Flooring Underlay: Its Role in Sound, Moisture, Levelness

What underlay does beneath laminate, parquet and vinyl, the main types, how to choose thickness, and the problems an underlay cannot fix.

When choosing laminate or parquet, attention usually goes to the colour and wear class, while what goes underneath is decided at the last minute. Yet the underlay affects how the floor sounds, how long it lasts, how well the locking joints hold and even the warranty terms. The wrong underlay undermines an expensive floor; the right one makes a noticeable difference to everyday comfort.

What an underlay does

An underlay has several jobs, and each is a separate question:

• reducing footstep noise, both the clatter heard in the room and the impact sound transmitted to the flat below;
• stopping moisture from the screed reaching the floor covering;
• compensating for very minor irregularities in the base;
• spreading the load on the locking joints;
• in some cases adding a little thermal insulation.

No single material does all of these equally well. So the choice starts with deciding which problem matters most to you.

Main types of underlay

Polyethylene foam underlay is common and simple to lay. It gives modest noise reduction and some cushioning, but it can compress over time and lose its properties.

Extruded polystyrene boards are firmer and smooth out small irregularities well. They support locking joints better, but they transmit heat poorly over underfloor heating.

Cork underlay is a natural material that absorbs sound well and resists compression. Because it is sensitive to moisture, a separate vapour barrier is laid beneath it on concrete.

Dense underlays with mineral or polymer fillers reduce impact noise more effectively thanks to their mass. In apartment buildings, where sound reaching the neighbours below is a concern, this type is worth considering.

Many underlays come with a built-in vapour barrier film. In that case a separate membrane may not be needed, but the seams must be taped.

Thicker is not always better

The most widespread misconception is that the thicker the underlay, the better. The locking systems of laminate and engineered parquet are designed to work on a base of a certain firmness. On a soft, thick underlay the boards flex underfoot, the joints eventually open or break, and gaps appear.

That is why underlay thickness and firmness should follow the floor manufacturer's recommendations. Some manufacturers accept only their own underlays or products with specified ratings; ignoring this can create warranty problems.

What an underlay will not fix

An underlay is not a substitute for a proper screed. Visible dips, humps and level differences cannot be hidden by it; the problem is simply postponed. The board flexes over the void, the joint is stressed, and the floor starts to creak.

Check the base with a long straightedge before laying. If flatness is outside the tolerance the manufacturer allows, correct it first with a levelling compound and let it dry completely.

Screed moisture should be checked separately. A vapour barrier does not dry a wet screed; the moisture stays trapped and can later distort a timber-based floor.

Underlay over underfloor heating

Where there is underfloor heating, the underlay must not block heat transfer. Thick foam and insulating boards hold heat back and reduce the system's efficiency. Thin underlays with low thermal resistance, specifically designed for heated floors, are used instead; they are usually marked accordingly.

Vinyl floors such as SPC and LVT behave differently from laminate. Many of them already have an underlay bonded to the back of the plank at the factory, and adding a second layer can weaken the locking joints. Glue-down vinyl uses no underlay at all: it is bonded directly to a smooth, primed base. Because thin vinyl reveals every flaw in the base, the flatness requirement here is even stricter.

Points to watch during installation

The underlay itself has to be laid properly:

• rolls are butted edge to edge rather than overlapped, and seams are taped;
• the vapour barrier is turned up slightly at the walls and hidden behind the skirting;
• underlay strips run perpendicular to the direction of the boards;
• it is rolled out progressively as the floor goes down, so it is not damaged.

In its renovation projects Zaferoğlu İnşaat plans the floor finish together with base preparation and the underlay system. Send us the make of the floor covering you picked, and we will tell you which underlay suits it and underfloor heating.