Top 7 fire-resistive construction types

Despite the recent rains in northern Arizona, the drought conditions we've experienced here since the late 1990s have unfortunately become the norm. Some believe this unwelcome pattern is attributable to the onset of climate change, rather than a temporary blip in annual precipitation amounts[1]. Whatever the cause- warming temperatures, increased evaporation, and less moisture in our plants, trees and in the ground- makes the prospect of increased wildfire activity more likely, and presents a very real risk to those of us with homes and businesses in the area. Whether you're considering a new home, or just an addition or renovation project, there are several methods of construction that can help protect your home and family from wildfire risk.

While firewise landscaping is important[2], the basic materials used to construct your home or business also plays an important role in the survivability of a structure in the event of a wildfire. Here are some of the construction types that offer varying degrees of fire-resistance - from moderately fire-resistive, to the most fire-resistive:

1) Strawbale

Fire testing has shown that strawbale construction can be up to three times more fire resistant than conventional wood framing[3]. While this might seem surprising given the common association of hay with the out-of-control barn fires from years past, strawbale construction derives its fire-resistant qualities from the straw that is very densely packed (unlike the loose, and readily combustible hay found in old barns), and is almost always covered with a thick layer of stucco or plaster as an exterior finish. It is the combination of densely packed straw and a thick non-combustible finish, which gives this popular and thermally-efficient construction material its favorable fire-resistant properties.

2) Adobe

Adobe is one of the oldest construction materials in the world. Adobe construction typically consists of sun or kiln-dried bricks formed out of a mixture sand, clay and straw. While adobe represents a definite improvement compared to conventional construction, the fire resistive qualities of this material can vary depending upon the combustibility of attached building elements such as lintels, vegas, roof eaves and soffits. Though adobe construction is usually load bearing, steel supported adobe walls provide a considerably higher fire rating than adobe that is supported with wood members[4].

3) Earth Bag

One of the most innovative and sustainable construction materials that is quickly gaining popularity is earth-bag construction. Pioneered in California by Dr. Nader Khalili[5], earth-bag construction consists of manually stacked tubes or bags of earth, which can be formed into most any shape or configuration. Quick to construct and valued for its organic beauty, earth-bag construction derives its fire-resistive qualities from its non-combustible earthen core, covered with an equally non-combustible stucco or thick plaster finish.

4) Rammed Earth

Thermally-efficient, rammed-earth buildings are simple to construct, fire-resistant, and durable. With walls typically 12- to 18-inch thick, rammed earth construction is considered one of the most fire-safe materials available due to the non-flammable nature of its constituent materials- earth, sand, gravel, and clay. At 18 inches thick, rammed earth construction is able to withstand flames for up to four hours[6].

5) ICF (Insulated Concrete Forms)

Because concrete is one of the most fire-resistant materials when exposed to heat and flame, energy-saving ICF's offer another highly fire-resistant construction option. ICF's are composed of poured concrete with a internal layer of insulating foam. Flame-retardant additives to the foam in recent years have improved the overall fire-resistive qualities of ICF's[7].

6) AAC (Autoclaved Aerated Concrete)

AAC is a homogenous, fine-aggregate cement block that is light-weight, non-toxic, highly energy efficient, and completely non-combustible[8]. Less costly than ICF's or other poured concrete options, AAC offers an extremely high level of fire-resistance. Coupled with concrete-composite siding, fire-resistive roofing, and firesafe landscaping, an AAC home is one of the most fire-resistant construction options available.

7) Earth Sheltered

Durable, low-maintenance, and thermally-efficient earth sheltered construction is considered to be one of the most fire-resistant construction strategies available. The characteristic low-slung, sweeping lines of an earth-sheltered building, typically constructed of cast-in-place reinforced concrete and then covered with a layer of earth, provides a highly desirable solution in climates of extreme heat or cold, or in areas vulnerable to windstorms or wildfire risk.

Fire-Smart Design Considerations

Constructing your home out of the right materials is only part of the picture. A fire-smart architectural response to your home or business will also include specifying non-combustible products and finishes such as heat-resistant glazing, as well as flame-resistant finishes such as stucco, plaster, or concrete-composite sidings; fire-resistant roofing materials such as clay tile, concrete tile or metal roofing; with roof eaves, overhangs and soffit materials designed to resist or delay the spread of flames. Similarly, out-structures such as fencing, covered decks, balconies, carports, and sheds must also be selected with fire-smart characteristics in mind.

Fire-Wise Landscaping

Fire-wise landscape design is also known as "Defensible Space"[9] and is a critically important to your home's chances of surviving a wildfire. Defensible Space is the buffer between your home and any combustible trees, shrubs, and/or groundcover that surrounds your property. With limited fire-fighting resources, municipal fire departments must often make choices about which homes to save. Fire Department officials have affirmed that homes with clear defensible space are most likely to be defended, should such a difficult choice become necessary.

While the information above is by no means exhaustive, being smart about your building choices, or hiring a professional to assist you with the best options available for your particular budget and site, is an important first step to giving your home and belongings the very best chances of surviving the next wildfire.

Footnotes:

[1] SW Drought linked to Climate Change - http://www.livescience.com/38099-southwest-drought-linked-ocean-temperature.html

[2] City of Prescott's Firewise Communities - http://www.cityofprescott.net/_d/fire_wise.pdf

[3] Strawbale Construction Protects Against Fire - http://www.strawbale.com/straw-bale-fire-resistant-southern-california

[4] How Fire Resistive Is Adobe Construction? - http://www.ehow.com/facts_7263761_fire-resistive-adobe-construction_.html

[5] Cal-Earth Institute - http://www.calearth.org

[6] The Benefits of Rammed Earth Construction - http://home.howstuffworks.com/rammed-earth-home2.htm

[7] How Well Do ICF's Hold Up in a Fire? - http://www.cement.org/homes/brief03.asp

[8] Autoclaved Aerated Concrete - http://www.cement.org/homes/ch_bs_autoclaved.asp

[9] Wildfire is Coming: Are You Ready? - http://www.readyforwildfire.org/defensible_space