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Community and Q&A

Positive pressurized attic space for fire protection and summer cooling?

Elliott Nelson | Posted in Mechanicals on

I have a 4 year old Deltec home in Central Washington State. Zone 5B. Deltec homes are essentially round houses that have a center cupola that is vented. (Deltec homes has a web site if you need photos) This house has 2 wings that were stick built on site. The wings have continuous soffit vents and continuous ridge vents. The ceilings between the conditioned space and the attic were meticulously sealed. I am attaching a photo of the cupola from Deltec’s web site showing the vents.

This portion of Washington has huge wildfires. Just this year, the city I live in, (Wenatchee) lost 24 homes and 4 commercial buildings in one fire. All of those buildings were ignited by burning embers. Every year that we have been here we have had large fires within 5 miles. Some within 3/4 of a mile. The house has a class 1 roof and fiber cement siding. The area around the house is Firewise landscaped and has exterior sprinklers. Everything is designed to protect from wildfires except for the attic ventilation.

Burning embers entering the attic is a known method of ignition. Testing done at the Insurance Institute for Business and Home Safety shows burnig embers entering attic vents with 1/8 inch screens. There is a fascinating video on You Tube “IBHS Research Center Ember Storm Test Highlights”

The house is well protected with the exception of attic vents. My questions are centered around creating a positive pressurize attic by building a dedicated air intake that would draw air through a yet to be designed filter. The filter would need to filter out burning embers.

I was wondering — we buried a large portion of the intake pipe to take advantage of lower ground temperatures. Summer temperatures here are routinely in triple digits. There is also an underground 10,000 gallon water tank that we might want to use as an air intake, drawing the air from the space above the water. We would need to do some testing but I think embers would have a difficult time making it through the water collection system into the tank.

I understand that some of this is really out there, but I can’t see another way to protect the attic space short of trying to cover every vent. That would take hours (that we might not have) and isn’t going to happen if I’m away from the house. My thoughts are that the system should be running most of the time in fire weather, as opposed to having to switch it on as a fire approached.

What do you think? Thanks for your input.

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  1. Expert Member
    Malcolm Taylor | | #1

    The best summary of wild fire prevention strategies I've seen was a Fine Homebuilding article from about a decade ago.

    I don't know enough about the topic to comment intelligently on your plan, but I wonder what level of pressurization you would need to counteract the strong, fire induced winds? And given that main function of roof ventilation is the removal of excess moisture, it might not be a good ides to draw the supply from a water tank.

  2. Charlie Sullivan | | #2

    Are power outages common when there are fires? Would there need to be a backup system to power the positive pressure system during that time?

    The air coming out of the water tank might not have high humidity if the water is cold.

  3. Elliott Nelson | | #3

    Thanks for the link to the article. It is a bit outdated. Radiant heat from wildfire isn't the big problem that we once believed. Jack Cohen is one of the top researchers in wildfires. He explains the major ignition source is not normally radiant heat. He has an excellent video at

    Many of the recommendations in the article were incorporated into the house. I certainly agree that the details can make the difference between the house surviving or burning. Many of the 24 homes that burned here were stucco with tile roofs. If you look at an aerial photo of the fire you can see that houses blocks from the fire front burned. The warehouses that burned were over a mile away.

    The climate here is very dry, 9 inches of precipitation per year, almost all of that comes as snow. Average humidity is in the teens so I don't know how much moisture would be a problem if it did pick up some from the air over the tank.

    The question of how much pressure is a good one. High winds are common here without the fire. A fire generates wind.

  4. Elliott Nelson | | #4

    Power here is very stable. Most electric service is undergrand. It is possible that during a big wildfire power could go down. Maybe standby power is a good idea. Thanks.

  5. Expert Member
    Malcolm Taylor | | #5

    Thanks for the link. Please keep us posted as to what solutions you come up with. It's an interesting subject reconciling the various demands of the building envelope with those of fire resistance.

  6. GBA Editor
    Martin Holladay | | #6

    I don't understand why these vents can't simply be sealed. I'm guessing that they are part of a natural ventilation system -- but it seems to me that the advantages of natural ventilation in your area are grossly outweighed by the fire risk. So I would seal the vents.

    The pressure exerted by a blower door — 50 Pascals — equals about 1 lb. per square foot (about equal to the pressure of a 16-mph wind).

    Wind speed in miles per hour multiplied by itself and then by 0.004 gives the wind’s pressure in pounds per square foot.

    So a 40-mph wind exerts 6.4 pounds per square foot of pressure — more than 6 times the pressure exerted by a blower door. In other words, it's extremely difficult to overcome wind with a fan. You are talking about a really, really big fan.

  7. Charlie Sullivan | | #7

    There are two reasons to ventilate an attic. One is to help it dry, and the other is to help it stay cool, to avoid ice dams in the winter and to reduce cooling costs in the summer. If your ceiling is well sealed you shouldn't have ice dam problems. And for summer cooling, insulation is more important and effective anyway.

    So I agree with Martin that sealing the vents is a good idea. Optionally, you could add a few operable vents that you normally keep closed but could open when and if you want to air out the attic.

    Given your dry climate, I'm a little tempted to encourage the idea of getting ventilation air by blowing across the top of the water tank, because you'd get evaporative cooling combined with ground-coupled cooling. But if that works well, you might want to supply the house with that air rather than wasting it on the attic.

  8. Expert Member
    Malcolm Taylor | | #8

    Have you considered replacing your roof vents with fire resistant ones?

  9. Charlie Sullivan | | #9

    The brandguard vents look interesting. They say they also stop snow from blowing in. There was a question about that problem here last winter, I believe. Ah, yes, found it and will post a pointer to this thread. I didn't see any testing results confirming that the brandguard vents really work--you'd think that would be there somewhere.

  10. Elliott Nelson | | #10

    Fire ember resistant vents.

    I had looked at other "ember resistant" vents before. The ones I researched before, had no certification. Partly because the California code was new and they hadn't produced testing criteria. Even now there are some in the fire service that there hasn't enough testing done to ensure that they work. There have been other vents that were sold that didn't work.

    I'm not sure I want to be the guinea pig, paying the costs of buying new vents and retrofitting over a hundred feet of continuous soffit vents and then have them fail to work.

    "Issue: The California Building Standards Code provides for the option use of specialized eave vents for wildfire protection, however, there is no nationally recognized test standard upon which to base vent product acceptance. "

    There just isn't that much data yet on "ember resistant" vents. I'm still pouring over the tests, but some of the tests, test some things that don't seem relevant. I'm not sure that flame impingement makes a lot of sense. If you have flames lapping your vents, you're probably going to loose the structure. The California code specifies 1/4" screens, testing has shown that embers can penetrate 1/8" screens. We really need much more testing on a national level. As I am looking at some of the letters I see that they expired 12/31/13. So far I haven't found a letter that is current.

    You can view some of the California Fire Marshalls "acceptance letters"

  11. Expert Member
    Malcolm Taylor | | #11

    Yes, I can see why you wouldn't be too keen on being a guinea-pig if the consequences might be losing your house!
    You can buy metal mesh with holes small enough to be measured in microns. If the only concern is embers, wouldn't retrofitting an appropriate sized mesh work?

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