Underlayment for SPC: When You Need It and When You Don't

Underlayment is one of the most misunderstood parts of an SPC job. With laminate you almost always roll out a pad first. With rigid SPC the rules change, and adding the wrong pad can actually weaken the floor. The deciding factors are simple: does your plank already have backing, is the subfloor dry, and what are you trying to improve — comfort, sound or moisture protection?

Many SPC planks already have it built in

SPC is a rigid core, and a lot of it comes with an acoustic backing pre-attached. MILAT SPC carries an IXPE acoustic layer rated up to 21 dB, so the sound pad is already part of the plank. Stacking a second foam pad under a floor that already has backing is where people run into trouble.

Why a thick pad can hurt a rigid floor

Click joints are engineered for a firm, flat base. Put a soft or spongy layer under a rigid plank and the floor gets a little bounce; that flex works the joints every time you step on them and can lead to gaps, peaking or clicks failing. If you add anything, it should be thin and firm — made for rigid SPC, not a squishy laminate underlay.

When a separate layer earns its place

• The plank has no backing at all — then a thin SPC-rated acoustic pad makes sense
• A concrete slab that could pass moisture — a thin poly moisture barrier is cheap insurance
• You want extra footfall-sound reduction in a flat above neighbours
• The subfloor has slight, consistent unevenness a firm pad can even out

Moisture barrier vs acoustic pad

These are two different jobs and people mix them up. An acoustic pad quietens footsteps. A moisture barrier — usually a thin polyethylene sheet — stops vapour rising out of concrete into the floor. Over a slab, a barrier can matter even when a sound pad doesn't. Over a dry timber subfloor upstairs, you often need neither.

Check the product guidance

Because MILAT SPC already includes acoustic backing, the safe default is no extra pad unless the subfloor calls for a moisture barrier. Whatever you add, confirm it's compatible with the click system and within any thickness limit in MILAT's installation guidance — that document is the final word for your specific product.

The subfloor does the job people expect from the pad

Half of the questions about underlayment are really questions about flatness. A pad will not rescue a wavy screed; it just lets the rigid planks rock on softer humps. What a floating SPC floor actually needs underneath is a subfloor flat within the tolerance the manufacturer states, dry, and clean of debris. Time spent grinding a high spot or filling a dip with levelling compound does more for the finished floor than any acoustic layer. If you feel movement underfoot after installation, the answer is almost never a thicker pad; it is a flatter base.

Apartments and upper floors: the impact-sound question

In multi-storey buildings, the neighbour below hears impact sound: heels, chairs, dropped toys travelling through the slab. This is where an acoustic layer earns real money, and it is why MILAT ships SPC with an attached IXPE layer offering sound absorption up to 21 dB. Many apartment buildings and management companies set a minimum impact-sound requirement for hard flooring; check yours before choosing a product, and keep the acoustic data sheet, because approvals committees ask for it. An attached layer has one more advantage here: nobody can accidentally install the floor without it.

Underfloor heating changes the maths

Every layer between the heating pipes and the room is insulation working against you. With underfloor heating, the goal is the thinnest acceptable build-up: SPC with its factory-attached IXPE and nothing extra. Adding a foam pad on top of a heated screed slows response and wastes energy, and some pad materials are not rated for continuous warmth at all. If a moisture barrier is required over a fresh screed, use a thin film rated for heated floors rather than a combined foam product. Always cross-check the flooring's temperature limit against the heating system's design temperature.

The double-padding mistake

The most common site error we hear about is stacking layers: a plank with attached IXPE installed over a separate foam roll the installer had in the van from a laminate job. The floor feels fine for a week. Then the click joints, flexing over too soft a base every time someone walks, begin to creak and eventually open at the ends. Rigid click systems, including licensed Unilin profiles, are engineered around a firm base. One attached layer is the design; two layers is a fault. If extra acoustic performance is genuinely required, solve it with a product rated for rigid floors, not by improvising.

Why attached beats loose-laid, from the factory's side

Manufacturers attach the IXPE layer for reasons beyond convenience. Lamination in the factory controls the bond and thickness across every plank, so the acoustic value on the data sheet is the value in your room, not a variable that depends on which roll the installer bought. It keeps the layer exactly as thick as the click joint tolerates. And it removes a whole category of site errors: wrinkled pads, overlapped seams, missed strips along walls. When the pad is part of the plank, the certified system that was tested is the system that gets installed.