12. Fire Risk

A Builder/Firefighter’s Thoughts on Fire Issues in Residential Construction and Remodeling

Convenience Store after Catastrophic Second Fire

Convenience Store after Catastrophic Second Fire

Spray foam is only one of a number of building products and methods which increase the risk of catastrophic fire loss (and potential loss of life).

There has been some media and regulatory attention given lately to auto-ignition of spray polyurethane foam (SPF) insulation during the application process. [see Massachusetts Fires Tied to Spray Foam Incite Debate.]  While the SPF industry has, understandably, reacted defensively to this development, there are legitimate reasons for caution and concern.

The SPF industry has taken an industrial, chemical process that works quite well in the controlled setting of a highly-regulated factory with in-house and outside safety specialists monitoring the process — and set it loose in the field in an uncontrolled environment, with far less consistent applicator training and an impossibly difficult arena for safety monitoring. A significant increase in application errors is to be expected, particularly when thickness limits are ignored in order to complete a job or to meet IECC R-value standards.

However SPF is only one of a number of relatively “modern” building products and methods which increase the risk of catastrophic fire loss (and potential loss of life). As with almost all technological advances, there are unintended consequences that are not either recognized or considered in the rush toward market acceptance.

As one who has been both a designer/builder and a volunteer firefighter for 30 years, as well as an instructor in sustainable design and construction, I can shed some light on the issue of fire safety in construction and remodeling.

Dangers of truss and engineered lumber construction

One common construction practice has been the use of light-frame floor and roof trusses. These have often been replaced, at least in floors and sometimes roof assemblies, with “engineered lumber” such as wood I-beams. In both cases, structural integrity depends on the synergy of all parts of the assembly, including chords, webs, struts & ties and their connectors (typically metal truss plates). With smaller elements, these engineered structural units fail much more quickly in a fire than either light wood frame or timber frame construction. In trusses, the metal gang nailing plates quickly overheat, char the wood they penetrate and fail.

Even small-town fire departments often have to engage in pre-planning to note which buildings in their jurisdiction are made from truss or engineered lumber construction. An I-joist, like a truss, will also fail more quickly than a comparable 2x joist or rafter. We typically will not enter such a building for interior fire suppression and will avoid going onto the roof to ventilate the heat. This leaves only outside fire suppression and makes the building more vulnerable to fire and water damage as well as collapse.

Ventilating a fire in progress

Total Loss of Multiple Businesses Exacerbated by Generations of Remodeling

Total Loss of Multiple Businesses Exacerbated by Generations of Remodeling

Early morning of April Fools Day, 2008, my department responded to a structure fire in our central village — a former home and barn used as a spa/salon and an antique store. The fire was triggered by a propane explosion, and the building was fully involved at our arrival. The building had been renovated so many times that we discovered (after the fact) a number of double walls and a double roof built up over an existing roof. Attempting to ventilate the roof did not work because of the secondary roof below. Outside attack hose streams were blocked by extra walls just inside the windows, and the fire chases created by the doubled interior partitions increased the rate of fire spread. Unintended consequences.

Cellulose implicated in convenience store fire

In August of 2006, our department was called to a convenience store under construction after the original one burnt down and a new model replacement (which was meant to be the prototype for all similar stores for this locally-owned Vermont chain) also burnt to the ground just two months earlier. It seems the same cellulose insulation contractor who had blown the suspended ceiling the day before the previous fire was at the site filling the ceiling again. This time, an employee noticed smoke coming from the hopper of the truck-mounted blowing machine, so he stopped the operation and emptied out the hopper. At the bottom, just above the overheated muffler from the compressor, was some charred cellulose.

If properly treated with borates, cellulose is one of the most fire-proof insulations on the market. While it won’t support combustion, it will char and smolder. Apparently, two months earlier and with the same blower, this contractor had put some smoldering cellulose into the ceiling. Overnight, it ignited some non-protected framing member, which spread the fire to the entire hollow ceiling and metal-clad roof. Almost as soon as the fire department arrived, the entire roof collapsed. Unintended consequences.

I was the firefighter who collected samples of the charred cellulose from the second reconstruction and got it to the state fire inspector. The earlier fire was then determined to be caused by a faulty cellulose machine and that contractor went out of business. We provided water while a septic pumper sucked all the new cellulose out of the ceiling and saved the store from a third fire.

Charred Cellulose Insulation from Blower Hopper

Charred Cellulose Insulation from Blower Hopper

Choose building materials with care – and consider unintended consequences

The lesson is that care and consideration are required, both in choosing materials and methods and in maintaining a safe construction site environment. Some materials, such as plastic foams, can dramatically increase fire risk – both in application (ASTM E85, referenced in IRC R316, sets flammability standards and thickness limits, which are often less than code-minimum R-values) and during occupancy. If foam insulation is part of a structural panel system, then the structure can collapse once the melting point is reached or the required 15-minute ignition barrier is breached. Once burning, plastics accelerate fire spread, smoke development and building damage.

Using non-flammable materials and avoiding fire chases (including the exterior chase created by a rainscreen cladding system) can help reduce both the likelihood of fire and the extent of fire damage. Make sure there are adequate fire stops in all enclosed soffits, attic kneewall areas, under first floor bathtubs and around chimneys. Install wood-burning appliances (and all combustion appliances) to factory specifications and according to applicable fire codes.  Design in fire egress windows in all sleeping areas, and a safe route to the ground. Beware of wildfire zone issues and codes. Of course, install hard-wired smoke and CO alarms and make sure any exhaust flue exits the house above maximum snow level to avoid CO back-drafting.

But, above all, think about potential unintended consequences. Every design and construction decision has the potential to increase or reduce the risk, intensity and extent of fire. About 4,000 people die every year from fire in the US, and the almost 400,000 annual residential fires create nearly $8 billion in losses. These are consequences best to anticipate and avoid.


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by Robert Riversong: may be reproduced only with author attribution for non-commercial purposes and a link to this page

2 Responses to 12. Fire Risk

  1. paula says:

    What roofing material do you recommend? I thought metal roofs were best, but in a fire, it sounds like they could make things worse. Do you recommend wood shingles? Aren’t they flammable too?

  2. Riversong says:


    As noted in the article, the answer (as with most answers) is “it depends”. If the primary fire risk is from a woodstove chimney or wildfire, then a non-flammable roof (like metal or slate or tile) might be best (and no soffit vents). Otherwise, since most residential fires are of interior origin, a self-venting roof might limit the fire damage. Wood shingles or shakes can be pre-treated with (toxic) fire retardant, but that undermines the “natural” low-ecological footprint value, and probably the recyclability, of the material.

    Roofing materials are UL rated as Class A (high degree), Class B (moderate degree) or Class C (light degree) of fire spread protection from externally-caused fires. Class A fiberglass-mat asphalt shingles are the most common roofing material, and the one I almost always use in new construction. The “architectural”, or layered, shingles are thicker and more durable, and some come with a warranty nearly as long as a painted metal roof.

    But there are other, perhaps more important, considerations for roofing material choice than the low probability of house fire, including aesthetics, cost, durability, wind resistance, water vapor breathabiity, ecological impact, ease of repair, and whether it holds or releases snow and ice. The primary reason I avoid metal roofs on homes in snowy New England is that they produce “avalanches” which can tear off rain gutters, plumbing vents and metal chimneys as well as damage things on the ground and make snow shoveling difficult.

    On the other hand, if you’re going to install roof-top PV panels, then a standing-seam metal roof allows attachment without drilling holes through the roof that then rely on caulking to prevent rain infiltration.

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