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Can a rainscreen become a chimney for fire?

Josh Engle | Posted in Green Building Techniques on

I have a client who is concerned about fire in a rain screen I recently put up to go under siding.
This is a wall of rough cut 1 inch board sheathing covered in typar and taped at all penetrations and seams. The typar is the primary air barrier. The rain screen is 1X3 (3/4 thick) strapping running vertically every 16″ oc. The bottom is open and covered with fiberglass mosquito netting and the top is open to the soffit. The soffit is not vented into the attic. I am planning to insulate with dense packed cellulose. The siding will be vertical grain unpainted cedar.
My client is concerned that the 3/4″ space between the siding and the sheathing will be able to act as a chimney to spread fire, should a fire occur. I’ve never seen anything written about this. Any ideas?
This building is in southern Vermont, so there is no great danger of wildfire.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Josh,
    The issue came up in the comments posted on this page: All About Rainscreens.

    In response to a similar question posted there, I wrote, "In most U.S. jurisdictions, building inspectors approve of rainscreen siding installations.

    "In some areas of the country with high wildfire risk, questions have been raised concerning whether a rainscreen gap poses a risk during a wildfire. The code is somewhat ambiguous on this issue. When the issue was discussed on the GBA website, the controversy generated strong emotions. You can read the previous threads here:

    Rainscreen performance during wildfires

    Rainscreens, Wildfire Hazard and Other Unintended Consequences

    "The bottom line: if you are worried about this issue, talk to your local building department."

  2. Mark Stevens | | #2

    Here in Massachusetts, where we've adopted the IRC, Section R302.11( Firebocking) therein seems fairly definitive: "In combustible construction, fireblocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space." As a residential architect, I can't in good conscious vent a rainscreen into a roof.

  3. Andrew C | | #3

    @ Mark:
    I can't tell for sure if you are distinguishing between a) designing a wall with a rain screen, and b) designing a wall with a rain screen that vents into a roof. Those would be two different things. Option b would be ill-advised, IMO.

  4. John Clark | | #5

    Per Joseph Lstiburek , the founding principal of Building Science Corporation, ".... When the air gap is small…less than ¾ inch…the friction from both surfaces bounding the air gap limits the air flow. The boundary layer on both surfaces is an effective fire stop[9]. ..."

    Search for the report titled bsi-098-great-fire-london on Building Science Corp website.

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