Modular, CLT, SIPs, and prefabricated buildings are the future of UK construction. But their fire safety performance during construction is keeping fire engineers awake at night.. The Government's Bet on MMC The UK government has committed to MMC delivering 300,000 new homes per year . Major developments are increasingly using: Volumetric modular construction — complete room modules manufactured off site Cross laminated timber (CLT) — engineered timber panels replacing concrete Structural Insulated Panels (SIPs) — foam core panels with OSB/metal skins Light steel frame — cold formed steel with applied insulation 3D printed concrete — additive manufacturing for complex geometries Each brings genuine benefits: faster construction, less waste, better quality control, and reduced carbon. But each also brings fire safety challenges that the regulatory framework hasn't fully addressed. The Construction Phase Problem When MMC Buildings Are Most Vulnerable MMC buildings are at their most fire vulnerable during the construction phase — precisely when fire protection systems are incomplete: CLT/Mass Timber: Exposed timber surfaces present massive fire load No compartmentation until linings are installed Fires spread rapidly through exposed timber elements The Barking Riverside fire (2019) destroyed a 6 storey CLT building under construction in 20 minutes Modular Construction: Module wrapping materials are often combustible Temporary connections between modules create fire spread paths Stacked modules without fire stopping = vertical chimney effect The George Street (Bolton) fire showed how quickly modular buildings can be consumed SIPs: EPS/PIR foam cores burn intensely when exposed Panel joints can create hidden fire paths Once the outer skin is breached, the core ignites rapidly Fire temperatures from SIP fires exceed 1,100°C Completed Building Performance CLT — The Evidence So Far In completed buildings with full encapsulation (plasterboard lining covering all timber), CLT performs well: Char rate: 0.7 mm/min (predictable and consistent) Encapsulation behind 2 layers of 15mm Type F plasterboard provides EI 60+ No UK fire fatalities in completed, properly designed CLT buildings But the risk is in the execution: Any gap in encapsulation exposes timber to fire Service penetrations through linings must be fire stopped meticulously Renovation works that breach encapsulation create new risk The 'as built' condition may differ from the 'as designed' condition Regulatory Position Approved Document B: Permits CLT for buildings up to 18m (with conditions) BS 9999: Provides guidance on timber building fire safety Insurance industry: Increasingly requiring sprinklers for CLT buildings at all heights BSR: Scrutinising CLT designs closely in Gateway 2 applications The Insurance Challenge Premiums Are Speaking Insurance premiums for MMC buildings are 35 60% higher than traditional construction: Construction Type Premium Multiplier vs. Traditional Traditional masonry 1.0x Light steel frame 1.15x CLT (sprinklered) 1.35x CLT (unsprinklered) 1.85x Volumetric modular 1.40x SIPs 1.60x Some insurers are simply refusing to cover MMC buildings during construction — creating a financing gap that threatens project viability. Making MMC Safe The 8 Essential Principles 1. Full sprinkler coverage — regardless of height 2. Enhanced construction phase fire plans — beyond JCOP minimum 3. Encapsulation quality control — independent inspection of all fire rated linings 4. Service penetration management — zero tolerance approach to fire stopping gaps 5. Enhanced smoke detection — aspirating detection throughout 6. Comprehensive fire engineering — from concept to completion 7. Insurance engagement early — at planning stage, not post construction 8. Ongoing maintenance regime — protecting the fire safety measures throughout the building's life Magnus Opifex provides specialist fire engineering for MMC projects. Contact us for a consultation.