Building Design

The Amazing Intumescent Paint

Years ago, I worked on a project where we did an addition above a large steel framed garage. The garage was attached to the side of a three story brick manufacturing building – a mattress factory that had been turned into a school. The addition above the garage was to be at the same floor level as the existing building’s third floor, which required the steel columns to be extended up through the existing garage roof to support the new floor. As a result, the steel columns had to be encased with fire-protection. The choice was to cover them with a couple inches thick fire-retardant foam insulation. The foam insulation was ugly and had an uneven finish. Since it was a garage, nobody minded. It was cheap. Problem solved.

A few years later, I worked on another project with exposed steel structure requiring fire-protection, but the steel was on the exterior face of the building, and the client insisted that the final product look good. What to do? The solution was to paint the steel with intumescent paint. A few coats – the final coat reasonably smooth, even, an architecturally acceptable color, and voila! – fire-protected yet nice-looking exposed exterior steel. Problem solved.

So what is intumescent paint? Intumescence is when material expands, the cells filing with air, when it is exposed to heat. Intumescent paint, while a thin (a few millimeters thick) when applied like paint to a surface, will expand out like foam when heated. The expanded foam providing fire-protection by thus preventing the heat of the fire from heating up the steel. Steel may not burn, but it does weaken when heated hot enough in a fire. Once it weakens, it fails structurally, leading to collapse of the structure. Part of fire-protection is preventing the collapse of the structure for a specific amount of time, typically two hours.

While intumescent paint maunfacturers have focused on fire-protecting steel, their products and paints can be used to protect other materials and other situations. For example, intumescent putties can provide fire-blocking when used to fill gaps where pipes and ducts, unavoidably, go through fire-rated walls.

Of particular interest to homeowners, is protecting the wood joists that are the structural support of the exterior walls of their homes. Many cities and states now require houses to be built with steel studs and two-hour fire-rated exterior wall assemblies. When a homeowner decides to enlarge his home, he may be required to make the entire house meet the current codes. If his existing house is built with wood stud exterior walls, he will need to replace the studs with new steel studs – this of course, is difficult and expensive. To resolve the problem, and retain the existing wood studs, the homeowner can choose to paint them with intumescent paint.

Another situation where intumescent paints are being put to a new use, is existing tall buildings with combustible aluminum cladding. Most aluminum exterior cladding on existing building is combustible. When a fire reaches the exterior wall via a window, it can race up the outside of the structure by burning up the aluminum cladding. Many countries are under considerable pressure to remove existing combustible exterior cladding and insulation from tall buildings after the 2017 Grenfell fire in London.

Removing the existing cladding and replacing it with non-combustible cladding is expensive. There is the cost to remove the old cladding, and install the new cladding, but the new non-combustible cladding is expensive. Many building owners simply do not have available money to get this work done.

Australia is permitting building owners to paint their existing combustible cladding with intumescent paint. While not a perfect solution, it does reduce the potential damage and danger if the building suffers a fire that gets to the exterior wall. It is also substantially cheaper than removing and replacing the existing cladding with non-combustible cladding.

New exterior cladding systems that are combustible are being installed with horizontal and vertical bars that are made from intumescent materials. The idea being to prevent a fire from being able to climb more than a few feet up the exterior, and not spread horizontally either. NFPA 285 provides for design of walls as assemblies, which can include combustible materials in fire-rated walls, if the flame and smoke spread are sufficiently limited, proven by testing that meets NFPA’s requirements.

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