Before 3pm on a working day, contacted the same day. After 3pm, the next working day. Contact, not a quotation. Telephone
Start a project brief
A modern UK office building at dusk with an open undercroft car park beneath it, the bare concrete soffit and its downstand beams visible above the parked cars.

Building type

Offices over an open undercroft: the cold floor above, fixed from below.

Offices and commercial buildings with exposed concrete soffits: undercrofts, car-park decks below occupied floors, walkways and loading bays with rooms above. The slab is the floor of the office and the ceiling of the car park at once, and it is the coldest floor in the building. What is lost through it, what is applied to it, and where fire performance and the EPC come in.

Concrete slab, downstand beams, servicesApplied to the soffit from below; the offices stay openUsually a U-value objective, priced per project

Same-day estimator

About a minute · no obligation

Step 1 of 4: your building.

Which building is it?

This page's building is pre-selected. Change it if yours is different.

Takes about 30 seconds. No contact details yet.

01What is happening

One slab, two jobs: the office floor and the car-park ceiling.

An uninsulated concrete slab over an open undercroft is an exposed floor. The car park is at outside temperature, the office above is heated, and the slab between them conducts. The people above feel it in their feet, the landlord sees it in the heating and the EPC, and the car park sees it as a cold, damp soffit.

An open-plan office on the ground floor of a commercial building, the desks nearest the window sitting over the open car park below, pale morning light across a cold floor.
The floor aboveCold underfoot
  • Cold underfoot, whatever the thermostat says. The floor is losing heat straight down through the slab into the car park.
  • The room the tenants complain about. A ground-floor office over an undercroft is the comfort problem a landlord hears first, and the one the heating bill shows.
  • Heating harder warms the air, not the slab. The floor stays cold until the slab stops conducting.
The concrete soffit of an undercroft car park, one half still bare grey concrete, the other half covered in a continuous cream closed-cell foam layer wrapping a downstand beam.
The soffitWhere the work is done
  • The coldest surface the car-park air meets. On a mild, wet day water forms on it: damp patches, drips onto cars, staining, corrosion on fixings and services.
  • Reached from the car park, not the office. The layer goes on from below; nothing is lifted, cut or fixed through on the tenant's side.
  • One layer, wrapping the beams. Where the layer stops, the slab conducts again, so beams, edges and column heads are carried in it.

It is not a structural defect. A cold slab is doing what concrete does; the fix is a layer beneath it, not a repair to it.

It is not solved from above. Insulating over the slab means lifting the floor of an occupied office. The soffit is reachable from the car park, which is why the work happens there.

It is not the same as a roof. The objective here is usually a calculated U-value for the floor, and the fire question has to be answered before the specification is written.

Heat loss and thermal performance U-values and Approved Document L

02Recent soffit projects

Real soffits, real car parks, the buildings above them open.

Installation footage and photographs from HOLVEM's own concrete soffit projects: an undercroft void, live retail soffits, a new-build fit-out and an external canopy. Every item is a project record; press play to watch a clip.

Project recordSoffit sprayed
Concrete soffit sprayed from belowClosed cell in passes, beams and edges wrapped.
A concrete deck soffit over an undercroft void fully sprayed in a continuous cream closed-cell layer, galvanised steel frame and red cable ducts below it, NexSeal drums and the spray gun on the gravel floor.
Project recordUndercroft soffit
Concrete soffit, undercroft voidThe whole soffit in one continuous layer; services stopped around.
An installer in full PPE and hood spraying closed-cell foam onto an overhead soffit, polythene masking hung beneath the work area.
Project recordLive retail soffits
Regional shopping centre, 3800 m² of soffitsClosed cell, 140 mm in three passes, with the centre trading throughout.
Project recordOverhead work
Soffit insulation, overheadThe layer built up to depth, checked as the work proceeds.
A concrete soffit sprayed in a pale closed-cell layer above a new-build fit-out interior, foil-lagged ductwork and cable trays hung beneath it.
Project recordNew build, fit-out
New-build soffit above a fit-outSprayed before the services went in; ductwork hung outside the layer.
The external canopy soffit of a new stone-clad building with glazed sliding doors, sprayed in a continuous cream closed-cell layer while the site is still in progress.
Project recordExternal soffit
External canopy soffit, new buildAn exposed overhang treated from below, the line carried to the wall.

Written records, with their figures and the basis each one rests on, live on the project pages.Project records

03What is applied

Closed-cell foam to the soffit, from below, wrapping the beams.

The layer is applied to the underside of the slab in passes, to the depth the calculation sets, and carried round downstand beams, columns heads and edges so the insulated line is continuous. Services are stopped around at a closed edge, or re-hung outside the layer where the project allows. Where fire performance calls for it, the DC315 coating goes onto the cured face. Five things in the section, keyed beneath it.

Schematic / not to scaleKeyed 1 to 5 below

1 / Existing slab and floor above

Stays as it is. Nothing is lifted, cut or fixed through from the office side.

2 / Prepared soffit

Sound, clean and dry: laitance, dust and loose material off, the surface within the temperature window. Adhesion is decided here, not later.

3 / Closed-cell layer, in passes

To the specified depth, verified as the work proceeds. λ 0.027 W/mK is the NexSeal LE product figure, not a HOLVEM performance figure.

4 / Beams, edges and services

Downstand beams and edges are wrapped so the line is continuous. Services are stopped around at a closed edge, or re-hung outside the layer.

5 / Coating, where specified

B-s2,d0 belongs to the coated NexSeal LE and DC315 system, as tested. Where the coating is not applied, that classification does not carry. Section 06 below.

The concrete soffit: the build-up in sequenceClosed-cell system data

04Enverge NexSeal LE

The material we spray, and why it matters.

Enverge NexSeal LE is a two component closed cell polyurethane, spray applied directly onto the prepared substrate. As it cures, it expands and sets into one rigid, continuous layer, bonding into corrugations, laps, fixings and awkward junctions without being cut, jointed or mechanically fixed. On a soffit that is the beam, the column head and the edge, wrapped in one layer with no joint to open.

That matters because most insulation problems start at the gaps.

Board systems can leave voids at ribs, laps and junctions, and every gap is a potential cold bridge where warm internal air meets a cold surface and condensation forms, hidden behind the insulation. NexSeal LE is applied as one continuous bonded layer, so there is nowhere for that to start: a sealed, stable building envelope that controls condensation, reduces heat loss and protects the structure underneath.

0.027 W/mKThermal conductivity · aged · BS EN 14315-1
Class 1Surface spread of flame · BS 476-7 · uncoated
B-s2,d0Coated, as tested · BS EN 13501-1
25 yearsWorkmanship warranty, plus the manufacturer’s

HOLVEM also installs open-cell foam. It is a different product with its own data sheet, specified for different objectives; it is not what goes onto a cold roof underside or a concrete soffit. Open-cell system data · Closed-cell system data

HOLVEM spray rig van with Enverge NexSeal LE A and B component drums plumbed into the proportioner.
HOLVEM rigA and B on board
The HOLVEM rig · Enverge NexSeal LE A and B components on board. HOLVEM's own photograph.

05Working around an occupied building

The offices stay open. The car park is released in bays, out of hours where it has to be.

Nothing about the work touches the tenant's floor. It is planned from the car park: which bays can be released and when, what has to be moved or protected beneath them, and which hours the building can spare. Three things are agreed before anyone arrives.

  1. The bays and the hours. The undercroft is released to us a few bays at a time, coned off and handed back in turn. Evening, weekend and out-of-hours working are planned where daytime parking cannot be lost.
  2. What is beneath the work. Vehicles are moved out of the released bays; services, lighting, signage and anything that stays are masked and sheeted. The spray zone is contained, so nothing outside it is touched.
  3. What the tenants notice. Nothing on their side of the slab: no access to the office floor is needed, nothing is lifted or drilled from above, and the office keeps its hours. They are told what is happening below them and when.

The building above never closes. The shopping-centre record below was done with the centre trading throughout, phased by zone; an office over a car park is phased the same way, by bay.

06Fire performance

A car park under an office is where the fire question gets asked first.

The soffit of a car park is the ceiling of a space people park and walk in, so the fire strategy has a view on what lines it. Approved Document B sets the classes for wall and ceiling linings by the use of the space, and the strategy for your building states which class the treated soffit must reach. The layer can be left as applied or coated; only the coated system carries a Euroclass. That question is answered before the specification is written, not after.

Coated · DC315 intumescent

B-s2,d0 to BS EN 13501-1, on the coated system.

The DC315 intumescent coating is applied to the cured layer; the foam depth under it depends on the scope and what the building has to achieve. The classification belongs to the coated NexSeal LE and DC315 system as tested: 35 to 40 mm of NexSeal LE on 9 mm plasterboard with DC315 at 0.7 mm wet film thickness (BTTG report 27/06638/12/24), confirmed for each project against the fire strategy. It is applied as one build-up and recorded as one.

  1. Layer cured and clean, depth verified.
  2. Coating applied to the specified thickness, coverage recorded.
  3. The record in the QA evidence pack at handover.

Uncoated · the layer as applied

Class 1 surface spread of flame to BS 476-7. No Euroclass claim.

Left as applied, the layer carries the product's Class 1 surface spread of flame and nothing more. Where the specification does not require a reaction-to-fire classification, this is a real option and a cheaper one; where it does, it is not an option at all.

  1. Layer applied and verified, face left as cured.
  2. No coating, no Euroclass: B-s2,d0 does not carry.
  3. The decision recorded in the specification, so nobody claims it later.

Whether your soffit needs a classification, and which one, is set by the building's fire strategy and the Building Regulations guidance that applies to it. HOLVEM answers what each system achieves and puts it in writing. Fire performance and coatings.

07The objective sets the depth

Four objectives. On a soffit, the third is usually the one.

What does the floor above need? Pick one below and you will see the depth that does it, and what it won’t do. On a soffit the beams and edges go into the calculation, and the survey confirms the figure.

Four depths, drawn to one scale and measured from the soffit. The objective and the calculation set the thickness on your building.

Pick an objective above to see the depth that does it, and what it won’t do.

Thickness and objectiveFire performance and coatings

08For landlords

An exposed floor is one of the elements the EPC is modelled on.

A let office has to hold a minimum EPC band to stay lettable, and the floor over an open car park is one of the worst-performing elements an assessor models. Insulating the soffit changes that element's U-value; what it does to the band is the assessor's calculation, not ours.

  1. The floor today. An uninsulated slab over outside air is modelled as an exposed floor at its as-built U-value. Older office soffits sit far above the figure a new floor would be built to.
  2. The floor after. HOLVEM supplies the calculated U-value for the treated slab, with the beams and edges in it, and the QA evidence pack: applied thickness measured and recorded, coverage logged. That is what the assessor needs to model the element as improved.
  3. The standard it is measured against. Since April 2023 a non-domestic property in England and Wales cannot continue to be let below EPC band E unless an exemption is registered. In June 2026 the government said privately rented non-domestic buildings over 1000 m² will need EPC B from 2031 where cost-effective, and dropped the C step proposed for 2027; that still needs regulations before it is law. EPC, MEES and compliance.

No band is promised. We publish the U-value we calculate and the record we hand over; the EPC is the assessor's, and it takes the whole building into account.

09The survey

Five things we check before anyone talks about depth.

A survey is one visit, from the undercroft, fitted around the building’s hours and its parking. Nothing is opened up and the car park keeps working. It gives us the five things a specification for this soffit has to stand on.

  1. The objective

    A calculated U-value for the floor above, condensation control on the soffit, or both. And whether the fire strategy asks for a classified surface.

  2. The slab and its soffit

    Condition, previous coatings, moisture and what the surface will hold. The layer bonds to what is there.

  3. Beams, edges and services

    The beams, edges, columns and services the layer has to wrap or stop at, and what can be re-hung outside it.

  4. Above and below

    Heating and occupancy in the offices above, parking below, and the hours the undercroft can be released.

  5. Access from below

    Which bays of the car park come out of use in turn, how the work is contained, and how vehicles keep moving while it is on.

What you get after the survey

Five things in writing, whether or not you go ahead.

  • A specificationWritten for this soffit, from what we found, with its assumptions named.
  • The depth, and whySet by the objective and confirmed by calculation for the element as built, bridges included.
  • The condensation-risk positionWhether anything will condense inside the build-up, to BS EN ISO 13788 with the BS 5250 climate.
  • The fire positionWhether your fire strategy calls for a classified surface, and the coated or uncoated build-up that answers it. Confirmed against the strategy, never assumed.
  • A firm priceFor your soffit, in writing. What moves it is explained in the cost guide.

Survey, or photographs?Send photographs of the soffit

A firm figure starts with the building. Yours takes a minute to describe.Building, soffit, what you are seeing, and how to reach you. A real figure for your building the same working day; nothing sent to anyone else.

Get my estimate

Project records

Work we have done on buildings like this.

All project records
Project recordPondswick substation, Luton127 m² of concrete soffit insulated and coated with DC315 under live switchgear.Read the record Project recordThe Box, Plymouth787 m² of concrete soffit at 120 mm, to a specified U-value of 0.18 W/m²K.Read the record

10Straight answers

Ask us the hard ones.

The questions facilities managers, landlords and engineers actually ask on the call, answered the way the specialist answers them. A question we have not answered? Ask it on the call. No obligation.

How much does it cost to insulate a concrete soffit?
On a soffit the depth is usually set by a U-value target, so it is priced per project once the calculation has set the depth; a contract-specified 0.18 W/m²K has needed 135 to 140 mm. Where condensation control or heat retention is the objective, the rates published in the cost guide apply. Access, substrate condition, preparation and the coating affect it as well. Send your project details and we will ring you the same working day with a real figure for your building.
Will it move the EPC band?
It changes one element the assessor models: the exposed floor, from its as-built U-value to the calculated figure for the treated slab. HOLVEM supplies that calculation and the QA evidence pack; the EPC assessor runs the whole building and holds the band. We do not promise a band.
Can the office stay open while you work?
Yes. The work is done from the undercroft, from below, and nothing is lifted, cut or fixed from the office side. The car park is released to us in bays, in turn, and handed back; the shopping-centre record above was done with the centre trading throughout.
Does the soffit have to be coated?
Only where the specification requires a reaction-to-fire classification. Coated, the NexSeal LE and DC315 system carries B-s2,d0 to BS EN 13501-1 in the build-up tested; uncoated, the layer carries Class 1 surface spread of flame to BS 476-7 and no Euroclass claim. Which one your building needs is the fire strategy's question, and we put the answer in writing before the work starts.
Why don't you recommend the same thickness for every building?
Buildings don't share heat-loss profiles, moisture risks or targets. Condensation control may need 25 mm; a contract-specified U-value needed 140 mm. The thickness is the output of analysis. A standardised number would mean the analysis never happened.
How do you prove the system was installed correctly?
Applied thickness measured and recorded, coverage logged, all compiled into a QA evidence pack handed over at completion. You hold the evidence, not our word.
Isn't spray foam a problem for surveyors and lenders?
Those concerns overwhelmingly involve residential lofts and undocumented installs. Our commercial work is the opposite case: BBA-certified products, a written specification from condensation risk analysis and U-value calculation, and a QA evidence pack at handover. The building file answers the surveyor's question before it's asked.

Your next step

Your building.
Your brief.
A specialist.

If you already know the thickness or U-value you need, we price to it and confirm it is right; if not, we work it out with you. The estimator at the top takes about a minute and asks for nothing until the reading is on screen. "Not sure" is a useful starting point.

Start a project brief About five minutes. Photographs help; nothing else is needed.

Or pick up the phone

Telephone . Before 3pm on a working day, you are contacted the same day. After 3pm, the next working day. A commitment to contact, not a quotation.

Photographs of the undercroft first? Upload photographs of the building