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Everything you've always wanted to know about underfloor heating and flooring, but were afraid to ask
- Question 1: What factors need to be taken into account when determining the type and depth of screed. E.g. thermal storage of concrete – how long the screed above takes to heat up, and how long does it retain heat?
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When used with warm water underfloor heating, the minimum thickness is defined by British Standard BS8204:2004 and is a minimum of 65 millimeters of hand-mix screed and 50 millimeters of liquid screed.
For domestic properties, it is not recommended to exceed 65 millimeters of screed as this will have a negative impact on a dwelling’s SAP rating, as the additional depth means the heating system will have to work harder to ‘warm-up’.
Warm-up and cool down times will be similar, but how long that will be depends on the water temperature, the amount of insulation, and the resistance of the floor covering. For a typical underfloor heating system in screed run by a boiler, approximately one hour would be considered usual.
- Question 2: Are there any specialist products Nu-Heat recommends, e.g. Flow Screed? Easy Screed?
- A cement-based screed may need up to 65mm depth whereas the denser substance of liquid screed requires only 50mm when used with underfloor heating.
The density of liquid screed means it is a good heat conductor, aiding heat transfer around the pipes and enhancing efficiency. This is particularly good when combining underfloor heating and heat pump technology as it maximizes the thermal output of the underfloor heating system, allowing the heat pump to work at a lower temperature, thereby improving its Coefficient of Performance.
- Question 3: How long does screed need to dry before fitting flooring on top?
- Hand mixed 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.
- Question 4: Is insulation needed beneath the screed? If so, how much?
- One consideration of particular importance to underfloor heating is the amount of insulation in the floor, and, more particularly, its position within the structure. There should always be insulation below the pipes resisting heat flow downwards, whereas any flooring material above the pipe will inevitably resist heat flow upwards. The intention is for the heat to flow upwards and into the room, so the floor construction should have as little resistance as possible (also bearing in mind floor coverings, such as carpet). However, given that it is impossible to make the floor structure perfectly conductive, the resistance provided by the insulation below the heating pipes must be significantly greater.
Floors must be insulated to Part L, which suggests 70mm of PIR (Celotex / Kingspan) or equivalent for ground floors. Floors over heated areas should be insulated with 30mm PIR or equivalent.
Edge insulation is sometimes supplied – this must be fitted around all walls to act as an expansion medium for the screed as it heats. Additional insulation should be used on external walls.
The position of the insulation within the floor structure is important too. Traditionally insulation is laid and has a concrete slab cast above it; with underfloor heating the slab should be below the insulation where possible so that the mass of material to be heated is less. This improves response times and reduces downward heat losses. It is much easier to have the build specified in this way by the architect, rather than having to make allowances in the heating design process.
- Question 5: Can UFH be used on any type of floor substrate? Concrete? Suspended timber? What are the various considerations?
- The choice of floor construction is usually influenced by the overall building construction, but is dependant on several factors, including: floor substructure, available height build-up, whether there is any weight restriction, required floor heat output (Low/Medium/High heat loss), acoustic requirement and joist spacing (if applicable).
The key to success is to choose the right floor heating system to suit the construction. For example, a sand and cement or liquid screed over a solid concrete or beam and block floor, or a plated system between a suspended timber joisted floor – as long as it is well supported and has no movement.
- Question 6: 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 and Nu-Heat always designs every system to suit the customer’s requirements.
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 Floors
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 mean leaving boards unpacked in a dry room for several weeks to acclimatise.
Carpet
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.
- Question 7: Should I use underlay? Is insulation needed between screed and flooring?
- Both wood flooring and carpet suppliers usually prefer underlay to be fitted with their products. Underfloor heating suppliers will generally prefer as little underlay as possible to minimize thermal resistance. Nu-Heat will take this, and all other relevant information, into account when designing the underfloor heating system.
- Question 8: If you might want to extend the system later, what needs to be built in at the outset?
- Extending the property and incorporating Nu-Heat underfloor heating into the additional living space is straightforward as any extra heating load can be met by: either allowing extra ports, or adding another manifold.
- Question 9: How can hotspots (where lots of pipes pass through a doorway, for example), be avoided?
- Hotspots can be avoided during the design process by locating the manifold in a sensible position. Nu-Heat always takes such things into consideration before making recommendations.
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0800 731 1976
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