Building for better fire resilience in WUI zones

by arslan_ahmed | February 10, 2023 10:00 am

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By Rick Roos

Wildfires continue to make headlines and the long-term prognosis is concerning, as experts forecast more of the same in coming years. Yet, building in wildland urban interface (WUI) zones continues at a fast pace. WUI zones are areas of transition between unoccupied land and human development, where structures and other human development meet or intermingle with undeveloped wildland or vegetative fuels. Communities adjacent to and surrounded by wildland are at varying degrees of risk from wildfires. WUI zones are not limited to densely forested areas; they can just as easily be inhabited areas close to brush or grassland. Dry, hot, and windy conditions—together with an ignition source—increase the risk of fire quickly spreading in any WUI zone.¹

Why are people still building, and rebuilding in WUI zones? Primarily for affordability. These zones continue to expand because of ongoing population growth and urban sprawl, driving significant housing development into environments in which fire can move readily from forests and grasslands into neighborhoods. It is important to note new builds in WUI zones are primarily residential, but commercial builds in these regions are not uncommon. The principles for building for optimal fire resilience are the same in both scenarios.

The Good Haus

The Good Haus, a zero net energy (ZNE), near passive, high-performance house in California’s Sierra Nevada foothills (see the section on page 4 titled “Could This Be the Perfect House for Rural California?”) exemplifies good design inclusive of consideration to fire resilience. Atmosphere Design Build principals Mela Breen and David Good (the home’s designer and builder) were drawn to rural northern California for different reasons: the natural beauty of the area and a desire for an active outdoor lifestyle that made the region ideal for their business and home. Their goal was to build a high-performance NZE home, which meant designing an exceptionally high performing building envelope. Central to its objective and in keeping with Passive House ideals, the plans called for a super-insulated home to ensure efficiency, occupant comfort, and building durability. An airtight and efficient thermal envelope would also dramatically reduce heating and cooling demands and permit a modest photovoltaic (PV) solar array to meet those demands. In keeping with the values of their practice, Breen and Good sought to incorporate sustainable, environmentally conscious building materials to support a healthy home with a smaller impact on the planet.

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However, construction of homes in WUI areas must subscribe to a higher standard of fire resilience to increase the odds of preserving structures and providing occupants additional time to find safety. Breen and Good understood this and made certain their material selection and the assembly design supported their needs for optimal fire resilience, as well as thermal performance. They chose to go beyond state/local code requirements to ensure their investment in their forever home would have greater protection in the event of fire.

The decision to look beyond state/local code requirements was a smart one, as the focus is often placed on materials alone, with the thought that if noncombustible siding materials are selected, the rest of the home is safe. However, fire protection is much more complicated than that.

What does the code say?

Building codes set minimum requirements for many construction features, such as fire protection. They outline the basic requirements a home must meet to be up to code, such as the performance expectations for allowable materials and assemblies, and the required technical standards for construction, such as those which have been adopted into the code and published by ASTM and the National Fire Protection Association (NFPA).

The bottom line: codes are not enough; building or rebuilding a home with the greatest possible fire resilience is voluntary. One’s approach to this is predicated on a concerted effort to learn and understand best practices to address fire resilience.

Take a holistic approach

Specifying fire-resistant materials for the building envelope is a good place to start. However, best practices today demand more. This includes considering fire safety at the start of the design process—it cannot be an afterthought—and taking a holistic approach to building in WUI zones. What does a holistic approach look like? Architects and builders need to think differently about the house design, especially if they are rebuilding after a fire. How can one design a better building to provide a greater opportunity to stop fire from penetrating the structure and, in the worst-case scenario, buy additional time for occupants to get to safety? It requires, in part, understanding how homes catch fire. Then, it is important to look at materials to determine whether they are combustible or noncombustible, if they are ignition or fire-resistant, and whether they have low or high flame-propagation rates. The next step is a challenging one: taking this information into account while designing fire-resistant assemblies.

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Canada’s National Research Council (NRC) published a new WUI guide, National Guide for Wildland-Urban Interface Fires, in July 2021, which provides detailed insight about WUI fire characteristics and associated hazards. For example, “the descent and landing of burning embers downwind, ahead of a burning fire, is referred to as spotting or, in the case of the WUI, ember rain.”² Since embers from wildfires fall, start by looking at the roof assembly, then walls, any exterior structures, and then to the outside landscaping around the home.

Roofs

A fire-resistant roof assembly includes not just the surface product, but also the underlayment, sheathing, insulation, joists, and other components that comprise a roof assembly. ASTM E108, Standard Test Methods for Fire Tests of Roof Coverings, Class A-rated surfaces such as clay, concrete, slate, and many types of asphalt or metal are good surface selections, but builders also need to consider components beneath the surface material. For example, noncombustible roof boards can improve the roof assembly’s fire resilience. There are noncombustible options, including several stone wool products. Builders should look for products with a flame-spread index of zero and a smoke-developed index of zero, which can improve a roof assembly’s ability to withstand conditions during a wildfire.

Other measures for creating a fire-resistant roof assembly include:

• Sealing eaves and covering valleys with an ASTM E108 Class A material to prevent embers from igniting the roof.
• When gaps exist within a roof system, filling the space at the eave ends to prevent ember and flame entry.
• Covering vents with 3.175-mm (0.125-in.) mesh to prevent embers from getting into the house.
• Using ignition-resistant or noncombustible materials, such as metal and fiber cement, for the soffit and fascia to further protect the eaves.

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Walls

Similarly for wall design, fire-resistant cladding is not a complete solution. One must consider all components of the wall assembly as its strength is a function of the combination of materials that make it up. Vinyl siding does not provide a lot of protection, nor is it the problem when it comes to fire-prone assemblies. Vinyl typically does not burn. It tends to melt and fall away from the structure. It is the materials behind the vinyl siding that can either make or break the system.

While noncombustible or ignition-resistant materials such as fiber cement, stucco, plaster, brick, and stone are preferred siding, it is possible to comply with state codes such as the California Building Code’s Chapter 7A—which stipulates materials and construction methods for wildfire exposure—even when using another product (e.g. vinyl siding). This means complying with the State Fire Marshal Standard SFM 12-7A-1, now adopted into ASTM as E2707, Standard Test Method for Determining Fire Penetration of Exterior Wall Assemblies Using a Direct Flame Impingement Exposure, which tests the ability of the wall assembly to resist the penetration of flames into the stud cavity.

Other considerations

It is also important to consider windows, doors, decks, and landscape design in the planning and design for fire-resistant building.

• Use fire rated windows and other glazing or feature multiple panes with at least one layer of tempered glass.
• Use fire rated doors.
• Construct decks using wood frames if necessary, but do so by using a fire-resistant finish material that extends to within 152.4 mm (6 in.) of the ground. Construct and finish cantilevers and other overhangs with noncombustible materials; and ensure that combustible materials are not stored beneath decks.
• Distance outdoor structures a safe distance from the house (this distance will vary regionally by code) so flames do not spread to the home, should they ignite.
• Consider landscape design as well. Homes should have a noncombustible area within 1.5 m (5 ft) of the home’s perimeter. This means no structures, combustible vegetation, or wood mulch against the home that could ignite and spread flames to the home itself. Gravel, brick, concrete, and stone are good options for noncombustible landscape features that can be used in this case.
• Space trees to avoid spreading fire from one another or to the home, and at least 3.05 m (10 ft) away from the home. These distances will vary by region, for example: within 3.05 to 9.14 m (10 to 30 ft) of the home, space trees 5.5 m (18 ft) apart. Between 9.14 and 18.3 m (30 and 60 ft) of the home, space trees 3.6 m (12 ft) apart, and between 18.3 and 30.5 m (60 and 100 ft) of the home, space trees 1.83 m (6 ft) apart.
• Include fuel breaks as landscape features. This includes noncombustible details such as concrete driveways, sidewalks, patios, and dry streams, each of which can help prevent the spread of flames.

COULD THIS BE THE PERFECT HOUSE FOR RURAL CALIFORNIA?

To achieve its goal as a zero net energy (ZNE) building, reaching the highest levels of efficiency while also opting for sustainable materials, the Good Haus relied on stone wool insulation to super-insulate the structure’s walls, roof, and slab foundation. Rigid board insulation (100.2-mm [4-in.] thick) creates a continuous layer of insulation on the exterior of the home, while stone wool insulation batt fills the interior wall cavities to achieve a wall assembly with an overall R-value of R39. The thick layers of stone wool insulation help to keep temperatures inside the home stable, protecting it from moisture and providing critical passive fire protection, an important consideration, given the Good Haus’s location within California’s WUI zone. Stone wool on the roof (127 mm [5 in.]) provides fire safety and maximum efficiency, achieving a cozy R80 for the roof system, while stone wool rigid boards also prevent heat loss below the home’s slab foundation, insulating the slab to R21. The slab insulation features stone wool batts between sleepers, plus a layer of R30. The floor system features a layer of R23 batts between the I-joists that bring the floor system to R68. In addition to its contribution to high energy efficiency and resistance to fire, the project team chose stone wool insulation for its ability to help create a vapor open assembly with high drying potential, to avoid moisture issues due to wind-driven rain and to prevent mold and mildew in northern California’s hot and humid climate. The sound absorbency of the stone wool products was also key to occupant comfort, given the home is near a state highway and sound blocking was an important consideration. For further acoustical comfort, stone wool batts were used in interior walls to help create a peaceful and relaxing spaces. Atmosphere Design Build principals Mela Breen and David Good (the home’s designer and builder) also favored stone wool as a sustainable insulation choice. The natural raw materials and high recycled content of stone wool products were an ideal fit with Breen and Good as to sustainability and responsible building practices. The complexity of the design, with its cantilevers and steel deck foundation, required heightened attention to the air sealing and insulation details. In addition to the highly insulated envelope with stone wool insulation, the details for connections between interior and exterior structural members were extensive to provide specific graphics to avoid thermal bridges. The house uses triple pane windows and doors to maximize light, comfort, and performance. A CO2 heat pump water heater provides efficient domestic hot water delivery. Balanced heat recovery ventilation offers high indoor air quality (IAQ) and uniform temperature distribution. Super-efficient ductless mini splits provide indoor climate control. A 6.5 kW roof mounted photovoltaic (PV) system offsets the home’s energy usage—creating a ZNE building. With the stone wool insulation, meticulous air-sealing, the combination of products, critical design work, and attention to detail, the Good Haus achieved an airtightness of 0.6 ACH50, making it one of the tightest homes in California.

Fire-resistant materials

When it comes to the building envelope, stone wool insulation is an ideal material in these fire-safe assemblies. Breen and Good recognized those benefits when selecting stone wool insulation for their build as it limits the spread of fire, minimizes heat transfer from the outside to the inside of the building, does not contribute to smoke development, and supports added design flexibility allowable in ASTM E2707 or NFPA 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Component, assemblies, including unlimited insulation thickness to meet increasingly stringent thermal requirements. Foam plastics, in contrast, are limited to maximum thicknesses.

The selection of stone wool insulation was a win-win solution for the challenges of building a high-performance home in a WUI zone. The project team achieved their aggressive goals for energy efficiency as well as fire resilience, and reaped additional benefits from the stone wool, including:

• Moisture resistivity—stone wool does not readily absorb moisture when exposed to water, while allowing vapor to pass through the assembly for superior drying capabilities.
• Dimensional stability—stone wool products avoid (prevent) gaps to maintain reliable thermal and mechanical property performance over time.
• Chemical inertness—stone wool insulation products are chemically inert and non-corrosive with no off-gassing, and are resistant to rot, mold, and fungi.
• Termite resistance—stone wool tests have demonstrated superior performance in areas with a high probability of termites.

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What about retrofits?

In some areas, it is not the new homes that are most at risk, but those built before codes were enacted or strengthened in that state/municipality. This separation between new and existing homes is problematic for regulators. For example, if a home has a wood shake roof, it is challenging for regulators to ask for a retrofit. Homeowners may not have the resources or inclination to replace the roof with something that has better fire resistance. Insurance companies may become a more powerful driving force by asking homeowners to make fire-resistant upgrades to maintain coverage, but for now, the onus is on homeowners to make upgrades themselves—and on builders to recommend products and assemblies that can increase a home’s overall fire resistance.

And this really is the overriding message about building in WUI zones. Meeting code is not enough. Building for better fire resilience is voluntary. As the industry and government work to improve awareness and understanding of best practices, it continues to move closer to making this a holistic approach and “top down” approach to fire resilience the new normal.

Notes

¹ Definitions and context from www.firesmartcanada.ca/
faqs and www.usfa.fema.gov/wui[6]

² Refer to nrc-publications.canada.ca/eng/view/ft/?id=3a0b337f-f980-418f-8ad8-6045d1abc3b3[7]

Author

Rick Roos uses his expertise in fire safety, hygrothermal building performance, and acoustic control to bring a holistic approach to codes and standards development. As senior manager, codes, standards, and fire safety at Rockwool, he works within the codes development process in the International Code Council (ICC) and Codes Canada. This role is complemented by his active membership across other industry organizations, including ASTM committee C16 on Thermal Insulation, ASTM E05 Fire Standards as associate member of CAN/ULC S700 Thermal Performance and Energy Use in the Built Environment, and as member of CANULC S100A committee on fire tests.

Source URL: https://www.constructionspecifier.com/building-for-better-fire-resilience-in-wui-zones/

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