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Rugs and Carpeted Floors

BSRIA Testing Confirms Suitability of Underfloor Heating for use with Carpets

carpetsUntil now, electric underfloor heating systems have generally been associated with hard floor coverings such as ceramic tiles and stone.

Until recently, ignorance and reticence have prevailed in respect of using underfloor heating for carpeted floors. The previous industry ‘standard’ has been to advise that the carpet and underlay should not exceed a tog rating of 1.5.

However, research conducted by BSRIA in Bracknell and funded by the Carpet Foundation in conjunction with the Underfloor Heating Manufacturers Association (UHMA) has proved conclusively that any carpet can be used with an underfloor heating system without impairing the performance of the system.

The work showed that none of the five carpets tested (wool-rich Axminster, wool-rich cut-pile Tufted, wool-rich loop-pile Tufted, synthetic loop-pile Tufted and synthetic cut-pile Tufted carpets) laid over waffle sponge rubber or crumb rubber underlay interfered with the efficient warming of the airspace in the 16 sq m test room above.

It is now a fact that any carpet and underlay system with combined thermal resistance of not exceeding 2.5 tog allows the underfloor heating system to operate efficiently.

New indications are that for the majority of carpet styles the thermal resistance will be less than 1 tog. Thus the majority of carpet and underlay combinations (excluding felt underlay) will generally be less than 2.0 tog and thus suitable for use with underfloor heating.

BSRIA’s findings show that the previous double plate test method was of no value when it came to underfloor heating. Similarly, the tested tog values of the carpets (using the BSRIA test chamber as a life size ‘tog’ meter) were significantly lower than the previous laboratory test values. The new findings discredit the previous industry standard of 1.5 tog.

Consumers can now be reassured that carpet and underfloor heating systems can operate effectively and efficiently together.

Suitability of Carpets

The latest recommendations from the Underfloor Heating Manufacturers Association (UHMA) and the Carpet Foundation are that the majority of carpet styles are suitable for use with underfloor heating systems.

All carpets and tiles act as a form of thermal insulation and this means that carpeted floors will take slightly longer to get warm initially, and stay warm longer. As with any floor covering care should be taken to ensure that thermal blocking does not occur. Please refer to the ‘thermal blocking’ section on this web site for more information.

Suitability of Underlay

There are essentially 4 main types of underlay
Felt
•Sponge rubber
•Recycled crumb rubber
•Recycled polymer foam.(Carpet Foundation member Ball & Young is market leader)

1) Felt underlay has a high thermal insulation value so is not recommended for use where there is an under floor heating system. Felt underlay is likely to seriously reduce the effectiveness of the underfloor heating system because it insulates the floor surface. Typical Tog values for felt underlays are of the order 2.5 to 2.9 tog.

2) Sponge rubber underlays with a waffle-pattern moulded into their underside are the most commonly sold in the UK. Radiant energy can easily pass through the air spaces in this waffle-pattern and sponge rubber underlays are recommended for use with underfloor heating systems.

3) Recycled crumb rubber underlay has been shown to present no detriment to the efficiency of underfloor heating systems and is also recommended.

4) Recycled polymer foam underlay is lighter in weight than other underlay types without detracting from its properties and is popular with carpet fitters. It too is recommended.

Each style of underlay is available in a range of thicknesses and weights. The thermal resistance of underlay is primarily dependent on what it is made of and its thickness.

Generally thinner rubber products perform best with underfloor heating systems. Two suitable underlays both from the point of view of carpet performance and for low tog values are Gates Technics 5 (tog value of 0.5) and Technics 6 (tog value 0.63). Both are doublestick underlays. Duralay have recently introduced a new range of underlays called Heatflow specifically designed for underfloor heating for domestic and commercial use. Their tog values are 0.75 and 0.92. The 0.75 tog domestic products are for stretch installation and the 0.92 tog products are for stretch or double stick installations.

There are two British Standards for underlay

BS 5808 Specifications for underlays for textile floor coverings
BS EN 14499 Textile floor coverings; minimum requirements for carpet underlays

You should be confident that your chosen underlay has been tested to one or both of these standards and is satisfactory for the intensity of use that it and the carpet will experience in use. This information should be on the product label or on an information leaflet or brochure available in the retailer’s store. If the product is unlabelled or has no supporting information the retailer should be questioned about the provenance of the product.

Maximum Combined tog Value

We recommend a maximum combined tog rating of 2.5 for use with Vysal’s underfloor heating systems provided that our installation instructions for a particular product and application have been followed.

Carpet Close Up

Guidelines for Installation of Carpets and Underlay

As a general rule we recommend that you follow advice from the Carpet Foundation in respect of carpet fitting. However, there are some aspects concerning underfloor heating that need to be clarified.

1.If you install Vysa-Mat heating mats we recommend that you apply a latex floor levelling compound as a suitable intermediate substrate upon which the underlay and carpet can be fitted. We recommend a fast-setting adhesive for carpet grippers if you wish to bond the carpet gripper strips to the floor in accordance with current regulations. Nailing them to concrete sub-floors is to be discouraged as services are increasingly being buried in the concrete sub-floors.

2.If you install Vysa-Film heating elements we recommend you use a Vysa-Floor free floating sub-floor which has been designed as a stable sub-floor specifically for fitted carpets. Vysa-Floor is much easier to lay than the 9mm ~ 12 mm thick plywood sub-floor that other manufacturers recommend the installer constructs as a base for fitted carpets.

3. If you install Vysa-Cable heating cables in the sand and cement floor screed as part of the building process during a new-build or refurbishment project, your carpet fitter can simply do as he always does once the floor screed is cured. We recommend 1 week for each 25mm of thickness under normal drying conditions. Do not be tempted to accelerate the curing by switching the heating system ON. We recommend a fast setting adhesive for carpet grippers if you wish to bond the carpet gripper strips to the floor in accordance with current regulations. Nailing them to concrete sub-floors is to be discouraged as services are increasingly being buried in the concrete sub-floors.

4. You may wish to follow the advice of others and follow the double stick installation method which they claim provides close contact between the floor, underlay and carpet allowing better heat transfer and avoiding uneven heating pockets. We have no opinion or comment regarding this as it is a matter of personal choice or preference.

Where the carpet is to be laid on a wooden sub-floor (such as Vysa-Floor) it is very important to ensure that adequate time has been allowed for the wood or Vysa-Floor to condition in the room before laying the sub-floor. Also, for it to condition after installation and whilst the underfloor heating is running to avoid variability in dimension from changes in moisture levels. Otherwise rucking or shrinkage of carpet and underlay may result from movement in the underlying floor surface.

Normal tackifiers and adhesives such as F Ball F41 & F3, are suitable for use at the normal operating temperature of 28 deg C. However, it is important to ensure that the heating is switched off 48 hours prior to installation and for 48 hours afterwards. The heating should gradually be brought up to full working temperature over a 7-day period.

Gates Technics and Durafit Information

Ball & Young Information

F Ball & Co