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Underfloor Heating

• Introduction
• Design responsibility and procurement
• Specification
• Examples of floor systems
• Comments

Introduction

The Warmafloor Sales Brochure, available from the Warmafloor website [http://www.warmafloor.co.uk/], is a good general introduction to underfloor heating, although this recommendation should not be taken as an endorsement of the Warmafloor product itself.

This page is intended as an introduction to the selection, specification, and procurement of warm water piped systems, for heavily-loaded floors, where the pipes are contained within a screed. The emphasis is towards ceramic and natural stone flooring.

Design responsibility and procurement

Underfloor heating systems should always be designed and installed by specialists and the contractual arrangements should ensure that the relevant design responsibility is passed from the specifier to the specialist. The scope of the design responsibility may, however, vary:

• As an absolute minimum, the specialist design should include the selection and sizing of the heat source, pipework, manifolds, valves, controls, and all ancilliary components related to the warm water system. The specialist should also provide advice on the selection of materials for, and the sizing of, the insulation and the screed. If the specifier is taking the ultimate design responsibility for the insulation and screed, the specialist should be made aware that the specifier is relying on the specialist's advice concerning these essential materials.
• As an ideal (for the specifier's liability), the specialist design should include everything from the structural slab up to and including the finished floor surface material. This level of contractor/sub-contractor design is generally possible only on large contracts employing major firms of specialist sub-contractors familiar with taking this level of design responsibility.

Even though the design responsibility for the complete floor system may be passed to the contractor/sub-contractor, the specifier should undertake research to ensure that they are able to properly evaluate the contractor's/sub-contractor's proposals. This research should cover, as a minimum:

• The bedding method(s) for the floor finish material.
• The type and thickness of the screed.
• The type and thickness of the insulation.
• The type and locations of movement joints, if any.

Specification

Bedding method for ceramic and mosaic tiles

• Reference : Refer to BS 5385-4:2009. Table 4 (Bedding methods) and paragraph 9.2.5.2 (Heated floors).
• Preferred bedding method (for underfloor heating) : Solid thick bed adhesive.
• Commentary : Paragraph 9.2.5.2.7 states that a separating membrane is required for all sand:cement fixing methods over underfloor heating but Table 4 states that this is unsuitable for heavy loads or impact. Solid thick bed adhesive is therefore the only bedding method recommended by BS 5385-4.

Bedding method for natural stone tiles

• Reference : Refer to BS 5385-5:2009. Table 2 (Bedding methods), Table 3 (Bedding thickness), and Section 7 (Underfloor heating)
• Preferred bedding method(s) (for underfloor heating) : The choice of bedding method depends on the construction programme and, most specifically, whether the programme allows for the drying and curing of the screed, and the running of the heating system, before the installation of the tiles.
• Solid thick bed adhesive : Design thickness: 5mm or as recommended by adhesive supplier. Use where (a) the curing and drying sequence referred to in BS 5385-5, paragraph 7.2, can be achieved, and (b) the stone tiles are of calibrated thickness and the dimensions do exceed 600 x 600 x 20mm.
• Cement:sand semidry mix, unbonded and reinforced : Design thickness: 50mm to 75mm. Use where (a) the curing and drying sequence referred to in BS 5385-5, paragraph 7.2, cannot be achieved or (b) the stone tiles are larger than 600 x 600mm and/or thicker than 20mm: Although the Standard (Table 3) states a minimum thickness of 40mm, this will be difficult to achieve with reinforcement. Above 75mm the bedding will take too long to dry out. The Standard (Table 3) states a maximum of 70mm.
• Commentary : Although a cursory reading of BS 5385-5:2009 might appear to suggest that the bedding under stone tiles should always be bonded in heavy traffic areas, the Standard is equally emphatic that (a) a separating layer should always be introduced when there is any likelihood of differential movement between the bedding and the substrate, and (b) the bedding should always be reinforced when laid over a separating layer in heavy load conditions (paragraph 6.4.2).

Screed

• Proprietary fibre reinforced quick drying cementititious screeds : Flowcrete K-Screed, Tarmac Truscreed, Mapei Topcem, and similar.
  Thickness: 60mm to 75mm.
• Proprietary calcium sulfate based screeds : Lafarge Gyvlon, Tarmac Truflow, RMC Supaflo, and similar.
  Thickness: 55mm to 70mm.
  For more see the Calcium Sulfate Screeds page.

Design issues

• The minimum thickness of the screed will depend on (a) the size of the heating pipework, (b) the method of fixing the heating pipework, and (c) the minimum cover over the heating pipework as recommended by the heating system supplier and/or the proprietary screed supplier.
• The maximum thickness of the screed will depend on the drying-out time, noting that it is possible to use the heating system to accelerate the drying-out subject to certain conditions. The use of the heating system to accelerate the drying-out may require special provisions such as the installation of temporary heat sources if the main boiler installation will not be complete at the time that screed drying is required.
• Floating cementitious screeds should always be laid unbonded and reinforced. 75mm (as stated above) is based on 40mm cover over 25mm external diameter pipework supported on 10mm thick clips. The minimum of 60mm is based on 35mm cover over 25mm external diameter pipework fixed directly to the insulation.
• Calcium Sulfate Screeds can generally be laid slightly thinner than cementitious screeds and are generally laid unreinforced.
• The screed must be separated from the surrounding walls by compressible joints. These are usually concealed below the skirting or wall lining.

Insulation

Design issues

• The under-screed insulation material should be specified, and its thickness calculated, by the underfloor heating system supplier.
• There are a number of insulation board products specifically intended for use under heating screeds. These often have a reinforced membrane applied to the upper surface, grid markings to facilitate the pipework layout, and may be designed to take direct fixing of the heating pipes without intermediate pipe clips. 50mm minimum thickness is typical for UK installations.
• BS 5385-5:2009, paragraph 7.2, states "Typically for commercial applications the insulation will require a minimum compressive strength of 350 MPa." We believe that this a "typo" and the figure should be 350 kPa, although even this may not be strong enough in certain applications. For more information about the compressive strength of under-screed insulation, see the Floor Strength for MEWPs page.

Movement joints

• Ceramic and mosaic tiles : BS 5385-4:2009, paragraph 9.2.5.2.7 states "not greater than 3.5 m centres".
• Natural stone tiles : BS 5385-5:2009, paragraph 7.2 states "Where underfloor heating exists, movement joints in the screed should limit bay sizes to 40m² with a maximum bay length of 8m."

Design issues

• Some specifiers believe that calibrated natural stone tiles, up to 600 x 600 x 20mm and laid in a solid thick bed adhesive, should be treated as ceramic tiles and laid with movement joints at maximum 3.5m centres.
• Movement joints in the tiling, bedding, and underlying screed, should always be provided in line with any movement joints in the supporting structure below. Specifiers should ensure that the joint materials/ products can accommodate expansion and contraction (some narrow joint products work only in compression) and are suitable for use with heated floors.
• Movement joints in the screed should always be provided in line with the movement joints in the tiling when adhesive bedding is used.
• Movement joints in the screed are not always required at the same centres as the movement joints in the tiling when cement:sand semidry bedding is used, but specialist advice should be sought in all such cases.
• Careful coordination is required between the location of the movement joints in the tiling/bedding and the layout of the heating pipework in the screed below. It is generally desirable for the heating pipework to be laid in bays which replicate the bays in the tiling above. This facilitates crack control if adjacent heating bays have different heating output or may be switched on and off at different times.
• Special provision must be made where pipework crosses the line of any movement joints in the screed. (Refer to the Warmafloor trade literature, as above, for various methods of dealing with movement joints in the screed.) Underfloor heating pipework cannot, in general, be laid across the line of structural movement joints.

Overall floor zones

• Minimum : After selecting the floor finish material, the minimum floor zone can be calculated by combining the minimum bed thickness with the minimum nominal screed thickness. Specifiers should be aware that minimum beds/screeds are likely to increase the possibility of cracking and a higher quality of workmanship is essential.
• Nominal/Optimum : The optimum thickness for a reinforced cement:sand semidry bedding is 65-70mm. The optimum thickness for a proprietary screed is also 65-70mm.
• Deep floor zones : The only effective solution for deep floor zones is to increase the thickness of the insulation, selecting a thickness (or combination of thicknesses) from the chosen manufacturer's standard range, and then adjusting the thicknesses of the bedding and screed as appropriate. For very deep floor zones, it is sometimes possible to use lightweight concrete blocks as a void filler, overlaying the blocks with the insulation board.

Examples of floor systems

Example 1 - Ceramic tiling in adhesive bed over cementitious screed

• Floor finish zone : 140mm (above concrete slab).
• Ceramic tiling : 10mm thick tiles - specified in NBS clause M40/110.
• Solid thick bed adhesive bedding : 5mm thick - specified in NBS clause M40/710.
• Proprietary (fibre) reinforced quick drying cementititious screed : 75mm thick - specified in NBS clause M10/130, laid unbonded. Reinforced with steel fabric mesh, or proprietary fibre reinforcement, specified in NBS clause M10/392.
• Polyethylene sheet separating layer : 500 gauge - specified in NBS clause M10/290.
• Expanded/Extruded polystyrene board insulation : 50mm thick - specified in NBS clause M10/290.
• Concrete slab substrate.

Example 2 - Stone tiling in semidry cement:sand bed over cementitious screed

• Floor finish zone : 200mm (above concrete slab).
• Natural stone tiling : 20mm thick calibrated tiles - specified in NBS clause M40/110 or 115.
• Semidry cement:sand bedding : 55mm thick - specified in NBS clause M40/730. Reinforced with D49 mesh specified in NBS clause M40/578.
• Polyethylene sheet separating layer - 500 gauge - specified in NBS clause M40/451.
• Proprietary (fibre) reinforced quick drying cementititious screed : 75mm thick - specified in NBS clause M10/130, laid unbonded. Reinforced with steel fabric mesh, or proprietary fibre reinforcement, specified in NBS clause M10/392.
• Polyethylene sheet separating layer : 500 gauge - specified in NBS clause M10/290.
• Expanded/Extruded polystyrene board insulation : 50mm thick - specified in NBS clause M10/290.
• Concrete slab substrate.

Example 3 - Vinyl sheet flooring over calcium sulfate-based screed (Light loading only)

• Floor finish zone : 110mm (above concrete slab).
• Vinyl sheeting - 5mm thick (including adhesive) - specified in NBS clause M50/150.
• Proprietary calcium sulfate based screed - 55mm thick - specified in NBS clause M10/130, laid unbonded and unreinforced.
• Polyethylene sheet separating layer - typically 500 gauge - specified in NBS clause M13/260.
• Expanded/extruded polystyrene board insulation - 50mm thick - specified in NBS clause M13/260.
• Concrete slab substrate.

Comments

12-Nov-2009 - Movement joints

• For ceramic and mosaic tiles it is noted that there is a discrepancy between the recommendations of BS 5385-4:2009 (quoted in the text of this page) and BS 5385-3:2007.
• BS 5385-3:2007, paragraph 7.1.6.4 states: "... bays of size not greater than 40 sq.m. with an edge length not greater than 8 m."
• The matter is being referred to BSi for clarification.

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