Best Practice > Floors
Find out the most suitable construction methods for flooring and the options for under floor heating and insulation.
For this type of flooring a void is provided based on measurements taken back from finish floor level i.e. the depth of screed, plus the depth of insulation, plus the vapour barrier, plus a layer of a vibrated blinding and at least 8 inches of aggregate. A continuous expansion joint runs between the containing wall and the proposed concrete floor. Steel mesh (if required) is placed on stands within the area to prevent it from being forced to the bottom of the slab when the concrete is poured. The concrete is then poured and tamped to expel any air that may be trapped and weaken the concrete. Once the concrete has set an additional vapour proof layer can be laid with insulation, followed by the screed.
Hollow core flooring units are constructed from reinforced concrete with hollow cylindrical voids running the full length of the beams. This makes the beams lighter while not altering their structural properties. The voids within these beams can be used to run services. Some manufacturers are providing prefabricated beams with insulation pre-bonded to the underside.
Suspended floors consisting of precast, pre-stressed concrete beams spaced at centres to suit the use of standard walling blocks as infill. Insulation and screed can be added as for in situ construction.
It is crucial that a continuous strip of insulation separates the new floor from the wall. If the concrete slab is allowed to continue to the wall, it will form a cold bridge.
Any high density rigid board is suitable. In a concrete slab construction, insulation should be incorporated between the slab/beam and screed, in order to avoid excessive thermal mass of the slab. An overall insulation thickness of 100 mm would be a good specification, given that the heat loss to the ground is much less than to the air at critical times (in winter, particularly at night). Insulation should be laid into layers, with all joints taped and staggered, to prevent a cold bridge being created by concrete running down between the boards when a wet screed is being laid.
A suspended timber ground floor should be insulated between the floor joists using either a rigid board secured against the joist sides (ideally with a concertina action at the edges for improved fit and retention) or mineral fibre quilt supported on flexible netting / mesh laid over the joists. This technique can be used to retrofit insulation into an existing suspended timber floor but would involve pulling up the floorboards. Alternatively, if you were not planning this level of disruption and have access to the subfloor void, the netting can be stapled to the underside of the joists and insulation retained in the same way. This practice should not cause any problems with interstitial condensation on the timbers, provided the subfloor void is ventilated, as it should be in any case.
I-beams are constructed with a thin flange of high-grade softwood timber (the size of which depends on the depth of joists specified) on the upper and lower parts of the I-beam. The web of the beam is formed using a continuous length of orientated strand board. They offer a strength equivalent to a normal flooring or roofing joist with significantly less timber employed for each span. The use of smaller timbers in the orientated strand board and flange mean that the timbers can be sourced from young forests and renewable sources with greater ease.
There are now I-beams being constructed where the flange is manufactured from longitudinal or cross laid softwood veneers and the web made from high-grade oriented strand board. This method of constructing I-beams produces a superior product which is longer and lighter.
A new development of the I-beam is where the flange is constructed as above, with the web being formed by a zigzagging galvanised steel strap running the full length of the I-beam. The benefit of this design is the large area of void, which will dramatically reduce build time due to the simplicity of running services through the flooring.
I-beams are quick and easy to install, even for long spans, reducing build time and increasing efficiency. Due to the significantly reduced width of an I-beam, a greater level of insulation can be applied between the beams in addition to the reduced cold bridging effect from the timbers themselves.
It is especially important with UFH to achieve the best possible floor insulation levels in order to prevent heat loss from the system. When installing UFH it is important that the floor covering is selected with some consideration to the flow of heat. The response time of the under-floor heating system will also be affected significantly by the floor covering. A thick carpet will have the effect of blocking heat from entering the room. For this reason hard finishes are recommended, such as wooden or slate floors or ceramic tiles. In addition, the top surface should be thermally well-connected to the floor screed in which the UFH pipes are laid. This can raise a problem where wooden floors require an intermediate layer (e.g. where fixing with nails is proposed).
British Standard BS EN1264 relating to under-floor heating with solid / timber floors, recommends a maximum floor covering resistance (above the screed) of 1.5 tog or 0.15 m2K/W. This indicates that the wooden floor should be laid direct onto the screed, which therefore must be very smooth and flat. Proprietary self-levelling compounds can be used to achieve this finish. Tiles laid onto a mortar bed will be thermally well-connected, provided there of mortar gaps are at a minimum. Some research indicates that carpet and underlay with a combined tog of 2.5 could actually be used, however it is recommended that the carpet manufacturer be consulted.