To ban or not to ban, that is the question

The much anticipated publication of the government’s consultation on banning combustible cladding materials has at least the merit of putting to rest any ambiguity about what materials it is safe to use on high-rise residential buildings.

The government makes its position absolutely clear in the consultation. Its preferred option is a ban on all combustible cladding materials used on buildings over 18m high. To avoid any confusion this will also apply to the entire building, not just the bit above 18m.

The ban will apply to the complete wall assembly including balconies and brises-soleil. The option to prove compliance with Approved Document B of the Building Regulations, which covers fire by using a full-scale test to BS 8414, will also be banned. This means the use of controversial desktop studies to prove compliance becomes redundant as these assessments were ostensibly based on BS 8414 test data.

The government is saying the industry isn’t capable of constructing buildings correctly and has very limited understanding of anything technical

The ban wouldn’t apply to the gaskets, seals and vapour membranes used for rainscreen cladding systems. A ban on these materials would have meant the end of rainscreen cladding systems until non-combustible alternatives could be developed, forcing the industry to adopt much more expensive masonry alternatives.

As it stands, the ban won’t have that much impact on external cladding panels: even the type of panel used on Grenfell Tower, the primary reason for the spread of the fire, is now available with a non-combustible core. Other materials, such as sheathing boards used in external cladding systems, are also available in non-combustible versions. This will all cost more – an extra £60/m² for the cladding alone, according to Arcadis.

Inevitably such an uncompromising ban will include some losers; in this case it is the sustainability agenda.

The ban means only mineral wool insulation type products will be acceptable within cladding systems. These are less thermally efficient than foamed plastic alternatives, meaning an 80% increase in thickness for the same thermal performance. This extra material means a heavier cladding support system is needed and the extra space taken up by the insulation will mean either smaller apartments or a bigger building footprint, assuming planning permission is forthcoming. It will also squeeze down the number of insulation manufacturers that produce compliant products, which could impact on production capacity and costs. All this means increasing thermal standards for external walls will become much harder to sell to the industry.

The ban could also present a challenge to the industry in its growing use of cross-laminated timber (CLT) for residential buildings. CLT has proved a popular alternative to concrete as CLT buildings are much faster to construct and have a far smaller carbon footprint. CLT is already being used for residential buildings over 18m – Dalston Works in London is billed as the world’s largest CLT building, at 33m tall.

The ban could also have an impact on lower-rise buildings under the 18m height threshold. Would you want to buy the top-floor flat in a 17m-high block built quite legally with combustible cladding, knowing the tall building next door contained non-combustible cladding all the way down to the ground? Even if buyers aren’t aware of the materials used for the walls of buildings, the highly sensitised, risk-averse insurance and mortgage lending industry will certainly want to know. And will developers of lower-rise residential blocks of flats want to take the risk of struggling to sell apartments if they use combustible materials for the structure or walls?

Ruling out the use of cross-laminated timber and thermoset insulation will have a major impact on construction and should be properly scrutinised. Both materials form a protective char in a fire and tend to stop burning when a flame is taken away from the surface. The expert reports into the fire at Grenfell concluded the external cladding was the primary cause of the spread of the fire, while the evidence was inconclusive on the role of the insulation.

In many ways the consultation is a throwback to a pre-Hackitt world. The consultation acknowledges systems tested to BS 8414 “provide a safe way to ensure the wall system will resist the spread of fire if correctly installed and maintained”. It says it wants a ban as BS 8414 is “not a straightforward way to achieve compliance”. Reading between the lines, the government is saying the industry isn’t capable of constructing buildings correctly and has very limited understanding of anything technical, a criticism that isn’t entirely unjustified in the light of Grenfell.

But aren’t Hackitt’s proposals meant to tackle this? Hackitt’s report is aimed squarely at making buildings safer from concept through to occupation by scrutinising designs and construction quality much more closely. It also aims to tackle the problem of maintenance work compromising building safety, through regular safety case reviews of occupied buildings. 

Hackitt also proposes independent third-party testing of products. If her recommendations were followed through, and BS 8414 was revised to make it more representative of real world conditions as called for by insurers, there could be a case for allowing cladding systems that passed this test. Otherwise it could be a case of intense political pressure trumping sound scientific evidence.

This week's poll: Do you agree with the government's proposal to ban combustible materials in cladding systems?

— Building News (@BuildingNews) June 20, 2018