Why and how to construct a floating floor

Insulating a floating floor

A floating floor is one that is not screwed or nail or fixed to the sub floor, instead the floor covering (usually chipboard, gypsum or cement fibre boards) floats on top of the insulation layer.

Why use a floating floor
This type of construction is usually used in domestic buildings especially when retrofitting, as it means that the existing floor surface doesn’t have to be ripped up. It can easily be installed on top of an existing concrete slab. Another major advantage of a floating floor is the speed of installation; it is lightweight and does not involved wet trades and therefore no drying out time is required.

Because the insulation is laid continuously across the floor there is no cold bridging through the battens, such as in timber floors. This means that a thinner layer of insulation can be used when compared to a floor constructed with timber battens, which will lead to a lower floor height when installed, which is useful if you need to match existing floors levels.

In a bathroom you should ensure that a moisture resistance chipboard is to construct the floating floor and in all cases soft wood battens should be positioned at doorways and partitions.

Example construction build up
An example of what layers you need in a floating floor can be seen below. The U-Values that this will achieve are available on our online U-Value calculator

  • 18mm tongue & grooved chipboard,
  • Vapour control layer,
  • Kingspan Thermafloor TF70,
  • 150mm concrete slab,
  • Damp proof membrane,

How to Install a Floating Floor
The steps below cover how to install a floating floor. Full details of the procedure are available in our product literature and this should be consulted prior to starting any work.

  • Concrete slabs should be allowed to dry out fully prior to the installation of Kingspan Thermafloor® TF70 (average 1 day per mm of slab thickness).
  • The surface of the slab should be smooth, flat and free from projections. Beam and block floors should be level and grouted. In accordance with BRE Good Building Guide 28 Part 1 (Domestic floors: construction insulation and damp –proofing), irregularities should not exceed 5 mm when measured with a 3 metre straight edge.
  • A thin layer of cement / sand mortar, a levelling screed, or a proprietary levelling compound can be used to achieve a level surface, and prevent the boards of Kingspan Thermafloor TF70 from slipping under the timber floor boards, if required. This should be allowed to set, harden and dry (approximately 1 day per mm) before proceeding further.
  • If there is no damp proof membrane in the concrete floor, one (minimum 300 micron / 1200 gauge polythene) should be laid with joints well lapped and folded, to prevent the passage of ground water, over the concrete floor slab, or beam and block floor, prior to installing the insulation boards.
  • The membrane should be brought up the surrounding foundation walls until it is sufficiently above the height of the wall DPC so that it will connect with or form the DPC.
  • To comply with NHBC recommendations, preservative treated softwood timber battens should be positioned at doorways, access panels and to support partitions. The size of the battens selected should ensure that, when installed, the top surface of the insulation boards are flush with the top of the battens.
  • The insulation boards should always be loose–laid break–bonded, with joints lightly butted.
  • Insulation boards should be overlaid with a polythene sheet (not less than 250 micron / 1000 gauge), to act as a slip layer, and a vapour control layer. Ensure the polythene sheet has 150 mm overlaps, taped at the joints, and is turned up 100 mm at the walls.
  • Timber floor boards e.g. tongue–and–groove 18 mm thick plywood, should then be laid over the insulation and battens with staggered cross–joints in accordance with DD ENV 12872 : 2000.
  • An expansion gap of 2 mm per metre run of floor, or a minimum of 10 mm overall, whichever is the greater, should be provided between the floor boards and the perimeter walls.
  • Where there are long (over 5 metres), uninterrupted lengths of timber floor boards, proprietary intermediate expansion joints should be installed on the basis of a 2 mm gap per metre run.
  • Before the timber floor boards are interlocked, apply a continuous bead of waterproof wood grade PVA adhesive to the top and bottom of the tongue and groove joints.
  • Once the timber floor boards have been laid, temporary wedges should be inserted between the walls and the floor, to maintain tight joints, until the adhesive has set.
  • Once the wedges are removed, they are replaced with strips of cork or polyethylene foam to act as a compressible filler and to help prevent cold bridging. Skirtings may then be fixed.

Visit our website for more information on floating floors.

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

0 comments on “Why and how to construct a floating floor
  1. hi i am converting a garage into a lounge 7.2.x 2.5 the floor is being raised about 150mm what is the best system using a visquine membrane and beams and chipboard flooring, can you please quote individual prices, regards Dave

  2. Hi, i wonder if you could offer me some technical advice.
    I’m intending to lay a floating floor over insulation in an existing kitchen. The old cement screed (around 60 – 70 mm thick) has been removed, exposing the slab which has no dpm beneath it. Previously the plastic dpm was between the slab and screed.
    Because of heavy units in the kitchen, particularly the cooker, freezer and washing machine, I feel I should perhaps put battens or small joists on to the slab after laying a new dpm. I was considering pouring a liquid dpm instead of a plastic membrane. What concerns me is how I will fix the battens down, as any fixing will breach the dpm. Short of creating a self supporting timber frame that sits on the slab, I cant see how I can do this. Can you advise?
    Also, I am intending to batten and insulate the walls internally before plasterboarding them, should this be installed down to the slab prior to the flooring going in.
    Regards, Peter.

    • Hi Peter

      With heavy appliances over a timber floor, installing timber joists to fit insulation between them will reduce the amount of movement in the floor and the load distribution will then be onto the joists rather than the insulation. The joists should be fixed down to the concrete, and the fixings will need to penetrate the DPM in order to do this. However there must be a DPM under the insulation you choose to use – whether its Thermafloor TF70 or Kooltherm K103 Floorboard. Kooltherm K118 Insulated Plasterboard could also be fixed to the timber battens to internally insulate the walls.

  3. hi, I have an attic which hasn’t had the roof insulated yet but as the joists were 300mm, the floor has been well filled with fibreglass and covered with chip board. I am considering lifting the chipboards and laying 50mm insulation boards (the larger sheets) on top of the joists and then relaying the chipboard flat on top. I test one sheet with chipboard on top and it felt strong enough to walk on. Does this make sense to do and if not what do you recommend?

    • Hi Michael

      If the floor is going to be walked on we wouldn’t recommend installing 50mm thick Kingspan Kooltherm K7 Pitched Roof Board directly on top of timber joists with chipboard directly on top of that. Instead, lay a layer of chipboard onto the joists first, lay insulation on top of that, with another layer of chipboard over the insulation to walk on. This will distribute the load for foot traffic.

      Also, the highest performing insulation layer should be on the unheated side of the construction in order to avoid interstitial condensation. As in this instance you are installing 50mm thick Kingspan insulation above the joists, you should considerably reduce the amount of fibre glass between the joists. Alternatively, you could increase the thickness of insulation above the joists.

      If you call into our technical department on 01544 387 384 with your full construction build up and project address we can run a full condensation risk analysis.

      Hope this helps.

  4. I have a loft that I don’t know how best to insulate. The ceiling joists are 4 inch and it is partly floored with tongue and timber. I was going to lift the floor and put 100mm fibreglass down and then replace the floor but having read some of the comments above I wonder if that might cause interstitial condensation. The house was built about 70 years ago. Would i be better to leave the floor down and lay 100mm of polystyrene insulation board and then chipboard on top? What is the relative U value of a given thickness of polystyrene compared with the same thickness of fibreglass? I can get a grant to insulate the loft but they will just put 300mm fibreglass over everything including the floor and i don’t want that.

    • Hi John

      We do not produce a polystyrene insulation that can has approval for use in a loft. We do manufacture rigid foam boards that can be used, but to advise on whether or not this cause a risk of condensation we would need to know what the roof build-up is at rafter level.

      Interstitial condensation will only be caused if there is further insulation somewhere within the roof construction. If there is no ventilation at rafter level, then it would be acceptable to use Kooltherm K107 Pitched Roof Board or Thermapitch TP10 between the joists at horizontal ceiling level. In this case, if a breathable membrane has been used over the top of the rafters, no further measures need to be taken. If there is the traditional sarking felt or bituminous felt over the rafters then the loft void will need to be ventilated, in order to avoid condensation forming.

      Hope this helps,

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