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The substation undercroft after the work: the concrete soffit and its downstand beams in a smooth ice-grey coated finish over the closed-cell layer, columns and cable routes below.
Project recordCoated soffit

Project record · Concrete soffit · Coated system

Pondswick substation, Luton: 127 m² of concrete soffit insulated and coated under live switchgear.

A primary electricity substation with an exposed concrete soffit under rooms of switchgear that must hold steady temperature and humidity. In October 2025 we applied an average of 90 mm of NexSeal LE closed-cell to the soffit and overcoated it with DC315 intumescent coating in ice grey, commissioned through the passive fire contractor for the site.

127m²
Concrete soffit treated
90mm
Closed-cell, average depth
DC315
Intumescent coating, ice grey
Oct 2025
Completed

01The building

A cold slab under rooms that cannot be cold.

A primary substation is a building whose interior conditions are not optional. The switchgear above this soffit needs tight control of temperature and humidity, and the exposed concrete slab beneath it was a large cold surface, losing heat downward and threatening to carry condensation into the rooms above.

The thermal risk. Concrete has high thermal mass and stays cold. An uninsulated soffit under a conditioned room is a continuous heat loss and a cold bridge along every beam and column head, with the plant above working against it.

The condensation risk. A cold slab and humid air meet at the surface. Condensation on or within the structure beneath switchgear is a risk the operator cannot carry, so the soffit had to be lifted above the dew point and kept there.

The fire requirement. An exposed foam surface in an electrical building needs the finish the fire specification calls for. Here that was DC315 intumescent coating, in ice grey, over the whole foamed soffit.

Building
Primary electricity substation, undercroft soffit
Commissioned through
Vulcan Passive Fire, for UK Power Networks
Location
Luton, Bedfordshire
Substrate
Concrete soffit, undersideBeams, columns and services worked around
Area
127 m²
Application
Enverge NexSeal LE closed-cell, 90 mm average
Finish
DC315 intumescent coating, ice grey
Date
October 2025

02What we specified, and why

90 mm of bonded closed-cell to the slab, then the coating the fire specification required.

Depth for the thermal target and the dew point, bonded to concrete without fixings, finished in the coating the fire specification named. On a soffit full of beams, columns and penetrations, boards were never an option.

  • The foam: Enverge NexSeal LE closed-cell, 90 mm on average. Sprayed directly to the concrete in passes, it bonds to the slab, follows every downstand and column head and closes around the penetrations. The depth was set by the design: enough to meet the thermal target and hold the surface above the dew point. Concrete soffits, the application.
  • Why closed-cell on concrete. It bonds, so there are no fixings into the slab and no voids behind the layer where cold air could reach cold concrete. NexSeal LE declares a water vapour resistance factor (μ) of 62 to BS EN 12086, used in the condensation-risk analysis to BS EN ISO 13788 and the design to BS 5250.
  • The coating: DC315 intumescent, ice grey. A water-based intumescent coating made for protecting spray foam insulation, applied by airless spray over the cured foam at a minimum of 0.7 mm wet film thickness and checked with a wet-film gauge. Ice grey is one of its three standard colours. The coating, explained.
  • Why the whole soffit. A cold bridge anywhere is a condensation point. The layer runs continuously across the slab, the beams and the column heads, and the coating runs with it.
Foam
Enverge NexSeal LE closed-cell, 90 mm average
Coating
DC315 intumescent, ice grey, min. 0.7 mm WFT
Substrate
Concrete soffit, beams and column heads
Vapour
μ 62 (BS EN 12086), in the risk analysis
The finished soffit in DC315 ice grey: a smooth, uniform coated surface across the slab and its downstand beams, the column heads wrapped in the same finish.

The finish. DC315 in ice grey over the whole foamed soffit, beams and column heads included: one uniform surface. Project record.

Photographs · Project record

The slab bare, the foam on, the coating over it.

The exposed concrete undercroft, the closed-cell going on around the beams, and the coated soffit as handed over.

Before: the exposed concrete soffit of the substation undercroft, downstand beams and column heads bare.
Project recordBefore: exposed slab
Before: the undercroft looking along the bays, concrete above, cable routes and columns below.
Project recordBefore: the undercroft
During: closed-cell foam being sprayed to the concrete soffit, the cream layer following a downstand beam.
Project recordDuring: foam to the slab
During: the application advancing across the soffit, beams and column heads wrapped as the layer grows.
Project recordDuring: around the beams
After: the coated soffit in DC315 ice grey, a uniform finish across the slab and beams.
Project recordAfter: ice grey
After: the finished undercroft, the whole soffit coated, columns and services below.
Project recordAfter: handed over

Photographs from the job, as it ran. Labelled Project record because they are.All project records

03How the work ran

Prepare, foam to depth, coat, gauge, hand over.

A live electrical site with a passive fire contractor in charge of the specification. The soffit was prepared, the foam built to an average of 90 mm, and the coating applied and checked for film thickness before the undercroft was handed back.

  1. Survey and specification

    The slab read, the fire spec fixed.

    Soffit geometry, beams, column heads and penetrations surveyed; the depth set to the thermal design and the DC315 finish confirmed with the passive fire contractor.

  2. Preparation

    Concrete cleaned, services masked.

    The slab prepared for adhesion, cable routes and fittings masked, and the undercroft set up for controlled access and ventilation.

  3. Foam

    In passes to 90 mm average.

    NexSeal LE sprayed directly to the concrete, continuously across the slab, the beams and the column heads, built up in passes to the design depth.

  4. Coating and hand-back

    DC315, gauged, then handed over.

    The intumescent coating applied by airless spray over the cured foam, wet film thickness checked with a gauge, and the undercroft handed back with the record of what was applied.

04The outcome

A continuous insulated soffit under the switchgear, in one coated finish.

Reported by the client in their terms, and visible in the photographs. No temperature or humidity readings were published and none are claimed.

Cold bridging

Eliminatedas reported

One continuous bonded layer across slab, beams and column heads, with no fixings and no voids.

The soffit surface

CoatedDC315 ice grey

A uniform intumescent finish over the whole foamed area, as the fire specification required.

Area

127m²at 90 mm average

Concrete soffit insulated and coated beneath the switchgear rooms.

The rooms above

Protectedthermal and moisture

The slab lifted above the dew point and kept there, so the switchgear environment is no longer at risk from below.

Where this record comes from

Reported, not metered by us. No readings from the switchgear rooms were published; the result is the client's report and the finish is in the photographs.

The fire evidence is stated as tested. Euroclass B-s2,d0 belongs to the NexSeal LE and DC315 build-up tested at BTTG, and the qualifier above is part of the claim.

05Where this record leads

If this looks like your building.

The application this record used, the coating that finished it, and the building type for concrete soffits.

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