There are three basic types of carpet cushion underlay:

• fibre - also called 'felted fibre'
  
  
• rubber - also called 'sponge rubber'
  
  
• foam - also called 'polyurethane foam'.

Let's have a look at each of these in turn.

Fibre

Fibre underlay is generally made by needle punching natural or synthetic fibres into an interlocked sheet of felt.

Natural fibres include animal hair and jute (a vegetable fibre commonly used to make hessian bags).

Synthetic fibres include nylon, polyester, polypropylene and acrylic.

Most fibre underlays are dense and firm, and have excellent thermal and sound insulation.

They are also environmentally friendly, and can be made from 100% recycled fibres.

However, they are not as resilient as the more-popular rubber or foam underlays, and they tend to take longer to install - so they are rarely used these days in residential jobs.

Nonetheless, they are still used in commercial projects that require a hard-wearing, firm underlay.

Rubber

Sponge rubber is made by combining rubber with blowing agents.

The manufacturing processes vary, depending on the density required and whether the raw materials are natural or synthetic.

Natural rubber comes from rubber trees.

It has anti-microbial properties, meaning it resists the growth of bacteria, mould and mildew.

Sponge rubber underlay allows heat to transfer easily, and is suitable for use over underfloor heating systems.

Waffled rubber is mostly used in domestic applications.

The waffles allow air to circulate between the underlay and the subfloor, which helps to reduce dampness.

Flat sponge rubber is firmer and denser, and is generally used in large scale commercial installations.

Foam

Polyurethane foam is made from 'polymers' that are derived from crude oil. In an underlay, it has an open cellular structure that enables it to feel soft underfoot but still be very durable.

There are three different types of foam underlay:

Prime foam is similar to the foam used in upholstered cushions, but somewhat denser. It is made from non-recycled materials.

Bonded foam is also called 're-bond' or 'chip foam', because it is made from chopped bits of scrap foam which typically gives it a multi-coloured appearance.

Frothed foam generally has a higher density than the other foams, and includes 'memory foam', which is able to compress and return to its original shape.

Australian Standard 4288

Although cushion underlays are often categorised as fibre, rubber or foam, the Australian Standards use a slightly different system for grouping the different types of cushioning materials.

In AS 4288, underlays are categorised as 'fibrous', 'non-fibrous' or 'combination'.