What is the minimum depth liquid screed can be
applied over insulation?
Liquid Screed can be applied over insulation (floating application) either thermal or acoustic at a depth of 40mm in domestic applications and 40mm in commercial applications. In combination with Underfloor Heating the minimum cover required over the pipe is 30mm, as per .BSEN 13813:2002
Preparation for Liquid Floor Screed
The preparation required for a liquid floor screed is no different to what is required for a conventional sand and cement floor screed. Although note that we say "required" as opposed to what a lot of people try to "get away with".
The build up of a ground floor would typically be:
DPM, Slab, Insulation, Vapour Barrier, Floor Screed.
Or if underfloor heating is to be installed the build up would be:
DPM, Slab, Insulation, Vapour Barrier, Underfloor Heating Loops, Floor Screed.
Naturally this can also be extended to upper floors where a beam and block construction, or concrete plank is used, and in these cases the DPM would not be present although the vapour barrier is still required.
The purposes of the vapour barrier is not directly related to damp from the ground. The vapour barrier is required by the manufacturers of the insulation, such as Celotex or Kingspan, to prevent contamination by floor screed. The vapour barrier also forms a separation layer to minimise cracking should movement occur below the screed.
For this purpose, the most convenient and safest form to use is a roll format. Folded vapour barrier has creases in it which take time to remove fully and if not removed could result in induced cracking of the screed.
Edge insulation, also known as perimeter strip or flanking strip, must be placed around all walls to separate the floor screed from the walls and any vertical pillars that extend into the floor. Depending on the insulation used, and other purposes required, it may also provide for thermal and acoustic insulation. Once again we stress that these measures should be taken regardless of whether liquid screed or conventional screed is used, and also whether underfloor heating is installed or not.
Underfloor Heating Screed - What is it?
Underfloor heating screed, as such, does not exist as as an individual or specific screed product. Yes, along with others we refer to \"underfloor heating screed\" but really we should say \"screed for underfloor heating\".
All of the liquid screeds mentioned above are ideally suited to underfloor heating screeding due to the characteristics of the gypsum based screeds. In particular their greater strength allows for screeding at a reduced depth, compared to traditional sand / cement screed, whilst maintaining the strength of the screed. Typically you would need a gypsum screed of 50mm when used with underfloor heating pipes, this would eaquate to 50mm on top of the insulation 15/16mm for the pipe on top of the floor insulation with a 34/35mm covering on the underfloor heating pipe itself. Generally, when specified with underfloor heating, a sand / cement screed would need to be 75mm in depth.
Therefore using a gypsum liquid screed reduces the amount of screed required and more importantly it means less screed on top of the underfloor heating loop resulting in a more responsive heating system. In addition there may be construction benefits from the saving in headroom.
How long does screed need to dry before fitting flooring on top?
Sand and Cement Screed
Drying times: The British Standard gives recommended drying times of 1 day / mm for the first 50mm, and 2 days / mm for every mm over 50. In other words for a 65mm screed there is a recommended drying time of 80 days. In practise 4 weeks is a sensible minimum, but if the covering is impermeable then the moisture content should be assessed, either with a hygrometer, or by covering an area of the floor with plastic sheet overnight, and seeing if there is condensation under it in the morning.
Liquid (self-levelling) screed
Drying times: Drying times will be significantly reduced by using liquid screeds. Not only is the thickness less, but liquid screed has better drying characteristics, and can be dried using dehumidifiers.
What flooring is best for UFH. Can wood be laid? Are there any special requirements?
The group of floor coverings consisting of ceramic tiles, natural stone, slate, marble, limestone, flagstones and any similar material work extremely well with underfloor heating as they are good thermal conductors. Most other coverings are also suitable.
Screeded floor constructions
Because the underfloor heating causes the floor to act as a huge radiator, it expands and contracts with the varying amounts of heat present. In the screed, this is absorbed by an expansion gap and edge insulation; any tiles fitted on top may well expand at a different rate to the screed and should, therefore, also have an expansion gap around the perimeter of the room (the bigger the room, the bigger the gap should be) – this will usually be covered by a skirting board. Flexible adhesive and grout should also be used when fixing down tiles to help mitigate expansion.
Suspended timber floor constructions
The nature of suspended timber floors means they are, inevitably, more prone to movement than screed. This said, much can be done to minimise movement and allow successful use of tiles. Well fitted joists are extremely strong and, when covered with a minimum of 18mm (22mm is recommended) staggered chipboard or ply panels with glued joints that has also been screwed down securely, any movement is much reduced. When using this floor construction it is vital to use flexible tile adhesive and grout.
Timber flooring is generally supplied in two forms: solid wood planks available in various widths and types of timber, and engineered board which includes everything from a thin veneered panel available from most DIY stores, to specialised solid wood laminates.
Because wood is a natural product it reacts to the surrounding atmosphere and this should be borne in mind when selecting the correct timber flooring for your home. Engineered board (as opposed to thin laminates available from most DIY chains), is a high quality product made up of several layers laminated together, often with the wood grain running in opposite directions in each layer, finished with a thick hardwood surface. It is an inherently stable product with minimal expansion and contraction when used with underfloor heating. Solid timber is also suitable, although narrower boards are recommended as they expand and contract less with atmospheric changes than wide boards. Where thin laminates are used over a suspended timber floor, a chipboard or ply sub-deck will be necessary for stability. Both engineered board and solid timber flooring are available in a wide variety of finishes, widths, thicknesses and qualities to suit the style required.
Whilst good quality laminate floors and engineered boards may have their moisture content corrected before lamination, care should always be taken to ensure conditions on site are suitable for both acclimatisation and laying any timber floor. Moisture levels in a building prior to laying a wood floor are critical. To avoid excessive movement of the finished floor the tradesmen laying it must be satisfied that the screed, timber flooring and internal construction (wall and ceiling plaster) are suitably moisture-free, which may meanleaving boards unpacked in a dry room for several weeks to acclimatise.
Despite popular misconceptions, it is perfectly possible to fit carpet over underfloor heating, it is just a question of choosing the correct combination of carpet and underlay for the project.
The thermal resistance value of carpet and underlay is measured in togs with thickness, density and composition all affecting the tog rating achieved. To get the best performance from underfloor heating, the combined value of the carpet and underlay should not exceed 2.5 tog. Typically, the tog value of an 80%/20%, wool/nylon carpet will be between 1.5 and 2.0 and the tog value of underlay varies enormously so it is perfectly possible to source a combination to suit. Quality manufacturers will have technical information on their products available to customers.