drainage behind siding. Small gaps in what can be provided by crinkled house warps. Joe L even proposes that siding manufacturers put bumps on the back side of the siding. I believe there was a house wrap blog that talked one that had nubbies on it for that reason

  7. Aj Builder, Upstate NY Zone 6a | | #7

    Why rainscreens in Colorado? A lessor approach is all you need there.

  8. Taylor Webb | | #8

    Thanks to everyone for your input, it is much appreciated.

    A few responses:
    -Regarding the rainscreen in Colorado: I agree that it could be a little bit of overkill considering our dry climate. However it does still seem to have some benefits. We are looking at doing a zero overhang detail; the intent is to avoid thermal bridging and air barrier penetrations with the cantilevered roof joists. Also, eaves tend to trap hot gasses and embers as they move up the face of the wall and so make the area just below them more prone to combustion. It seems that the rainscreen would provide additional moisture protection/drying capability to help offset the lack of an overhang. Plus, as Lucas points out, the cement board would act as more of a buffer to the wall behind if it has an air space between it and the insulation. (it turns out that Roxul burns at almost the exact temperature as the lead wall of flame in a wildfire, which is the hottest part. Intuitively it seems that any measure to delay that heat from penetrating the wall assembly would be beneficial as that lead wall of flame tends to move past fairly quickly)
    -3. Malcolm, I'm thinking that the rainscreen would be vented at the top. Without an eave or vented attic space, seems like it would be a non-issue.
    -4. Albert, that is great info on the different types of Roxul, The devil is in the details. It is possible to get fire retardant treated wood furring.
    -5. Kevin, I'm glad to hear that you are working on this as well! We should put our heads together sometime soon to compare notes. Our plan is to wrap the insulation/air barrier up the wall and around the roof sheathing so that there is no attic space to vent. (it's incredible how many of the Passive House strategies are directly related to fire-safe design; i've made a fairly long list of them...)
    -6 & 7. Robert and AJ, you are probably right that rainscreens are not usually needed here; but i'm still leaning towards using one for the reasons above.

    Thanks again for your comments! I'm going to keep researching this and will likely have more questions...

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