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A bright, empty UK commercial interior with a bare timber floor, pale boards running toward tall windows, soft morning daylight across them.

Application

Floors, sealed from beneath: the boards insulated from the void, the draughts stopped, the void still breathing.

A suspended timber floor loses heat and lets cold air up through every joint. Closed-cell foam applied to the underside of the boards between the joists seals the floor from below and leaves the void ventilated, as it must be.

Start a project brief What it costs: the cost guide
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Is this your floor?

01The build-up

The boards stay. The layer seals them from the void.

A suspended floor sits over cold, moving air on purpose: the void is ventilated to keep the timber dry. That same air reaches the room through every board joint and gap at the skirting. Switch the drawing between the bare floor and the sprayed floor.

Two states. Switch between them; the marks and the reading follow.

Swipe across to see the whole section.

BARE BOARDS · COLD AIR THROUGH THE JOINTSSection, schematic, not to scale. Three marks in each state; the cards beneath read them.

Cold boards · draughts throughThe dew point · water formsWarm, dry, sealed

1The boards

Thin timber over cold air.

Floorboards and a void that runs at close to outside temperature all winter. The floor is the coldest surface in the room, and the room's heat goes straight down through it.

2Draughts through the joints

Every joint is an air path.

Ventilated void air is drawn up through the board joints and the gap under the skirting by the warm room above. A quilt laid between the joists slumps, leaves gaps, and does nothing about the air.

3The ventilated void

It must stay ventilated.

The air bricks keep the timber dry. Any insulation that blocks them, or that traps moisture against the joists, creates the rot it was meant to prevent.

The short version: a suspended floor needs to be sealed and ventilated at the same time, which is why the layer goes on the boards and not in the void. Closed-cell foam is the one insulation that seals the joints, stays put and leaves the timber free to dry.

02The order of work

Five steps, from the void.

The work is done from beneath the floor, through the void or from a basement or undercroft, with the floor finish untouched. Each step is checked before the next begins. Floors are less commonly specified than roofs, so the survey does more of the work here.

  1. Substrate and access

    The boards, joists, sleeper walls and ventilation are inspected from below: rot, damp, the state of the air bricks, and whether there is room to work. Access is through a hatch, the void or an undercroft; where there is no safe access, the job changes.

    HOLVEM checksSound, dry timber; ventilation adequate; safe access.

  2. Preparation

    Debris is cleared from the void. Old quilt or netting that would trap moisture against the timber comes out. Pipes and cables are protected or re-routed, and the air bricks are kept clear.

    HOLVEM checksClean bays; services protected; ventilation paths kept.

  3. Passes and depth

    Closed-cell foam is sprayed onto the underside of the boards between the joists, in passes, to the specified depth. It fills the joints and the perimeter gaps in the same pass.

    HOLVEM checksApplied thickness measured and recorded, bay by bay.

  4. Coating, where specified

    In a ventilated void the foam is normally left as applied. Where the underside is exposed in an occupied basement or undercroft and a reaction-to-fire classification is required, an intumescent coating is applied.

    HOLVEM checksCoverage logged where a coating is specified.

  5. Terminations

    The perimeter at the sleeper walls, hatches, service penetrations and any change of construction are detailed per project. The layer stops at a closed edge, and the air bricks are left open.

    HOLVEM checksEvery stop closed; every vent still open.

A close detail of the underside of bare floorboards between two joists, seen from the void, gaps of light at the board joints and cobwebs on the timber.
BeforeThe bare boards, from below
The substrate as it is inspected: the underside of the boards, the joints the draughts come through, and the joists that must stay free to dry.

Every step is checked before the next. Thickness measured, coverage logged, compiled into a QA record at completion.

03Preparation

Before the first pass: what the floor has to be.

A floor void is the least visited space in a building and the easiest to get wrong. Three things HOLVEM establishes first.

Ventilated

The void keeps breathing.

Air bricks are counted, cleared and kept. Where ventilation is inadequate it is improved before the layer goes on, because a sealed floor over a stagnant void puts the joists at risk.

Dry and sound

Timber in good order.

Rot, damp joist ends and failed sleeper walls are put right first. Foam is not a structural repair, and it is not applied to wet timber.

Reachable

There is room to work.

Access, headroom and the route in are established at survey. Where the void cannot be worked safely from below, HOLVEM says so; the floor is not lifted to force it.

Survey, or photographs?How HOLVEM works: we understand your project before we price it

04Thickness and objective

The depth is set by what the floor has to do.

Three objectives, three depths, each confirmed on your floor with the joist fraction in the calculation. What it costs is on the cost guide, not here.

25 mmDraught and condensation control

Seal the joints; keep the boards dry.

Enough to close the air paths through the floor and put the void-facing surface above the dew point. The objective where a floor is draughty but the building is otherwise adequate.

50 mmHeat retention

A warmer floor, a warmer room.

Controls draughts and condensation and cuts the heat going down through the floor. The depth most occupied buildings notice.

75 mm and aboveA calculated U-value

A figure to meet, and to prove.

Set by the U-value the project has to reach, calculated for this floor as built with the joists included. Confirmed by the calculation, not by a rule of thumb.

The underside of a suspended timber floor with a continuous cream closed-cell layer against the boards between every joist, the joist bottoms left bare, seen from the void.
AfterThe layer, against the boards
The finished face: foam against the boards in every bay, the joists still exposed to the ventilated void.

Thickness and objectiveWhat it costs: the cost guide

05Where it suits

The buildings whose floors need it.

Suspended timber floors sit under offices, shops, schools and public buildings of a certain age across the UK. Three building pages where this application is common.

Building

Offices and commercial

Older office buildings with suspended ground floors that are cold underfoot and draughty at the skirting.

Read the building page

Building

Retail units

Shops in older buildings where the sales floor is cold from below and the heating fights a draught all day.

Read the building page

Building

Schools

Suspended floors under classrooms and halls in older school buildings, worked in the holidays.

Read the building page

Every building type HOLVEM works on

06What HOLVEM checks

Three things checked before anyone talks about depth.

HOLVEM does not quote a depth from a photograph. The floor is understood first: how moisture moves through it, whether fire performance is required, and what U-value it must reach.

Condensation risk analysis

Will the timber stay dry?

A condensation risk analysis to BS EN ISO 13788 for the floor as built, with the void conditions and the internal conditions the building actually runs at. Where the simple method is inconclusive, HOLVEM runs a dynamic simulation to BS EN 15026.

Fire

Is a reaction-to-fire classification required?

In a closed void, usually not. Where the underside is exposed in an occupied space and a classification is required, the coated closed-cell system is specified, classified B-s2,d0 in the build-up tested. Established before the depth.

U-value

What figure does the floor have to reach?

Calculated to BS EN ISO 6946 for this build-up, joists included, not read off the foam's own conductivity. The calculation is yours to keep.

Condensation risk analysisU-values and Approved Document LFire performance and coatings

07Straight answers

Ask us the hard ones.

The questions people with a cold floor actually ask on the call, answered the way the specialist answers them.

Do you have to lift the floor?
Normally no. The work is done from the void, through a hatch, from a basement or from an undercroft. Where there is no safe way in from below, HOLVEM says so at survey rather than forcing it.
Will sealing the floor cause rot in the joists?
Not when it is done this way. The foam goes on the boards, not in the void, so the joist bottoms stay exposed to ventilated air and the air bricks stay open. Sealing the floor and ventilating the void are done together, on purpose.
Does the foam stop the draughts as well as the cold?
Yes. That is the point of a sprayed layer on a floor: it fills the board joints and the perimeter gaps in the same pass as it insulates. A quilt between the joists does not stop air.
Can you insulate a concrete floor?
A solid ground-bearing slab cannot be sprayed from below. A concrete floor over an undercroft, car park or basement can: the layer goes on the soffit beneath it. See the concrete soffit page.
How do you prove it was done properly?
Applied thickness is measured and recorded, coverage is logged, and the evidence is compiled into a QA record handed over at completion.

The next step

Your floor.
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. Tell us what the floor is, what is underneath it and how you can get to the void. Photographs from below, if you have them, are the most useful thing you can send.

Start a project brief Send photographs first

Or call . Enquiries received before 3pm on a working day are contacted the same day. After 3pm, the next working day. We understand your project before we price it.