The best underlay for a floating floor is the one that addresses the room’s actual problem: minor unevenness, footfall noise, cold from below or very limited build-up height. Do not choose solely by thickness or by the cheapest roll. A damp substrate, a door with no clearance or an uneven slab can undermine a new floor before the room is furnished again.
A thin layer may improve comfort, but it cannot replace a properly designed thermal upgrade or repair. The Portuguese Law no. 52/2018 published in the Official Gazette amended the building-energy-performance regime; use it as context rather than as a specification for selecting an underlay.
Contents
- What does underlay actually do?
- Cork, foam and rubber: where each one makes sense
- Moisture comes before the mat
- How to decide without doors scraping or untidy skirtings
- Mistakes that make a simple floor expensive
What does underlay actually do?
Underlay is the layer placed between the substrate and a floating floor finish. It can accommodate very slight imperfections, reduce impact sound transferred into the slab and improve the feeling underfoot. It does not repair loose screed, hollow tiles, an active crack or a level difference that is plainly visible.
It helps to separate three effects that product brochures often blend together:
- Walking comfort: this depends on density, thickness, floor rigidity and the quality of the locking system.
- Noise within the room and to the neighbour below: this needs a solution compatible with the floor, slab and perimeter details.
- Cold coming from below: this may justify thermal insulation, but a thin mat beneath laminate rarely replaces a properly designed intervention over a basement, garage or ground.
Before comparing rolls, establish whether levelling must be dealt with first. See how to choose between self-levelling compound and manual levelling before new flooring; it is a substrate decision, not a finish decision.
Cork, foam and rubber: where each one makes sense
Cork often appeals to homeowners looking for a natural-based option with some thermal and acoustic comfort. It can work well where the substrate is dry, reasonably flat and the full manufacturer-approved system allows that layer. It should never be used to conceal rising damp or an unresolved leak.
Technical foams, including polyethylene options and denser foam composites, are common in fast refurbishments because they come in rolls, cut easily and add little height. The difference between a lightweight foam and a denser technical mat matters: the former may compress more under load and feel less stable in heavily used areas.
Rubber or elastomeric compounds are more demanding choices where acoustic performance and resistance to compression are key priorities. They can suit living rooms, bedrooms or home offices where the aim is to reduce footfall noise, provided their thickness works with doors, thresholds and skirtings.
The 2021 regulation for the second phase of Portugal’s More Sustainable Buildings support programme included floor insulation among eligible measures. For any refurbishment, ask for the technical data sheet, declaration of performance and written confirmation that the layer is compatible with the chosen finish.
| Material | Usually works best when | Check before installation |
|---|---|---|
| Cork | You want modest thermal comfort and a natural-based solution over a dry, stable substrate. | Confirm density, compression behaviour and compatibility with any vapour-control layer. |
| Technical foam | The substrate is already well prepared and a thin solution is needed for compatible laminate or click vinyl. | Do not use it to compensate for unevenness or assume every foam reduces sound transmission downstairs. |
| Rubber or elastomeric mat | Footfall noise and stability under frequent use are key priorities in the room. | Check final build-up height, edge details, underfloor-heating compatibility and manufacturer requirements. |
Moisture comes before the mat
On a ground floor, above a garage, next to a balcony or in a home with a history of leaks, the first question is not “which underlay?”. It is “is the substrate dry, and has the source of water been removed?”. A vapour-control layer may be necessary in certain systems, but it does not cure a substrate that is still receiving water.
Look for straightforward warning signs: recent staining at walls, darkened tile joints, persistent odour, swollen skirtings, salt deposits or moisture readings that do not settle. If there has been a leak, penetration or extensive wet work during the refurbishment, allow drying time and follow the floor and adhesive manufacturer’s stated conditions where adhesive is involved.
What lies below also matters. Over a cold slab above a garage, a very thin mat will have limited effect on overall thermal performance. In its 2023 overview of energy and water-efficiency support, ADENE listed floor insulation among supported interventions. In practice, the assembly must be designed for the particular situation and available heights.
How to decide without doors scraping or untidy skirtings
Measure the whole build-up before ordering: levelling layer, any membrane, underlay, finish and transition profile. An extra five millimetres throughout a home can mean trimming doors, altering thresholds, raising architraves or remaking the junction to the kitchen and bathroom.
- Check flatness. Use a long straightedge and mark high areas, hollows and degraded joints.
- Choose the finish. Underlay follows the choice of laminate, engineered timber or click vinyl; it does not lead it.
- Test door clearance. Open each door over a complete sample, including the mat.
- Plan transitions. Set out profiles between rooms, the junction with existing tile and movement joints.
- Read the full system requirements. Some floors already have an attached backing and do not accept a second underlay.
Where the level difference is more substantial, deal with the substrate first. Our guide to dry and wet screed in a renovation explains why an underlay layer cannot replace proper levelling.
Mistakes that make a simple floor expensive
The most common mistake is installing quickly over a substrate that is not ready. Other frequent problems include fixing the floating floor tightly against walls, omitting the perimeter gap, leaving mortar debris beneath the mat, layering products without manufacturer approval and breaking the vapour-control layer at service penetrations.
Do not select a very soft mat to disguise defects either. The floor may move at the joints, open gaps or show marks beneath heavy furniture. In kitchens, entrances and areas with chairs, suitability for load and the chosen finish usually matter more than a generic “extra comfort” claim.
Call a professional where moisture persists, the substrate has cracks or hollow areas, a large level increase is needed, underfloor heating is involved, transitions are complex or the stability of an old timber floor is uncertain. Before appointing a contractor, include substrate preparation, the exact material, edge details and responsibility for compatibility in the scope; compare those items with our guide to renovation quotes in 2026.
Key takeaways
- Prepare and dry the substrate before choosing any underlay.
- Compare density, compression resistance and compatibility, not thickness alone.
- Measure doors, thresholds and transitions with every planned layer included.
- Use underlay for its stated purpose; do not use it to conceal a defective base.
If you are replacing flooring as part of a refurbishment, speak with Obra Lisboa about reviewing the substrate, build-up heights and edge details before the order is finalised. A clearly defined solution reduces corrections once the finish has reached site.
Sources and further reading
- Portuguese Law no. 52/2018 published in the Official Gazette
- 2021 regulation for the second phase of Portugal’s More Sustainable Buildings support programme
- 2023 overview of energy and water-efficiency support
Editorial image; it does not depict an Obra Lisboa project.
Obra Lisboa
