Passive Houses and Fire Resistance
In California and throughout the world, wildfires are threatening more and more homes and people’s lives. Fortunately, many fundamental Passive House principles and techniques can make a home more resistant to catching on fire—and can also protect the health of people living inside the home during a fire event.
In the simplest terms, one of the key elements of a Passive House is a strong separation between inside air and outside air—and that separation helps prevent fire and smoke from entering the home. But let’s dive into some of the details of how that all actually works.
Air Sealing: Separating Outside from Inside
The primary fire resistance feature of Passive House building is Air Sealing. This is accomplished during construction by using various products to completely seal all holes and cracks in the floor, walls, and roof. The overall goal is to eliminate any place air could leak from inside to outside, or vice versa.
In Passive House building, air sealing is done mainly to save the energy that would be lost if that heated or cooled air leaked out. But this technique can also make a house less likely to catch fire.
Attics and Crawlspaces: In or Out?
Conditioned Space in a building is the area that is either heated or cooled directly. When air sealing, a builder needs to decide where the edges of the conditioned space will be. This may seem obvious, but there are a few places inside the building that may or may not be inside the conditioned space. These places include the attic space (above the living area) and the crawl space (below the living area).
If insulation in the attic is installed between the ceiling joists, then the plane of the ceiling effectively becomes the edge of the conditioned space. That means that the attic is then outside the conditioned space. It is effectively connected to the outside air, rather than to the inside (conditioned) air. In a case like this, the plane of the ceiling would also become the edge of the pressure boundary, and therefore the place where the builder would air seal.
When a house is built like this, with the attic outside the conditioned space, then the attic space must be vented. Here’s why: the attic contains a volume of air that is enclosed but is not conditioned. In the summer, that air can get very hot, and in winter, it can get very cold.
If the space is not vented, then in the summer, it could get so hot that it would actually add heat (through the insulation) to the air you are trying to keep cool in the conditioned space. Similarly, in the winter when it gets cold, the air could reach dew point. That results in liquid water condensing out of the air and dripping onto the wood framing, insulation, and sheetrock below. Over time, this can result in rot, mold, and hastened deterioration of the structure.
Crawl spaces that are not part of the conditioned space are also vented, mostly to prevent the build-up of moisture.
Flying Embers: The Problem with Vents
In a fire event, however, these same vents become places where embers can enter the structure. As the incidence of fire events, and their intensity, increases, we are becoming more aware of the details about how buildings catch fire.
In a wildfire, most buildings that burn catch on fire due to embers that are blowing ahead of the fire and landing on buildings. In the strong winds that often accompany a fire (and sometimes are even caused by the fire itself), embers can be sucked through the vents—and start a fire.
How Passive House Construction Keeps Embers Out
If, instead, the attic space and crawlspace are included in the conditioned space, then there is no need to vent those spaces—and those pathways for embers to enter the house are eliminated.
In Passive House construction, the attic and crawl space areas are often (but not always) made part of the conditioned space. The key reasons for this include:
- It’s much easier for a builder to air seal the building around the entire outside.
- The heating/cooling system can be located inside the conditioned space, which makes the home much more energy efficient.
- The structure will be more durable if condensation problems can be avoided.
While fire prevention is not the primary reason for this decision, it is clearly an important “side” benefit, especially when building in high fire-risk areas.
Other Building Strategies to Prevent Fires
Of course, there are other ways that buildings catch on fire other than embers entering through vents. And fortunately, there are also other strategies that builders can use to mitigate those risks. Many of these are not unique to Passive House construction, but are simply common-sense principles for anyone living or building in fire-prone regions.
Fire-Resistant Roofs and Exterior Walls
Most residential buildings are made of 2×4 lumber and other sizes of wood framing parts. During a fire event, these can act like kindling—fast and easy to burn. To protect these materials from fire, it makes sense to cover them on the outside with less-flammable materials on the roof and exterior walls.
In the past, it was very common to cover the framing with wood shingles on the roof and wood siding on the walls. While some homes are still built this way, we now know it is better to use fire-resistant materials on the roof, such as asphalt shingles or, even better, fire-proof materials such as clay tile, concrete shingles, or some kind of metal roof. And for exterior walls, there are a variety of fire-resistant and fire-proof materials to choose from, including stucco, cement siding, and metal siding. Any of these can make your home more resistant to fire.
Fire-Resistant Eaves and Overhangs
Sadly, some of the architectural features that make homes attractive and comfortable also lend themselves to catching embers. Even houses that have fire-resistant roofing and siding may have weak points on the eaves, where rafter tails may stick out past the exterior wall. The eaves are sometimes the most flammable element exposed. For this reason, many experts advise minimizing or eliminating eaves and overhangs.
If this is not possible or desirable, the alternative is to cover the bottom of the rafter tails with a fire-resistant material and use a similar material for the fascia boards that run along the edge of the roof at the ends of the rafter tails. Also, fill the voids between the rafter tails with a non-flammable type of insulation. If there is no air between the rafter tails, then there is no need to provide venting there.
Fire-Resistant Decks and Fences
Wood meets the WUI (Wildland Urban Interface) construction rules if it is 2x material (2 inches thick nominally, actually 1½ inches thick). But 2×4 or 4×4 lumber will definitely burn if it is exposed to fire for long enough. If you have wood decks or fences that terminate at the exterior wall of your house, these are areas of concern during a fire event.
I visited an area in Colorado that was recovering from a massive wildfire and was in the process of rebuilding. The houses being rebuilt there all had decks made entirely of steel framing and corrugated deck plates like you might find in an industrial setting. I have not seen anything like that in California yet—but I bet we will soon.
Exterior Insulation: An Extra Layer of Protection
The conventional way to install insulation is between the framing members such as wall studs, floor joists, and roof rafters. However, a common practice in Passive House construction is to install exterior insulation in addition, on top of the framing but below the exterior finish material. There are many benefits to this approach (which I will explain in a future article), but suffice to say it can improve home performance in many ways.
The point here is that any exterior insulation should be made of a non-flammable material to give you an extra layer of protection over your very flammable framing. Examples of non-flammable or fire-resistant exterior insulation would be Rockwool, which comes from a mineral by-product of steel smelting, so it is post-industrial waste as well as non-flammable. Gutex is another product we have used. It is made of wood fiber (waste product from lumber production) but is manufactured in such a way as to render it very fire-resistant. These are better options than Styrofoam or other foam-based insulation products that are petroleum-based, very flammable, and produce toxic gasses when burning.
Fire-Resistant Windows and Doors
Once a fire is very close, then the windows and exterior doors become a possible entry point. These assemblies have two major components: the opaque parts (such as jambs, frames, sashes, sills and trim) and the glazing (glass panels). All jambs and frames, like any other component that can be seen from the outside, should be made of a fire-resistant or non-combustible material. Aluminum-clad wood windows or fiberglass are good choices; unclad wood or vinyl are not as good.
For glass, the main concerns during a fire event are breakage and heat transfer. In California, new windows are required to have dual glazing, because they are more resistant to heat transfer and therefore more energy efficient. In the WUI zone, windows are required to be tempered, as tempering increases the temperature at which they will break.
Passive Houses, because they are designed to achieve very high energy efficiency, often use triple-glazed windows, which are even more resistant to heat transfer than typical dual-glazed windows. In the event of a fire, there is also one more pane of glass that would need to heat up and break before fire could enter the building.
Exterior rolling shutters can also be installed to cover the windows during a fire event. These can also be used to reduce solar heat gain if your house is too hot.
Defensible Space Around Your Home
The last area to consider—and this is as important as anything on the house—is the outdoor space that surrounds your house. Here in California’s Central Coast, the rule of thumb is to consider everything within 150 feet of your house as your “defensible space.” The goal is to reduce the flammable fuel load as much as possible in that zone.
For those that are motivated and have the means, exterior fire sprinklers can be installed and activated when a fire is approaching. These systems often have a computer interface that monitors fire activity and activates the system when needed.
No Smoking Zone: Air Quality During a Fire
There is one other subject related to fire events that I should mention here, and that is air quality during a fire. As many of us in California have learned, any time there is a fire in the area (or even hundreds miles of away), the air quality can become unhealthy. We’ve all experienced times when local officials advised residents to stay home and stay indoors with the windows closed because the outside air had become unhealthy to breathe.
As I mentioned in the beginning, Passive Houses are designed and built specifically to separate inside and outside air. The sad truth is that many conventionally built structures are very leaky, so that the inside and outside air are constantly mixing. In a Passive House, that is not the case. The ventilation system provides fresh air that goes through a filter. The filtration system can even be augmented during the time when exterior air quality is poor. As a result, you can go a lot longer inside a Passive House with good air quality, even during an extended period of smoke and other pollutants outdoors.
Contact Carmel Building & Design
If you’re considering building a custom home in the Monterey Bay / Central Coast area and are concerned about wildfire resistance, contact us to learn more about Passive House building. At Carmel Building & Design, we specialize in creating homes that are beautiful, comfortable and durable—while also being more energy-efficient, environmentally sustainable, and fire-resilient than conventionally built homes. We look forward to partnering with you to bring your dream home to life.
