SPC Flooring Underlayment: Choosing the Right Layer

Underlay is the layer between an SPC floor and the subfloor. It does four jobs, and only one of them is about comfort.

• Impact sound. It reduces the noise of footfall transmitted into the structure below, which is the figure building regulations in most European markets actually measure.
• Surface tolerance. It bridges small undulations in a screed that a rigid plank will not follow on its own.
• Joint protection. It gives the click joint a consistent bed, so a plank does not rock over a high point.
• Moisture separation. Some underlays carry a vapour barrier, which matters on a ground-bearing slab.

It does not make a floor waterproof, it does not replace subfloor preparation, and it cannot rescue a slab that is out of flatness. The two most common complaints after installation — creaking joints and a hollow sound underfoot — are usually underlay or subfloor problems rather than product problems.

These four families cover most of the European market. The cost index is relative, with closed-cell polyethylene foam set at 100. The acoustic ranges are those commonly declared by manufacturers, not guarantees for a particular building.

Two variants sit outside the table. Acoustic felt, typically 3–5 mm of bonded recycled fibre, delivers high impact reduction for its thickness but is not a moisture barrier. And many SPC planks now ship with an IXPE backing already attached: that backing is the underlay, and adding a second layer above it is usually the wrong decision.

The acoustic figure on a datasheet is almost always ΔLw, and it is almost always read as a property of the underlay alone. It is not. ΔLw is measured in a laboratory to the ISO 10140 series and rated to ISO 717-2, and the result belongs to the whole assembly: the floor covering, the underlay, and a reference concrete slab.

That has two consequences. A supplier quoting 22 dB without describing the build-up has quoted a number that cannot be compared with a competitor's number. And the laboratory figure is not the field result: in a real building, transmission through walls, screeds and service penetrations often dominates, so the measured performance is usually lower than the sheet suggests.

Thickness is not the variable either. Impact reduction depends on the density and dynamic stiffness of the layer, and beyond a point a softer layer lets the plank deflect. That raises low-frequency drumming and puts stress into the joint, which is the opposite of what was intended.

For project work, the requirement is normally written as a maximum impact sound level in the finished room rather than as an underlay specification. That figure belongs to the acoustic engineer, and the way to meet it is to obtain the laboratory report for the exact floor-plus-underlay assembly and hand it over, rather than to argue about millimetres.

The installation system decides most of the answer before the underlay is discussed.

• Floating click SPC rigid core. The joint is held by friction and geometry. A soft or thick layer lets the plank move vertically at the joint, which is how click locks break. Most manufacturers state a maximum underlay thickness and density, and exceeding it voids the warranty. Where the plank already carries an attached backing, a second layer is usually outside the specification.
• Bonded glue-down LVT. These are adhered to the subfloor, so a separate underlay is not used at all. Acoustic performance comes from the adhesive, the slab and, in refurbishment, a smoothing compound.
• Loose lay. Needs high friction and full contact with the subfloor, so a compressible layer works against the product.
• Underfloor heating. Ask for thermal resistance rather than thickness. A thick, low-density layer insulates the pipe from the room, which raises running cost and slows response.

Subfloor flatness belongs here too. A rigid plank telegraphs a ridge rather than bending over it, so the usual trade tolerance — often around 3 mm under a 2 m straightedge — should be met before anything is unrolled. Underlay bridges millimetres, not centimetres. If the choice is between a different construction and a different underlay, the construction comparison in our SPC vs LVT guide is the better place to start.

Underlay is cheap relative to the floor and expensive to get wrong, so it is worth two or three specific questions at quotation stage.

• The EN 16354 declaration. The European standard for floor underlays requires the manufacturer to declare thickness and tolerance, compressive behaviour and impact sound reduction. A supplier who cannot produce that sheet has not specified the product.
• The build-up behind the dB figure. Which slab, which floor covering, which layer and at what thickness. Without that, the number is marketing.
• Compression set. How much thickness is lost after sustained load. A layer that compresses permanently leaves the click joints with nothing to hold, and the failure appears months later.
• Density and thickness tolerance. Low-cost foam is frequently thinner than declared, and the difference is invisible once the floor is down.
• Approval for the floor being installed. Get written confirmation that the layer is compatible with the specific range. In a claim, the floor supplier's warranty is the one that will be tested, and most exclude unapproved underlays.
• Vapour barrier. On a ground-bearing slab, confirm whether the layer includes one or whether a separate membrane is required at joints and perimeters.
• Indoor air quality documentation. A floor with a clean declaration can sit on an underlay that has none, which matters where the project is assessed as a whole.

Do I need an underlay if the planks already have a backing attached?

Usually not, and often you should not add one. The attached IXPE or EVA backing is the designed underlay for that plank, and a second compressible layer changes the joint behaviour the manufacturer tested. Most warranties exclude installations built that way, so confirm with the floor supplier before adding anything, and never add a layer to compensate for a subfloor that needs levelling.

Does a thicker underlay reduce impact noise more?

Not reliably. Impact reduction depends on density and dynamic stiffness as well as thickness, and beyond a point a softer layer lets the plank deflect, which can increase low-frequency drumming and stress the click joint. Compare declared ΔLw figures measured on comparable build-ups rather than comparing millimetres on two datasheets.

Can cork underlay be used over underfloor heating?

Cork is not automatically disqualified, but it is the wrong default. Its thermal resistance is higher than a thin closed-cell foam, so it slows heat transfer into the room, and it is sensitive to moisture. Ask for the thermal resistance figure in writing and check it against the heating system manufacturer's limit before specifying it.