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PU Underlay Guide

l Lay the underlay as instructed by the manufacturer and cut to the shape of the room. Ensure there is sufficient underlay against the skirting board to trim or adjust as required.
l Fold back one half of the underlay and, depending upon the tackifier being used, apply a coat of tackifier with a roller or trowel to the subfloor. The whole of the exposed floor area should be covered and when the tackifier is dry fold the underlay back over the tackified surface. Note: Do not lay the underlay into the tackifier whilst it’s still wet otherwise it becomes a permanent adhesive.
l Repeat the process with the other half of the room. (Always refer to manufacturer’s instructions).
Trim the excess underlay around the skirting board, leaving a slight gap to allow a tuck finish. Check the thickness of the carpet to calculate the size of this gap.
l Position the carpet on top of the underlay, carefully checking all corners, recesses and pattern matching on seams before cutting. It may be preferable to pattern match and seam prior to the doublestick installation. This will help handling and will strengthen the seams. It is recommended that any carpet seams are laid at right angles to underlay seams.
All selvedges should be stripped and sealed. All seams should be sealed using seaming cement.
Fold back one half of the carpet to expose the underlay and apply the permanent adhesive using the notched trowel. (See fig.3 and fig.4).
The size of the notches will depend upon the backing of the carpet, but should be a minimum of 2mm x 6mm.
Unfold the carpet back onto the wet adhesive and use the glider to smooth out any wrinkles or bubbles, creating a good bond between carpet and underlay. (see fig.5)
Repeat the process with the other half of the room.
Trim the carpet around the skirting board and carefully tuck into the gap you have created between the underlay and the skirting board.
Note: Some carpets, in particular natural floorcoverings such as jute or sisal, are prone to movement. Using gripper around the perimeter of the room will prevent this, creating a ‘belt and braces’ safety margin.
Rubber, sponge: This is made using chemical blowing agents to provide an open cell structure. Waffle (corrugated) is available in different thicknesses and colours. Flat offers better wear protection than sponge. Known as sponge or sponge rubber, these underlays tend to be guaranteed for the life of the carpet. This type of underlay is said to be Europe’s best seller and its ease of cutting makes it a favourite with many installers.
PU (polyurethane) foam: This is made from recycled polyurethane foam. The foam is sorted and then granulated into crumb form. It is then chemically bonded together under pressure in a mould, precision peeled to the required thickness and then laminated with the required style of reinforcement netting and laminates. The underlay is available in various densities and thicknesses typically from 6mm to 12mm, suitable for both contract and domestic installations.
This type of underlay has an excellent point of load recovery rate and rolls weigh much less than rubber equivalents. Some PU underlays are suitable for double-stick applications. Polyurethane underlays are also recyclable at the end of their life.
Note: Most PU underlays are permeable and can be used over a wood block floor, however always check with the manufacturer or supplier as some can contain a membrane which can cause wood blocks to sweat.
Crumb rubber: This underlay is made from re-cycled tyres with a scrim bonded to it and is very firm and is considered to be excellently dimensionally stable. It is primarily used in high footfall areas and is an ideal product for contract installations, however it doesn’t offer the same level of comfort as domestic underlay.
This underlay is an ideal choice for double stick installations and is available in thicknesses from 5mm to 10mm. When used in domestic properties it provides excellent support to the carpet when used on stairs. This product can be a bit messy during cutting as it is prone to crumbling. It is often sold in smaller roll sizes due to its weight.
Felt: This was one of the first types of underlay to be produced and is made from re-cycled or waste yarn such as wool and jute as well as synthetic products. It is regarded as a very environmentally friendly product and provides excellent thermal insulation. The quality of this product is gauged by its weight per square yard. These typically range from 28oz up to 65oz and are produced in a number of thicknesses.
This type of underlay has excellent fire retardant properties and it is still widely used in the contract sector due to its dimensional stability, and because in the event of fire it gives off a non-toxic white smoke as opposed to the toxic black when rubber burns. It is also a good choice to use where seams are necessary on woven carpet as it allows the seams to bed in.
Combination: This combisnes felt and crumb rubber. The crumb rubber forms the base of the product with the felt on the surface. Again, this product is produced in a number of thicknesses and qualities. It is a good choice where sound insulation and thermal insulation are considering factors as well as when the subfloor has irregularities.
Stick down underlays: This type of underlay is produced to be used in conjunction with adhesives – generally referred to as a release or a double stick system underlay. They are produced in most of the different varieties such as polyurethane, needlefelt, crumb rubber and solid rubber. Some are available with the tackifier adhesive already applied to them. This type of underlay is normally used in areas of high foot fall or where wheeled traffic such as pushchairs and wheelchairs are likely.
The underlay is fixed to the floor using a tackifier then the carpet is glued to the underlay using a permanent adhesive reducing the chances of delamination or carpet stretching.
Underfloor heating underlays: It is most important that the correct underlay is sourced when installing floorcoverings over underfloor heating, these underlays are produced in sponge rubber, crumb rubber and polyurethane and are designed not to degrade or emit odours when heated, they also have low tog values.
Typically the combined tog value of underlay and floorcovering should not exceed 2.5 tog though this can vary slightly depending on the type of UFH system that is installed. We would always recommend that you seek the advice of the heating system manufacturer to identify the combined maximum tog rating that can be used with the system.
Wood and laminate underlays: There are many types of underlays for use with laminate and timber products. The presence of moisture, underfloor heating and sound issues all need to be considered. Although combination underlays that can resolve more than one problem are available, most of those currently available are designed to combat only one. Though many of these types of underlay incorporate a moisture/vapour barrier, this is no substitute for a damp proof membrane. The subfloor should always be checked using a moisture meter prior to any installation taking place.
Coconut fibre: Developed for laminate click systems, 4mm thick.
Fibreboard: Produced in tile form as an underlayment for parquet and laminated panels, and to improve sound and thermal insulation.
Polyolefin foam: For impact sound insulation.
Polythene film: Polymer granules bonded between polythene films. Airflows between the granules aims to reduce mould growth. It provides a vapour barrier while minimizing sound.
Polystyrene foam: In panel format for use under chipboard, prefabricated cork, parquet, laminate and wood floating floors. Polystyrene foam offers thermal, impact and sound insulation.
Polythene foam: Usually in 2, 2.5 and 3mm thicknesses, the foam structure is formulated to provide thermal and sound insulating properties.
It becomes a vapour barrier when combined with a foil backing. It can be made with an overlap and integral joint strip for use as a damp proof membrane combined with puncture resistance and impact sound insulation. A kraft paper backing can add to the insulation properties.
Rubber: High density formula for noise reduction and to be used with underfloor heating. Without a backing it is not suitable where moisture is present unless a barrier has previously been installed. It is also made with vapour proof backings.

This article has been reproduced from the Contract Flooring Journal. You can find them at www.contractflooringjournal.co.uk.