Framing and exterior layers
The homes first layer of protection is the framing material itself. Metal is no-combustible, wood will burn however we can make wood-frame homes incredibly resistant to fire without chemicals. It comes down to keeping the fire and heat away from the wall framing.
Fires which start inside the home are much more common than wildfire exposure. This is where metal framing far outshines wood but metal is more expensive to build with and most builders are not familiar with it. So, how do we best protect the wood framing?
From the interior, nothing is much different between metal and wood. Walls are covered with 1/2" sheetrock. You can reduce interior combustibles in your design and use fire safe practices. This home is intended to be all-electric with a more than sufficient amount of circuiting to reduce overloading of any one circuit. Here metal framing helps by creating a grounding pathway if a circuit shorts and reduces the likelihood a fire will start in the walls or attic.
We can significantly reduce fire risk from the exterior whether building with metal or wood. This image shows the layers of an exterior non-combustible wall. The framing is metal (gray), the only combustible layer is the 1/2" plywood (orange) which is sandwiched between the wall framing, insulation and sheetrock. The next exterior layer is 1-1/2" Polyisocyanurate insulation board or Mineral Wool insulation (yellow). These are not combustible. On the exterior we use a metal, brick or stone or cement-fiber siding (green). These 2 outer layers prevent fire ignition and reduces the heat exposure to the studs. This is just as important in metal framing as in wood. Metal, exposed to high heat for enough time will soften and deform.
At the bottom and top of the walls these layers are overlapped to prevent embers from reaching the plywood. On the interior side we use fiberglass or mineral wool cavity insulation and sheetrock. I do not ever recommend a wood or paper fiber or spray foam insulation in the wall cavities.
Attic insulation
Attic Insulation not only performs as an insulating material but must also be non-combustible. The roof trusses are made from light-gauge steel regardless of the main framing material used in the home. This is done because the roof trusses represent the greatest exposure to ignition. The top of each structural frame is plywood and so is the roof sheeting.
To minimize fire risk there is 12" of insulation on top of the structural frame and stuffed into the metal trusses. This insulation must be fiberglass or mineral wool. Rigid Polyiso will leave too many gaps to seal effectively. This leaves only the roof plywood sheeting as a combustible material, the roofing itself is metal or tile. If the roof does catch on fire there is a much better chance it will not spread to the rest of the home.
The exterior wall sheeting, insulation and siding overlap the attic insulation and is capped with a metal strip or non-combustible trim material. This essentially isolates the structural frames from external fires and this same configuration makes the home very energy efficient. We have to insulate a home, let's add fire resistance to that requirement.
Siding and Roofing materials
There are many good options for non-combustible siding and roofing materials. These come in many looks and finishes from natural wood panels and boards to faux stone and brick. Just verify they are non-combustible as many of the same looking options also are made from plastics. Beware of materials, including insulating materials which claim a fire resistance because they are treated with a fire retardant chemical. These chemicals breakdown over time, losing their overall fire resistance yet they are never re-treated or replaced.
A common and very fire resistant siding is stucco. This is a good option to work with, just make sure you have a qualified installer and pay close attention to the finish details in areas where stucco meets trim and where the top of the wall stops under the eves. Stucco finishes must be high-quality and leave no gaps.
Siding and trim materials made from compressed wood fiber or plastics must be avoided. This is especially true for the roofing. Because you will always have a waterproofing layer and plywood sheeting under the roof material the only layer of protection is the roofing material itself.
Even if you don't live in a high-risk wildfire area we should build with these types of materials. They often perform better and last longer.
