Carpet Underlay Types Explained: Foam, Rubber, and Felt

When carpet is installed, an underlay is laid beneath it on the subfloor. The underlay is largely invisible in use but has a significant effect on how the carpet feels, how well it performs, and how long it lasts. A good underlay adds cushioning, insulates against temperature and sound transfer between floors, and protects the carpet backing from abrasion against the subfloor. Choosing the wrong underlay — or skipping it to save cost — shortens carpet life and reduces comfort.

Foam Underlay

Foam underlay is the most common type in Australian residential installations. It is made from polyurethane foam and available in a range of densities and thicknesses. Key characteristics:

• Cushioning: foam provides soft, cushioned underfoot feel. Higher density foam (measured in kg/m³) provides firmer support that better protects carpet backing; lower density foam feels softer but compresses more quickly under heavy foot traffic.
• Thickness: common residential thicknesses range from 6mm to 12mm. Thicker foam adds comfort but can affect door clearances and threshold transitions.
• Cost: foam is the most affordable underlay option, which is why it is standard in most rental and builder-grade installations.
• Limitations: foam underlay degrades over time, particularly under heavy use. It can break down into crumbling material after ten to fifteen years, which creates a bumpy surface and reduces support. Moisture resistance varies by product — standard foam is not suitable for areas with moisture risk without a moisture barrier layer.

Rubber Underlay

Rubber underlay (also referred to as waffle rubber or crumb rubber) is denser and heavier than foam. It is made from natural or recycled rubber and has a characteristic waffle or ribbed surface that helps with air circulation and grip. Key characteristics:

• Durability: rubber underlay is significantly more durable than foam and does not degrade in the same way. It maintains its properties longer under heavy use, making it suitable for high-traffic areas and commercial applications.
• Sound insulation: the density of rubber provides better airborne sound insulation (between floors) than foam underlay of the same thickness.
• Firmness: rubber underlay feels firmer underfoot than foam. It provides strong support for the carpet backing, which reduces pile crushing and extends carpet appearance life.
• Moisture: natural rubber has reasonable moisture resistance. Crumb rubber underlay made from recycled tyres is used in areas with moderate moisture exposure.
• Cost: rubber underlay is more expensive than foam but less expensive than premium felt. It is a common choice for quality residential installations and commercial carpet.

Felt and Wool Felt Underlay

Felt underlay is made from compressed synthetic fibres, recycled textile fibres, or — in premium products — wool felt. It is the firmest of the three main underlay types. Key characteristics:

• Thermal insulation: felt, particularly wool felt, provides excellent thermal insulation — significantly better than foam or rubber of similar thickness. This is the underlay of choice for rooms where floor temperature is a comfort consideration.
• Firmness: felt underlay provides a firm, supportive underfoot feel. It is commonly used under wool carpet and in formal rooms where a firm, stable surface is preferred over cushioned bounce.
• Premium feel: wool felt underlay gives carpet an exceptionally dense, quality feel that is noticeable underfoot. It is significantly more expensive than foam or rubber.
• Applications: felt underlay is not suitable for areas with moisture risk — it absorbs water readily and takes a long time to dry. It is best suited to dry indoor rooms with good ventilation.