Underlay is one of those purchases that most homeowners want to spend as little as possible on, which is understandable since it disappears under the floor and is never seen again. The problem is that the wrong underlay, or an underlay that is too thin or too dense for the application, affects the performance of the floor covering above it in ways that become apparent quickly.
Good underlay reduces impact sound transmission between floors, provides a small degree of thermal insulation, accommodates minor subfloor irregularities, and in some cases provides a moisture barrier between the subfloor and the floor covering. This guide explains what different types of underlay offer and which to specify for different flooring categories and subfloor conditions.
What Underlay Does
Acoustic underlay reduces the transmission of impact sound (footsteps, dropped objects) through the floor to rooms below. This is measured in decibels of impact sound reduction, often expressed as delta Lw. Higher delta Lw values indicate better impact sound reduction. In flats and multi-storey buildings, good acoustic underlay is important for neighbourly relations and is sometimes required by building regulations.
Thermal underlay adds a degree of insulation beneath the floor covering, which can improve thermal comfort and reduce heat loss to a cold subfloor below. However, thermal underlay over underfloor heating systems is counterproductive; it insulates against the heat you are trying to get into the room and forces the heating system to work harder.
Moisture barrier underlays incorporate a foil or polyethylene membrane that prevents residual moisture from a concrete subfloor from migrating into the floor covering above. This is particularly important for engineered wood on concrete ground-floor slabs where moisture is a risk.
Underlay for Floating Engineered Wood
Floating engineered wood floors require an underlay that provides some cushioning and acoustic reduction without being too soft, which can cause the floor to feel springy underfoot. The standard specification is a foam or foam-and-film underlay in the 2mm to 3mm range. Products combining a polyethylene foam layer with an integral DPM (damp-proof membrane) film are widely used and appropriate for most concrete subfloor situations.
For floating engineered wood over underfloor heating, only UFH-compatible underlay should be used. Standard foam underlay has a TOG rating of approximately 0.5 to 1.5, which acts as insulation against the heat. UFH-compatible underlays have a low TOG rating (below 0.15 typically) achieved through a thin, dense construction that conducts heat but still provides some acoustic benefit.
Underlay for Laminate and LVT
Many click LVT and SPC products come with an attached underlay layer already bonded to the board. In these cases, no additional underlay should be used; adding underlay beneath a product with an attached underlay creates too much flex in the floor and can cause the click joints to work loose over time.
For LVT without an attached underlay, a thin, dense underlay of 1.5mm to 2mm is appropriate. The underlay must be flat and consistent; LVT without an attached underlay is less tolerant of thickness variation than a floating wood floor and benefits from a very uniform underlay surface.
Underlay for Carpet
Carpet underlay is more varied than underlay for hard floors. Foam, rubber and felt underlays all perform differently. Rubber crumb underlay, made from recycled rubber and available in various weights (100g/m2 to 250g/m2), is the most common choice for residential carpet installation. Heavier-weight rubber underlay provides better cushioning and acoustic performance. Felt underlay is firmer and is sometimes preferred for loop-pile commercial carpets where a dense, stable base is more appropriate than cushioning.
- Floating engineered wood: 2-3mm foam with integral DPM for concrete subfloors
- UFH floors: use only UFH-rated underlay with low TOG rating
- LVT/SPC with attached underlay: no additional underlay required
- LVT without attached underlay: 1.5-2mm thin dense underlay
- Carpet: rubber crumb underlay by room use and carpet weight
Specifying underlay correctly costs very little more than buying whatever is cheapest, and the acoustic and performance benefits over the lifetime of the floor are significant. For floating wood floors in upper-storey flats and apartments, investing in a quality acoustic underlay is particularly worthwhile given the social impact of transmitted footstep noise between floors.
