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Last week the government published its consultation into a ban on combustible cladding materials. The proposals are uncompromising: it suggests a ban on all combustible materials in the walls of buildings over 18m high. The government also wants to see an end to full-scale testing of cladding systems as an alternative means of compliance. This means the end of desktop studies, as these were meant to be based on the results from these tests. 

These proposals are hardly a surprise, given the way fire spread rapidly up the exterior of the Grenfell Tower. Since then, there has been pressure for a ban on combustible cladding from relatives of the victims of the tragedy, as well as the media and some in the industry. But what are the implications of the proposals?

The consultation says that buildings over 18m high should not include combustible materials in the 鈥渃omplete wall assembly, including the inner leaf, insulation and the facade鈥�. This ban includes the areas of the building below the 18m cut-off too. The consultation acknowledges that the external wall in a high-rise building is usually separate from the wall assembly, but does not explicitly say the ban excludes the structure. Some interpret this as excluding the structure from the ban, while others take a different view.

It could potentially mean a complete collapse of CLT in this country 鈥� we would go from a world leader to a backwater

Anthony Thistleton, Waugh Thistleton

Combustible paints, wallpaper, laminated glass and seals would still be permitted. 鈥淭here are certain materials that aren鈥檛 available in non-combustible versions at the moment, which include vapour membranes, gaskets and seals,鈥� explains Andrew Mellor, a partner at PRP Architects. 鈥淚f there was a blanket ban, rainscreen cladding systems would disappear until the manufacturers developed new products.鈥�

The ban would also apply to balconies and brises-soleil attached to facades. It would have a limited impact on the choice of external cladding panels, as there are plenty of compliant materials available. Cladding components such as sheathing boards are already available in non-combustible versions. Masonry construction would be unaffected.

There are potentially two big losers from the proposed ban on combustible materials: foamed plastic insulation and cross-laminated timber construction. Foamed plastic is popular because of its high performance and light weight. Non-combustible mineral wool needs to be 80% thicker for the same thermal performance and is much heavier. It requires a much more substantial cladding support system and thicker window reveals, which reduces the amount of light reaching inside apartments. This comes at a cost. 鈥淥n cramped sites, this will result in loss of floor area, with potential significant impacts on viability,鈥� warns James Knight, the partner in charge of residential at Arcadis.

Moreover, Knight has studied the government鈥檚 cost assessment of the change to non-combustible materials. 鈥淲e think that the modelled assumptions around both cladding area and costs are optimistic,鈥� he says. According to the consultation, a 15-storey building has a cladding area of 1,700/m², pushing up the cost by 拢25,000 to 拢75,000. According to Arcadis, a 15-storey building has nearly double that cladding area, with a non-combustible rainscreen system costing substantially more 鈥� an extra 拢350,000 to 拢400,000. A switch to a masonry system would push the cost up by 拢850,000 to 拢900,000. Arcadis warns that the ban would limit supply and choice, with the result that prices would go up even more.

The biggest losers

Unsurprisingly, foamed plastic insulation manufacturers are not happy. According to John Garbutt, Kingspan Insulation鈥檚 divisional marketing director, there is 鈥渘o way鈥� the company鈥檚 foamed plastic products could pass a test that met the consultation criteria because it is 100% organic. The test for non-combustibility, called EN 1716, involves heating the material

in an enclosed chamber and measuring how much additional heat is produced by the test sample. Garbutt says the phenolic insulation manufactured by Kingspan chars when exposed to a flame and goes out when this is taken away. 鈥淭he test doesn鈥檛 tell you how quickly the sample burns or reacts to a fire.鈥�

The full-scale test of a complete cladding system to BS 8414 was used by manufacturers such as Kingspan Insulation to demonstrate compliance with Part B, an option that would no longer be available. 鈥淢aterial testing can鈥檛 be a proxy for system performance,鈥� Garbutt says. 鈥淭here is a cavity in rainscreen cladding systems. You can鈥檛 do a material test that measures the impact of the width of that cavity on how the system performs.鈥�

Waugh Thistleton Architects specialises in designing cross-laminated timber residential buildings. This includes Dalston Works, a 33m-high apartment block in London鈥檚 Hackney that was completed last year. Director Anthony Thistleton interprets the ban as applying to CLT construction and warns it could spell the end of a rapidly growing form of construction because the 鈥渟weet spot鈥� for CLT is on buildings more than five storeys high. Below that height, other materials such as SIP panels make more financial sense. 鈥淣othing in the wall from the inner to the outer surface can be combustible. We think it will preclude the use of CLT from being used above six storeys,鈥� he says. 鈥淚t could potentially mean a complete collapse of CLT in this country 鈥� we would go from a world leader to a backwater.鈥� 

Thistleton points to the benefits of CLT: it is lightweight, which means that less concrete is needed for the foundations; it has low embodied carbon content; and it enables a rapid construction process.

CLT is seen as offering better fire performance than traditional timber frame because of its relatively small surface area to volume ratio.

鈥淲e are confident CLT doesn鈥檛 offer any additional risk at height and fire doesn鈥檛 progress up a building in the same way as at Grenfell,鈥� Thistleton says. Unfortunately, there is a paucity of CLT cladding system test data to back this up and only eight weeks until the consultation closes in August to find it. 鈥淲e can demonstrate it is safe but we need to be given a chance to do it,鈥� he says. Waugh Thistleton is co-ordinating a CLT-industry-wide response to the proposal.

Thistleton and others warn that a ban on combustible materials in cladding systems above 18m could shake confidence in lower-rise buildings. 鈥淭he minute it is banned on buildings over 18m, it has an impact on lower-rise buildings too,鈥� he says. PRP鈥檚 Mellor reckons this is already happening. 鈥淲e are already seeing solicitors and mortgage providers asking questions about lower-rise buildings.鈥� He adds that some clients were switching to non-combustible cladding systems for buildings still on the drawing board before the proposed ban was announced. This trend is likely to accelerate as the consultation proposes any building that has not started on site would be covered by the ban, even if the design had already been signed off by building control. 

Mellor thinks the ban could also have ramifications for existing buildings, as the regular safety case reviews proposed by the Hackitt report for existing high-rise buildings will pick up combustible cladding materials. 鈥淭his has major implications for viability, cost and the impact on the ability to sell,鈥� he says.

Full system testing

Both Thistleton and Kingspan鈥檚 Garbutt believe the answer to the problem of fires in cladding systems is full system testing. Thistleton points out that the proposed ban is a prime example of prescriptive regulation, an approach Hackitt warned against in her report. He draws attention to her suggestion that the industry should, instead, be moving towards systems that are certified and tested. Garbutt says full-scale testing to BS 8414 is the only way to establish system performance. BS 8414 is intended to demonstrate that fires will not spread externally faster than inside the building. 鈥淵ou show me a building anywhere in the world where there has been an out-of-control cladding fire where the system has passed or would pass a BS 8414 test. I don鈥檛 think you will find one.鈥�

Like Thistleton, Garbutt says the answers are in Hackitt鈥檚 final report. 鈥淭he elephant in the room is that people don鈥檛 build things properly,鈥� he says. 鈥淗ackitt was bang on: you can regulate such that we build everything out of steel and concrete, but people will still build things that don鈥檛 meet the regulations.鈥� He concedes that Hackitt鈥檚 recommendations will take several years to implement and suggests one way to tackle poor build quality in the interim is to train up a crack force of building control officers to focus purely on high-rise residential buildings. Given the highly politicised nature of this debate, Thistleton and Garbutt are likely to be disappointed by the government鈥檚 final decision.