Case Study – Soffit Insulation | Luton Pondswick Primary Substation
From failing roof to finished system
The Challenge
Duratite were engaged by Vulcan Passive Fire, appointed by UK Power Networks, to undertake the soffit insulation works to the undercroft of the newly constructed Luton Pondswick Primary Substation. The new substation forms part of UK Power Networks’ ongoing programme to expand power capacity and improve infrastructure resilience across the South of England.
The structure features a raised concrete floor slab supported by a steel frame to provide undercroft access for cabling, drainage and ventilation. However, the exposed soffit presented a significant risk of thermal loss and condensation transfer to the internal switchgear above — equipment which requires strict control of ambient temperature and humidity to operate safely and efficiently.
Ensuring the soffit slab achieved both thermal equilibrium and fire compliance was therefore a critical design requirement for the client. Any fluctuation in temperature could result in thermal shock, electrical malfunction or equipment degradation, and as such, insulation integrity and fire certification were paramount.
Project file
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The Solution — step by step
Scope of Works
Duratite designed and specified their approved fire-rated soffit insulation system, utilising Enverge Nexseal LE closed-cell spray foam insulation applied directly to the underside of the concrete soffit, overcoated with DC315 Intumescent Coating in Ice Grey to achieve full Euroclass B-s2-d0 fire compliance.
An average thickness of 90mm Nexseal LE was applied to 127m² of concrete soffit by Spray Foam Tech, a Duratite Approved Contractor, during October 2025.
The spray foam formed a continuous, fully bonded insulation layer, eliminating air gaps and thermal bridging across the surface. Once cured, the DC315 Intumescent Coating was spray-applied to the foam to provide an enhanced fire-rated finish while simultaneously protecting the system against the potential effects of interstitial condensation within the undercroft environment.
This approach provided an integrated solution meeting the thermal, fire safety and environmental performance requirements of UK Power Networks.
The Outcome
- Thermal Conductivity (λ): 0.027 W/m·K
- Fire Rating: Euroclass B-s2-d0 (with DC315 Coating)
- Insulation Thickness: 90mm Average
- Coverage Area: 127m²
- System Components: Enverge Nexseal LE Closed-Cell Spray Foam + DC315 Intumescent Coating
- Colour Finish: Ice Grey
The completed system delivered a robust, seamless insulation barrier directly bonded to the concrete substrate, providing long-term thermal stability and eliminating cold bridging through the soffit. The DC315 Intumescent Coating ensured the system achieved its specified fire classification while providing a neat, uniform finish suited to an exposed service area.
By specifying the Duratite-approved Nexseal LE and DC315 combination, Vulcan Passive Fire and UK Power Networks gained an installation that not only met the project’s stringent fire safety and performance standards, but also offered long-term durability and resistance to environmental degradation.
This project demonstrates Duratite’s capability in delivering technically advanced spray-applied insulation systems for critical electrical infrastructure — where fire protection, thermal efficiency, and installation quality are non-negotiable.
Products used
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