Staying Fire Wise: Attic Vents and Wildfire – StrawBale.com

I live in the country and am quite aware of the places fires start: leaves under the deck, debris in foundation vents, trees not properly maintained near the house, etc. If you live in fire prone areas, be sure to read on about attic vents and wildfires.

forest wildfireDear Andrew,

My sister was an evacuee in the major San Diego Fire. She lives in Rancho Bernardo. She is an area coordinator for neighborhood watch. At the last meeting after the fire an attending fire captain explained an interesting phenomena many of the homes that were destroyed and had tile roofs had something happen to them that most other homes did not.

The fires started in the middle of the building not the out side. After a through investigation the fire dept. came up with two causes. The first was that homes that had curved tile roofs and did not have bird blocks embers blew between the tiles and started the roofs on fire under the tiles. The second cause was that older tile roof home had attic vents that had 1/4 ” mesh screen or larger and the embers just blew through the screens into the attic causing the fires. When the winds came through the Rancho Bernardo area they were around 40 to 60 mile an hour.

It was like a blast furnace and temperatures in some areas exceeded 2000 degrees. With the wind blowing in a horizontal direction the heat and the winds blew embers and extremely hot air into the attics of the homes. The fire dept. is now recommending that new homes and older ones be fitted with attic vents of 1/8 ” to prevent small embers from being blown into the attics. The only problem here is that the mesh being extremely thin my also melt or burn at lower temperatures. You may want to pass this information on to straw bale builders and run it by your local fire dept.

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16 Responses to Staying Fire Wise: Attic Vents and Wildfire – StrawBale.com

  1. Avatar
    Lani Ball Wed, January 16, 2008 at 9:43 am #

    Andrew: Very much appreciated this latest article on attic vents/wildfires.
    Wouldn’t some kind of metal louvres block the embers coming through the wider wiremesh screens, wider mesh being more efficient for actual ventilation of moving air.
    Could metal louvres (installed as a backup to the wire mesh coverings) be controlled by an electric switch mechanism somewhere in the house below for quick manipulation of closing louvres in an emergency situation?
    Could someone explain what “birdblocks” are? Would that be something to keep birds from nesting in the open rooftile ends??

  2. Avatar
    Ray Smith Wed, January 16, 2008 at 1:11 pm #

    Attic vents have been known to be a serious wildfire problem for a long time. The Wildland-Urban Interface Code has prohibited soffit vents for years since hot gasses tend to concentrate beneath soffits and then circulate up through the vents and into the roof structure causing ignition. Soffit vents are now illegal in California and some other jurisdictions. There’s no good workaround that I know of. As a result, some experts now recommend unvented roofs, which of course introduces additional complexities to roof design.

  3. Avatar
    Mark Frazier Thu, January 17, 2008 at 10:39 am #

    Andrew,

    our new (Straw Bale) house in California was NOT ALLOWED to have soffit vents. We had to change the plans for low roof vents along the edge. The strange part is the house next door was built with sofit vents. The only reason I can think of is that their plans were approved in the prior year, maybe before the regulation took affect?

  4. Avatar
    Jered Bonbrake Thu, January 17, 2008 at 4:17 pm #

    Why are we considering vents as merely a hole cut with some sort of covering? If you want to vent an attic in a fire prone area, who says the vent cant be ducted… from the ground up; or perhaps be channeled with a couple of 90 degree turns before it opens into the attic? True convection will take place if we properly duct or plan a diversion pathway into and out of the attic… it is merely a function of hot air rising and cold air then filling the vacuum created. I am sure you could find a venting method if you would think along the lines of hvac ducting, only external.

  5. Avatar
    Adrien Fri, January 18, 2008 at 8:02 am #

    I think this is a special case when Santa Ana winds are present. If you don’t live in an area where 70 to 100 mph winds drive wildfires as they do in southern california(I am from Poway but live in Northern Arizona), I don’t think it is as much an issue. The winds are what pushed the embers into the attic, not the nature of wildfire itself.

  6. Avatar
    Ray Smith Tue, January 22, 2008 at 2:32 pm #

    Hey, this is an interesting conversation. I’m just surprised it’s news to so many on the list.

    Adrien’s hunch is incorrect — the problem is an inherent aspect of most wild fires, and, in fact, Arizona jurisdictions (see Pima County website, for example) have the same prohibition as California on soffit vents. A wild fire creates its own wind, which drives embers forward of the fire front by long distances. These can ignite a building a half mile or so from the main fire.

    Jared’s idea of providing an alternative cool air source from ground level is one that I thought of also (running the stacks from the crawlspace up through the bale wall), but when I ran it past an expert was told the “stack effect” of having a long “chimney” bringing the air up from the ground would likely make the situation even worse, and might even suck fire into the crawlspace as well. Anyway, since the air has to exit the roof someplace, this vent air arrangement still doesn’t deal with the likelihood of embers getting into the roof structure through those openings — be they at the ridge or at the dormer ends.

    Are there any experts out there who can help with this?

  7. Avatar
    Jim Reiland Wed, January 23, 2008 at 9:57 pm #

    Great discussion!

    Fine Homebuilding magazine published an article about designing for wild fires in their April-May 2004 issue that addresses some of these questions. Authors Stephen Mead and Jim Wheeler recommended locating soffit vents in the facia instead of the blocking under the eves, theory being that hot gasses and embers wouldn’t be circulating and trapped there. Bob Theis, an architect and active member of the California Straw Building Association (www.strawbuilding.org) has been suggesting that detail for a long time now, using cement board and/or plastering the underside of the soffits/eves so hot gasses and embers don’t have anything to ignite as they swirl around under the eve.

    Because gutters occupy most our facia boards we’re installing soffit vents directly behind the facia board, and running cement board right up to the junction of the box beam over the straw bale wall. We also used 1/8″ screens…something recommended in a series on wild land fires that appeared in our local newspaper, the Applegater, a few years ago. I’ve seen people use corrugated steel instead of cement board, too.

    Jim Reiland
    Jacksonville, OR

  8. Avatar
    Brian Vick Sun, January 27, 2008 at 9:17 pm #

    How about using structual integrated panels and have vaulted ceilings. The couple I’ve seen don’t have sofit venting at all.

    http://www.sips.org/

  9. Andrew
    Andrew Mon, January 28, 2008 at 9:09 am #

    Brian,
    Yes. SIPs are a great idea and like you say, do not require venting. Another option is to use spray foam insulation which also does not require ventilation. There is actually a spray foam insulation that is water blown and soy based which is therefore quite benign. You can find out more at http://www.biobased.net.

  10. Avatar
    Anne Atkins Fri, August 8, 2008 at 9:33 am #

    I have just moved into my straw bale home in Fallbrook, where the Rice fire burned through. My roof is red tile. I have the required clay bird stops and the soffits are closed and finished with stucco. The attic vents through small mesh vents on top of the roof. The vents look like red Spanish tiles. Our roof is covered in a peel-and-stick tar instead of the usual flamable tar paper. Inside our attic space we have 8 inches of spray foam insulation that is extremely fire resistant (a lighter held to an edge of the foam did not catch flame.) This is part of our fire prevention program. We will not sustain an attic fire. We do not have any horizontal surfaces that can catch fire and burn down our home. The house is entirely stucco coated. We are a green building that is built to withstand wild fires.

  11. Avatar
    Brent Berkompas Tue, November 18, 2008 at 12:15 pm #

    Hello my name is Brent Berkompas a firefighter in southern California and owner of Brandguard vents a newly designed vent for new and retofit applications that resist the intrusion of flames and embers.
    We are based in Socal and can be reached for questions via the website or on my business line at 949 294-5429

    Respectfully,
    Brent Berkompas
    http://www.brandguardvents.com

  12. Avatar
    seo Fri, December 27, 2013 at 5:55 am #

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    new scheme in our community. Your site provided us with useful info to work on.
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  13. Avatar
    Steven Sun, July 22, 2018 at 3:13 pm #

    Funny to read these replies, my house wont sustain an attic fire, we can close a louver. After being in the witch creak and cedar fires in So Cal, I can tell you this. Fire will do the unthinkable and if you think your smarter than that, good luck!

  14. Andrew Morrison
    Andrew Morrison Mon, July 23, 2018 at 8:51 am #

    Hi Steven. You are so right on. All we can do is build the most fire resistant structure possible and prepare as best we can. Like you say, the fire will do what the fire will do and closing a louver won’t likely be the saving grace for a home. The raw power of a wild fire is something to behold…

  15. Avatar
    Richard Mumper Tue, October 29, 2019 at 5:27 pm #

    I would like to chime in on the conversation. I live in Los Angeles and own a fire investigation firm that has been in the business of investigation the origin and cause of fires- structural, vehicle and wildfire – for over 25 years. Some basic prevention that just might save your home from igniting during a wildfire are broken out into two scenarios- ember ignition prevention during high winds along with dry conditions and a nearby fire, whether wildfire or other:
    Ember ignited fires.
    1. Remove all light combustible materials such as leaves, needles, debris, paper, cardboard, cloth, etc. away from any part of the exterior of the structure, including removing any of the above from nooks and crannies on the roof, gutters and around the perimeter of the building.
    2. Move trashcans and other unsecured storage containers a significant distance away from the structure (wind can nock them over resulting in combustible material being spread where you don’t want it).
    3. Remove shrubs and palms away from the structure that easily ignite. Certain shrubs, palms and trees contain a lot of dry material are more prone to ignite. They are not worth it in fire prone areas of the west.
    4. Block off and seal all attic and crawlspace vent openings with plywood, metal or ember/fire approved vents, including ridge and turbine vents. Attempt to prevent any airflow in the attic during fire hazard conditions. You want the attic to become a dead space during a fire.
    5. Close all windows and doors to the structure and nearby vehicles.
    6. Move vehicles away from the structure and make sure that if a vehicle cannot be moved away from the building that number 1 above is applied to the vehicle, including underneath.
    7. If you choose to stay with your home instead of evacuate, be on the lookout for flying embers. If you see them land, immediately put them out with a garden hose.

    Approaching or nearby fire.
    All of the above apply except for #7. With an approaching or adjacent fire, do as much of 1-6 as possible beforehand (prevention). Do not attempt to fight a structure or vehicle fire with a garden hose. A garden hose will only be effective when the fire is very small.

    Most of all remember that winds and fire are very unpredictable and can easily result in taking your life and the lives of loved ones. Therefore, always be extra, extra safe.

    One last thing, remember that a nearby fire or fire inside of structure consumes all oxygen very quickly. This may prevent you not only breathing but starting any machine or engine that needs air to operate, like your vehicle. Don’t assume you will escape in your vehicle at the last minute. It just might stop running while you are stopped and you will perish inside it.

    Again, be safe and live to tell about a fire and not become a victim of it.

  16. Avatar
    Richard Mumper Wed, October 30, 2019 at 9:27 am #

    Also…check out this NFPA webpage: https://www.nfpa.org/Public-Education/Fire-causes-and-risks/Wildfire/Wildfire-safety-tips.

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