Investigators in London are studying whether an aluminum-faced cladding that contained rigid foam insulation contributed to the speed at which fire engulfed a 24-story apartment building on June 14, killing at least 79 people.
The fire at Grenfell Tower, a 24-story building in West London that contained 120 apartments, raced up the sides of the building with a speed that amazed firefighters. Attention soon turned to the role of aluminum-faced insulation panels in the fire’s rapid spread. The Reynobond PE cladding panels were added to the building’s exterior during a renovation.
Reynobond consists of two sheets of aluminum, each 0.02 inches thick, with a core of extruded polyethylene insulation in between. (In the U.S., extruded polyethylene is more often used for pipe insulation or for cushioning delicate equipment in packing cases than for building insulation.)
This type of panel isn’t typically found on residential projects. A growing number of residential builders in the U.S., however, attach rigid foam insulation directly to the exterior of wall or roof sheathing, and cover it with a vented rainscreen and siding. That’s a relatively common approach in high-performance buildings where designers are looking for ways to increase the overall R-value of the wall and minimize thermal bridging through wood framing.
The fire in London with its horrific loss of life may strengthen arguments from foam opponents here, but it should not dissuade builders from continuing to use rigid foam in wall assemblies, one Massachusetts architect says, because the energy-efficiency advantages of foam outweigh its risks.
Details at Grenfell Tower
It may take many months before investigators in London are able to pinpoint the cause of the fire, and whether the cladding played a significant role in how quickly it spread. The tower, with 20 floors of apartments, was originally built in 1974. It underwent a renovation, completed in 2016, which included the installation of the Reynobond PE panels.
The Guardian reports that the wall assembly included Reynobond PE and a layer of Celotex RS5000 polyisocyanurate insulation behind it, the two separated by an air gap. Celotex manufacturer Saint-Gobain says that the insulation was “developed specifically to enhance the thermal performance of external facade constructions.” The foil-faced polyisocyanurate has a “Class O” fire performance rating, a category used for hospitals, schools and retirements homes.
The £10 million renovation involved a number of companies and subcontractors, including Harley Curtain Wall/Facades, which won the £2.6 million contract to install new cladding. The Guardian reported that Harley used products from both Celotex and Arconic, which manufactured Reynobond. Contractors saved about £2 per square meter by installing Reynobond PE instead of a more fire-resistant cladding panel called Reynobond FR, the newspaper said. The FR version has a mineral core that resists fire.
Residential use of foam is different
Will the fire give builders who use exterior rigid foam a reason to change their minds? Possibly. But
Steven Baczek, an architect in the Boston area who has designed a number of high-performance houses, says that rigid foam insulation is probably here to stay because of its many energy-efficiency advantages. The way foam is typically used in residential building — installed on the exterior side of structural sheathing — reduces the risk of injury and death in a fire because the sheathing offers an ignition barrier that should give occupants time to get out of the house.
“If I put up a Zip [System] wall and then put polyiso on the outside, I effectively have the rigid foam outside of at least a 30-minute, maybe one-hour, assembly,” he said in a call. “So what am I worried about there? … Would it go up quick? Probably, but it’s on the outside of the house.”
Spray foam used on an attic interior must be covered by a 15-minute ignition barrier, he added, “so the plywood in the wall is doing you at least double what the code requires you to do in the attic.”
If occupants couldn’t get out of a burning house in that time frame, he said, it probably means there’s something wrong with fire alarms inside the house.
“It’s certainly a tragedy,” he said of the London fire. “You don’t want any house to burn down. You don’t want any life to be lost. But foam provides comfort and energy efficiency over a very long time. You don’t want to say it’s worth a life. It isn’t. But the reality is the benefits seem to outweigh the risks.”
Are rainscreens a bigger risk?
Bazcek is more cautious about rainscreens, which are built-in gaps between the back of the siding and the insulation or sheathing that permit siding to dry faster and accumulated moisture to escape more readily. Ventilated rainscreens are becoming more common, but Bazcek likened them to
a “chimney every 16 inches on-center around the house.”
In the case of Grenfell Tower, an unbroken air gap would amount to a chimney more than 200 feet tall. It’s not clear whether fire breaks had been included in Grenfell’s walls to slow or stop the movement of fire from one floor to the next. But even in residential construction, with a rainscreen of only one or two stories, flame spread is enough of a concern in some jurisdictions that inspectors require it to end at the top of the wall — without connecting to the attic or roof ventilation, Bazcek said.
That said, he prefers to tie the two together because it’s more effective at removing moisture.
“You elongate the stack effect by connecting the wall to the roof ventilation system, and if you elongate the stack effect you increase the effectiveness,” he said. “It’s nice when it’s connected, but some jurisdictions don’t allow you to do it.”
Bazcek plans to continue using both rigid foam and vented rainscreens in the walls he designs, believing the advantages outweigh the risks. He also points to an inherent risk in all wood-framed buildings, whether they use rigid foam and a rainscreen or not: wood in old houses is very dry and very combustible.