Why and how to construct a floating floor

15 April 2016 Kingspan Insulation UK

What is a floating floor?

Why and how to construct 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.

It is typically taken to mean a lightweight construction where chipboard, gypsum or cement fibre board floor panels are ‘floated’ across the top of the rigid insulation layer. Floating floors are used most often in domestic buildings especially when retrofitting or refurbishing an existing building. This is because they are quick, lightweight and easy to install, thinner than screed options, with no wet trades and no drying-out time. They also allow for fast heating response times.

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.

How to insulate a floating floor

An example of what layers you need in a floating floor can be seen below from top down. The U-values that this will achieve are available on our online U-Value calculator.

  1. Floor finish (eg carpet/ laminate floor, timber boarding. As floating floors are by their nature flexible, care should be taken when specifying tile finishes which are more suited to less flexible substrates)
  2. Tongue and Groove flooring (typically 18 mm T&G chipboard or cement  / gypsum fibre boards)
  3. Separation layer (500 gauge polythene)
  4. Insulation laid continuously (eg Kingspan Thermafloor TF70)
  5. Damp proof membrane if required
  6. Screed (if existing)
  7. Concrete slab

What about for heavier items? For items such as bath footings or the base of toilets then a timber batten would be used. Similarly, where there is heavier point loadings such as in door thresholds or at the bottom of stairs, then again you would use a timber batten.

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.

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