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The Box, Plymouth, from the street: the museum’s chequered glass-and-tile facade above its glazed entrance.
Project recordThe soffit, finished

Project record · Concrete soffit · Specified U-value

The Box, Plymouth: 787 m² of concrete soffit at 120 mm, to a specified U-value of 0.18.

Plymouth's museum, gallery and archive, built under a main contractor to a specification that set a U-value of 0.18 W/m²K through its concrete soffits. Irregular profiles, tight access and a great many service penetrations ruled out boards. We applied 120 mm of NexSeal LE closed-cell, checked it for adhesion and thickness, and the work was signed off by site inspection on schedule.

787m²
Concrete soffit treated
120mm
Closed-cell, bonded to the slab
0.18W/m²K
Specified U-value, achieved
On schedule
Verified by site inspection

01The building

A cultural building with a cold slab and a hard specification.

The Box is a civic museum, gallery and archive: a building that has to hold its environment for the things it keeps as well as the people who visit. Its concrete soffits were specified to a U-value of 0.18 W/m²K, in zones where the profile was irregular, the access was tight and services passed through the slab everywhere.

Why the soffits mattered. Exposed concrete has high thermal mass and stays cold. Under a conditioned building that means continuous heat loss, a cold bridge at every beam and column head, and a condensation risk wherever humid air reaches the slab.

Why boards were out. An irregular soffit profile, many penetrations and tight working spaces leave a board system with cuts, gaps and voids. Each void is a thermal short-cut and a place for condensation to form behind the insulation. A sprayed, bonded layer has none of them.

The brief. Thermal performance to the specified figure, moisture control, airtightness at the slab, and all of it verified: adhesion, thickness, and a site inspection before sign-off.

Building
Museum, gallery and archive, civic
Client
Willmott Dixon Construction, for The Box
Location
Plymouth, Devon
Substrate
Concrete soffits, irregular profileTight access; many service penetrations
Area
787 m²
Application
120 mm Enverge NexSeal LE closed-cell
Specified U-value
0.18 W/m²K, achieved
Verification
Adhesion and thickness checks; site inspection

02What we specified, and why

120 mm of closed-cell bonded to the concrete: the depth the figure needed, with no voids behind it.

A specified U-value sets the depth. 0.18 W/m²K through a concrete slab with this product is a 120 mm layer, calculated with the slab and the bridges in it, and then applied to that depth and checked.

  • The system: Enverge NexSeal LE closed-cell, at 120 mm. Sprayed in passes directly to the prepared concrete, bonding to the slab and closing around every beam, column head and penetration. λ 0.027 W/mK aged to BS EN 14315-1 is the product figure the calculation used. Concrete soffits, the application.
  • Why closed-cell on a slab. It bonds and sets rigid, so the layer is continuous, airtight at the concrete and fixing-free. NexSeal LE declares a water vapour resistance factor (μ) of 62 to BS EN 12086, which we carry into the condensation-risk analysis to BS EN ISO 13788 for a building that controls its humidity.
  • Why the depth is checked, not assumed. A U-value is only met if the depth the calculation set is actually on the slab. Thickness was probed across the zones after application, and adhesion tested, before the work was offered for inspection. U-values and Approved Document L.
  • Why from the conditions up. Substrate temperature and humidity were monitored through the work, because foam applied to cold or damp concrete does not bond as the data sheet says it should.
Product
Enverge NexSeal LE, closed-cell
Depth
120 mm
Substrate
Concrete soffits, prepared
U-value
0.18 W/m²K specified, achieved
Checks
Adhesion, thickness, site inspection
Detail of the finished soffit at The Box: the closed-cell layer at full depth around a service penetration and a beam, continuous across the concrete.

Around the services. The layer closes around every penetration and beam at full depth, which is exactly where a board system would have left a gap. Project record.

Photographs · Project record

Bare concrete, the layer going on, the soffit at depth.

The exposed slab and its penetrations, the application in a tight zone, and the finished soffit before inspection.

Before: the exposed concrete soffit at The Box, beams and service penetrations bare.
Project recordBefore: exposed soffit
During: closed-cell foam being applied to the concrete soffit, the cream layer growing around the services.
Project recordDuring: application
After: the finished soffit in a continuous closed-cell layer at full depth.
Project recordAfter: finished
After: detail of the layer around a penetration and beam.
Project recordAfter: detail

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

03How the work ran

Clean, calibrate, monitor, spray, check, inspect.

A live construction site, a main contractor's programme, and a specification that would be inspected. The sequence was set up for that: preparation and calibration before the first pass, monitoring through it, and checks after it.

  1. Preparation

    Concrete cleaned and degreased, services masked.

    The slab prepared for adhesion across every zone; penetrations, fixings and adjacent finishes masked; the work areas set up for controlled access and ventilation.

  2. Set-up

    Rig calibrated, protection on.

    The proportioner calibrated for the product and the conditions; operatives in respiratory protective equipment; substrate temperature and humidity logged before spraying.

  3. Application

    In passes to 120 mm, conditions watched.

    NexSeal LE sprayed directly to the concrete across 787 m², built up in passes, with substrate temperature and humidity monitored throughout.

  4. Checks and inspection

    Adhesion, thickness, sign-off.

    Adhesion tested and depth probed across the zones after application, then the work offered for site inspection and signed off on schedule.

04The outcome

The specified U-value achieved across every zone, verified, on programme.

A specification met and inspected. The U-value is a calculated figure applied to the depth it needs; the elimination of cold bridging is what a continuous bonded layer does; the schedule is the contractor's.

U-value through the soffit

0.18W/m²Kspecified, achieved

Across all soffit zones, at 120 mm of closed-cell on the prepared concrete.

Cold bridging

Eliminatedas recorded

One continuous layer across slab, beams and column heads, closing around every penetration.

Verification

Inspectedadhesion and thickness

Checked after application and signed off by site inspection.

Programme

On schedulemain contractor's

Completed within the contractor's programme for the building.

Where this record comes from

Calculated and applied, not metered. A U-value is calculated for the build-up to BS EN ISO 6946 by a competent person and achieved by applying the depth it needs. HOLVEM supplies the calculation; the assessor holds the compliance position.

Yours will differ. A different slab, target or exposure gives a different depth. How thickness follows the objective.

05Where this record leads

If this looks like your building.

The application this record used, the building family, and the compliance route it met.

Your next step

Your building.
Your brief.
A specialist.

If you already know the U-value or thickness you need, tell us and we price to it. If you do not, tell us what you know about the building and the problem, and we work it out with you. Someone who does this every day reads it and comes back to you.

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