Throughout North America, officials are sounding the alarm about the rising risk of major fire events and the real threat that exists to life and property in North America’s wildfire “danger zones,” known as the Wildland Urban Interface. They hope to apply lessons learned from previous wildfire activity to strengthen fire and building codes to reduce risk and prevent losses. WUI areas are those where the built environment and wildfire-proned vegetation/forested areas meet.

WUI zones are expanding as a result of ongoing population growth and urban sprawl which is driving significant housing development in WUI designated areas. When more homes are built in WUI zones, the potential for wildfires sparked through human and natural ignition sources (i.e. lightning) rises, while the ability to battle these fires (or for controlled burns to occur) becomes ever more challenging due to a wide variety of factors including topography, density, community design and construction materials.

Approximately one in three houses in the United States are now in the Wildland Urban Interface. From 1990 to 2010, the number of homes in WUI swelled from 28 million to over 43 million, representing a 41 percent increase, while the WUI land area grew from nearly 225,000 mi2 to just over 297,000 mi2—up 33 percent. This rapid expansion continues unabated with approximately 60 percent of new housing in the United States being built in WUI areas. The result is increased risk and a firefighting community with stretched resources. Federal wildfire suppression expenditures rose significantly from 1985 to 2000 ($0.24 billion to $1.4 billion annually) and rose to just over $3.1 billion in 2018. Fire losses were expected to be at record levels in 2018 for the second straight year. In California alone, insured losses reported to date by the California Department of Insurance topped nearly $12 billion, a figure that may still rise as additional claims are filed. Severe fire seasons may be the new norm, given WUI growth, the trend toward higher temperatures, lower rainfall, and a longer fire season overall.

To mitigate risk, officials are seeking to improve the built environment, with a focus primarily on new construction, in an effort to improve the chances that homes in WUI areas and the people who occupy them might have greater chances of survival during a fire event. Building to higher standards in terms of fire resistance and ignition resistance are the primary goals of the WUI code.


Defensive Strategy

To design and build a house that has a greater chance of surviving a wildfire, carefully selected building materials and careful attention to details must play a significant role in prevention. Two of the greatest threats to homes include direct flames/radiant heat and embers. While the latter may seem like a lesser risk, high winds have been demonstrated to blow hot embers up to a mile from their source, igniting fires that can cause significant losses. The WUI code sets out the specific requirements for certain parts of the home/structure and property to ensure it is code compliant to WUI requirements. Building products used to achieve compliance or as part of a compliant assembly are often listed as approved products in the WUI Building Materials Listing Program or have undergone testing to demonstrate that the product/assembly meets WUI requirements.

Components covered by the WUI code include: exterior components such as roofs, walls and siding, decks, windows, doors, soffits vents and other details considered to be vulnerable ignition points. WUI code also addresses the requirement to create defensible spaces around homes/structures, including vegetation management and fuel management to prevent fire spread and make it easier and safer for firefighters to tackle a blaze.


About the Code

Local or state WUI codes are typically modelled after the International Wildland Urban Interface Code and adopted in whole or in part, often with additions accounting for specific regional considerations. That means that while WUI code can be very similar across most jurisdictions, WUI code and code requirements may vary depending on location and wildfire risk.

The requirements for fire and ignition resistance may be covered by a variety of code references. In California, for example, WUI Code construction requirements are contained in the California Building Standard’s code, including: California Building Code, Chapter 7A; California Residential Code, Section R327; and California Referenced Standards Code, Chapter 12-7A. If proposed solutions (product or assembly) is not currently approved to meet WUI requirements, they can be tested to prove they meet code requirements).

In wildfire-prone areas, WUI code generally sets out that the following major components of the home be addressed. Note: This is a sample, not a complete list of components and details covered by WUI code.


Roofs (CBC 705A / CRC R337.5)

Roofs can either be protected by a material with a Class A fire-resistance rating (required in State Responsibility areas, known as SRAs, or very High Hazard Fire Severity Zones). Class A materials include clay, concrete, and slate, as well as many types of asphalt and metal. In all other zones, a fire-resistant roofing assembly is allowed. For example, Class B wood roof shingles can be used over a Class A underlayment. Any spaces between the roof covering and the roof decking must be constructed to prevent flames and embers from entering, be fire-blocked with approved materials, or have one layer of cap sheet installed over the combustible decking. Valley flashing must be at least a 26-gauge galvanized sheet metal over a 36-inch wide No. 72 cap sheet, complying with ASTM D3909.


Eaves (CBC 704A.2.1 and CBC 704.2.3 / CRC 337.7.4. and R337.7.5)

Eaves must be built with ignition-resistant or non-combustible material or include a 5/8-inch Type X gypsum backing. Metal and fiber cement are popular for soffit and fascia details. Additional code requirements may apply depending on whether a building has open roof eaves or closed roof eaves and roof eave soffits.


Vents (CBC 706A / CRC R337.6.1 to CRC R337.6.3)

Attic vents and underfloor vent openings must resist the intrusion of flame and embers or shall be a minimum of 1/16-

and maximum 1/8-inch corrosion-resistant, noncombustible wire mesh or equivalent. Combustible vents on top of roofs may be covered with this material to comply. Ventilation openings on the underside of eaves are not permitted or eaves that are fire sprinklered, or vents that are 12-feet above a walking surface or grade below.


Windows and Exterior Doors (CBC 708A / CRC R337.8)

Windows are a weak point in the structure when it comes to wildfires. For this reason, under WUI Code the minimum standard is double glazing with tempered glass on the exterior.

Exterior doors should be fire-rated or built from non-combustible or fire-resistant materials. 1-3/8  inch solid core doors are required and generally offer 20 minutes of fire protection. But faced metal core doors can offer extra protection, along with a metal door jamb. While not required, metal panel garage doors are also recommended. Opt for a tight install to prevent embers from sliding under into any gaps.


Exterior Decking and Stairs (CBC 709A / CRC R337.9)

Walking surfaces of decks, porches, balconies and stairs within 10 feet of the building must be constructed of noncombustible or fire-retardant treated materials that extend down within 6 inches of the ground. Alternatively, desired materials can be tested to demonstrate that they meet WUI requirements. Heavy timber construction is also approved. Alternate materials can be used if they are ignition-resistant and pass performance requirements specified by the State Fire Marshal (California).


Exterior Walls and Cladding (CBC 707A.3 /CRC R337.7.3)

Wall components and cladding must be non-combustible, ignition resistant or part of the exterior portion of a 1-hour assembly. One-hour assemblies do serve to protect the occupants from wildfires, but do not address the phenomenon of flame propagation up the wall due to the engagement of components that can be ignited by a wildfire. Cladding material can include fiber cement, stucco, plaster, brick or natural or manufactured stone and should cover the house from the foundation to the roof. Heavy timbers, log wall construction or 5/8-inch Type X gypsum sheathing behind exterior covering is also permitted. Other non-combustible or ignition resistant options may be approved, if they can be shown to meet requirements. Ignition-resistant materials are those tested in accordance to ASTM E2768, by a facility recognized by the State Fire Marshall or ICC-Evaluation Service, to have a flame-spread rating not over 25 and that complies with accelerated weathering tests.


Design/Build Challenge

While meeting each requirement outlined in the WUI code may seem as straight forward as addressing it with an approved solution, WUI can present some challenges to the design/build community.

The first and most obvious challenge is that, any product used to satisfy a WUI code requirement must be demonstrated to meet that requirement—officials will require all supporting documentation including the results of recognized testing. In California, many approved products in compliance with Chapter 7A are listed in the Office of the State Fire Marshall’s Building Materials Listing Program.

Yet, it must be noted that not all products or solutions are listed in the WUI Handbook—even if they’ve been previously approved for use. It is recommended that designers and builders check with manufacturers regarding approvals/documentation. Products not listed may still be approved for use, if the case can be made to local or state-wide code officials that the product(s) or assembly are in compliance. To have a new product or assembly approved can be a challenging process, a reason it is typically initiated by manufacturers. But sometimes, the design community plays a leadership role in order to champion innovation, particularly when it will result in a more advantageous solution for their project/structure.


New Product Approvals

A recent case in the state of California stands out. Forward thinking design firm, Sagan Design Group, wanted to break from a status quo WUI solution to meet requirements for the exterior wall assembly (2016 CRC – R337.7.3). Typically, designers are limited to tested assemblies for various sidings (usually must use a rated assembly).

Generally, those who do not wish to use a rated assembly must instead use 5/8-inch Type X gypsum sheathing under the exterior cladding to act as an ignition-resistant barrier in order to achieve WUI code compliance or have an alternative assembly/solution tested. Sagan Design Group sought to use a continuous layer of exterior mineral wool semi-rigid board insulation instead of Type X gypsum, highlighting another key builder challenge: compliance with all the various codes—in this case the WUI code and the Energy Code, which do not consider one another. Often, solutions will meet one code, but will not help address the simultaneous requirements of another—and there isn’t necessarily a handbook or listings guide that tackles that.

In California and across the United States, energy codes have been growing stricter (and will continue to grow stricter still), requiring tighter building envelopes with lower U-values that address air leakage and thermal bridging respectively. Sagan Design Group was met with the challenge of achieving the 2016 Energy Standards, approximately 20-30% more stringent that the 2013 Standard, while also meeting R-values targets and satisfying WUI requirements. Tim Melin, P.E., a Civil Engineer with the firm, states that while a number of solutions exist, it is getting more difficult to satisfy all codes simultaneously. As result, he notes that the design/build community will need to champion innovative approaches to meet more complex design challenges particularly as the state moves toward Zero Net Energy (takes effect January 2020) and better energy performance is required with each new code cycle.

For low-rise residential buildings, U-values and R-values are outlined in the CA Energy Code Table 150.1-A. It notes that the prescriptive wall requirement in Climate Zone 16 requires a maximum U-value of 0.051. Yet, 2016 Reference Appendices state that, without continuous exterior insulation (2x6 @ 16-inch OC framing using R-25 cavity insulation), the lowest U-factor that can be achieved is 0.065 and is therefore, not acceptable. As a result, the ability to meet California Energy Code without continuous exterior insulation is becoming ever more difficult and in the future, as codes tighten, continuous exterior insulation is likely to become standard practice.

When continuous exterior insulation is present, a reference in the above stated table also notes that it is likely to be foam insulation. While it can meet energy code, there is some debate about adding to a home’s potential fuel load in WUI zones. Foam plastic products, when ignited, burn rapidly and produce both heat and toxic gases, contributed to the potential fuel load of a fire and allowing it to burn more intensely and for longer. Yet, foam insulation can be used within an assembly and still meet WUI requirements when covered with Type X gypsum. Although, it requires a wall system with more layers (foam insulation plus 5/8-inch type X gypsum). Melin notes that that’s where the case for continuous exterior mineral wool sheathing board proves to be strong. The mineral wool continuous exterior insulation can simultaneously WUI compliance without the need for gypsum—eliminating a layer within the assembly and potentially reducing install time, labor, and costs—and also achieve energy code requirements.


A Trailblazing Solution

In making its case to local code officials, Sagan Design Group and Rockwool demonstrated that the continuous exterior mineral wool sheathing board (Rockwool Comfortboard 80) provided the noncombustible, ignition-resistant performance benefits equivalent to Type X gypsum, while also delivering a wall system that reaches necessary R-values and contributes to the strict U-value rating. The mineral wool continuous insulation also proved further advantageous, allowing for a wide variety of cladding options, a benefit that would also sit well with homeowners who often insist on specific materials to achieve a certain aesthetic. Additionally, it helps achieve a wall assembly that is breathable and hydrophobic, helping to guard against moisture issues.

While many other options are possible (there are no one-size-fits-all solutions), designers do face challenges in meeting the array of more complex requirements, and there are barriers. For example, software modelling programs of performance based models make it difficult to model some solutions such as offset studs, making it more difficult and time consuming to get alternatives passed. Meanwhile, when meeting energy code, it is becoming near impossible to employ a straight prescriptive method. It’s no longer enough to target the low hanging fruit, such as improving mechanical systems, water heaters or components like windows to improve performance to the standard now required. Not to mention, this doesn’t even address simultaneous codes such as WUI regulations. As a result, the new low-hanging fruit is insulation—continuous exterior insulation to be precise.

Sagan Design Group worked directly with mineral wool insulation manufacturer Rockwool, building a case filled with data, testing, documentation, code references, certifications and more to show building officials that not only could mineral wool sheathing boards meet WUI requirements, they could also replace Type X drywall all together under cladding in the exterior wall system. In the end, local officials approved what might end up being a trailblazing solution.

Given the direction of energy codes and the need to satisfy both energy and WUI requirements in Wildlife Urban Interface Areas, it seems inevitable that the residential built environment may be on the cusp of change—driven largely by code compliance. It’s undeniable that how we build, is in fact, changing. Surprisingly, however, in embarking on the approval process, the teams from Sagan Design Group and Rockwool found that there were currently no approved assemblies utilizing continuous exterior insulation in existence in the WUI Building Materials Listing Program. Both the firm and the manufacturer have worked hard to amend that. In the meantime, documentation is available outlining the approval and the full testing results for ignition resistance and 1-hour system testing on relevant assemblies using Rockwool mineral wool for the California market.

“We need to build smarter to meet the more complex demands of today’s building codes and standards,” notes Melin. “We’re now building for structure, energy, safety, moisture management and air/vapor considerations. Products need to be versatile. Mineral wool semi-rigid exterior board sheathing in the form of continuous insulation is a serious multi-tasker. With it, we can meet WUI requirements, we get energy performance, comfort, sound control, breathability, hydrophobic properties and compatibility with a wide range of other components.”

This example shows that we can build better within WUI areas. WUI in itself isn’t the greatest challenge, although it can be a hurdle. The main challenge is meeting WUI plus the complex array of requirements inside the rest of the suite of building codes. What it means is that designers and builders will need to look to building products that do more. They’ll need to use their experience to continue to do what has always driven the built community forward: thinking beyond the status quo. Better solutions should also pay off for homeowners as well, with more durable, resilient buildings that require less maintenance and/or remediation down the road.

Codes and standards will continue to evolve. As the built environment moves forward, designers, builders, manufacturers and code officials all play a pivotal role in proving that as we build today—and more importantly, in how we build—we can reshape tomorrow. W&C

Radeloff, V.C.; Helmers, D.P.; Kramer, H.A.; Mockrin, M.H.; Alexandre, P.M.; Bar-Massada, A.; Butsic, V.; Hawbaker, T.J.; Martinuzzi, S.; Syphard, A.D.; et al. Rapid growth of the US wildland-urban interface raises wildfire risk. Proceedings of the National Academy of Sciences of the United States of America (PNAS) 2018.

National Interagency Fire Center (2017) Historical wildland fire information: suppression costs 1985 -2016.

Press release – California Department of Insurance – January 31, 2018

Heat Values of Various Fuels – World Nuclear Association - August 2018