Wood fibre underlay board 20 mm (thermal and impact sound insulation)

The boards go onto a prepared subfloor that is flat, solid and dry. They are butted tightly together without overlapping, laid in rows with staggered joints, much like laminate. An expansion gap of roughly 5 to 10 mm is left at the walls. A foam edge strip goes into this gap. It separates the floor from the wall and stops footsteps travelling into the masonry.

The boards are usually neither glued nor fixed down. Only use spot gluing or double-sided tape if the manufacturer specifically calls for it, for example under heavier loads or in a particular system build-up. Nothing needs to dry or cure, so you can carry on with the next layer straight away.

Why does it help with noise? A softer, springy layer between the floor covering and the slab absorbs part of the impact from walking. That is why this kind of layer is called impact sound insulation. At 20 mm, the thermal benefit is more of a bonus. The floor feels less cold underfoot, but the board does not replace proper floor insulation over an unheated cellar or a ground-bearing slab.

The manufacturer's instructions determine whether the floor covering can go directly on the board. Some laminates and vinyls allow it. Others need a load-spreading layer on the board first, typically OSB or gypsum fibre flooring boards. This layer matters most under heavy furniture, a run of kitchen units or thin vinyl, which would otherwise show every dent in the softer base. With click vinyl and laminate, also check that the flooring manufacturer accepts an underlay this thick. Many systems are designed only for thin underlays, and a thicker one can invalidate the warranty.

• Expanded polystyrene (EPS): Good thermal insulation, mainly used under sand-cement or anhydrite screeds. Under a dry floating floor without a load-spreading layer it dents at pressure points and is less effective at reducing impact sound.
• Mineral wool: Very good at absorbing sound, but you cannot walk on it directly. It belongs between battens and joists, not straight under the floor covering.
• Cork and thin underlays: Springy and thin, useful when you need to save height. They usually do not do enough for a noticeable gain in acoustics or warmth.
• Combined with an acoustic membrane: Between flats, a thin acoustic membrane is sometimes added to improve sound insulation further

With underfloor heating, do not lay wood fibre underlay board above the heated screed unless the manufacturer explicitly allows it. The insulation would hold back the heat that is meant to reach the room.

The subfloor causes the most work and the most trouble. A flat, firm and dry concrete slab or screed means quick laying. If there are bumps, hollows or old adhesive residue, it has to be cleaned and levelled first. Small irregularities can be fixed with self-levelling compound on a primed surface, larger height differences with a dry levelling screed (loose-fill levelling granules). The compound needs time to dry, though, so part of the advantage of dry laying is lost.

The job also gets more involved with:

• A damp subfloor, such as fresh screed or a floor on the ground, where the manufacturer may require a polythene damp-proof membrane (DPM) or vapour control layer
• Lots of corners, alcoves, thresholds and radiator pipes that need accurate cuts
• Doors that have to be trimmed and threshold transitions to deal with, because the floor level rises by the thickness of the board plus the new covering
• Clearing the room and removing the old floor and skirting boards

A large, regular room with a ready subfloor, on the other hand, is simple and goes quickly.

If your subfloor is firm, dry and flat, you can lay the boards yourself. If you are unsure about moisture or flatness, have a floor fitter check the subfloor first.

1. Bring the boards into the room and leave them there to acclimatise for as long as the manufacturer advises, often a day or more, so they adjust to the temperature and humidity.
2. Remove the old skirting, vacuum the subfloor and scrape off adhesive and mortar residue.
3. Check flatness with a long straightedge or spirit level and level any unevenness beforehand.
4. Lay polythene sheeting with overlaps and turned up the walls, if the manufacturer requires it.
5. Stick the edge expansion strip around the perimeter of the room.
6. Start in a corner and lay the boards tightly butted in rows, with staggered joints.
7. Cut the edge pieces with a sharp knife or saw so that the expansion gap remains at the wall.
8. Walk over the whole area and replace any pieces that move or sit higher.
9. Continue with the load-spreading layer or the floor covering as the manufacturer instructs.

In practice, a fault under the board often only appears once the floor is finished, when putting it right is far more work than it needed to be.

• Grit and dust under the boards: They create high spots, and the floor flexes and creaks there
• No expansion gap: The floor has no room to move, lifts at walls and corners, and footsteps carry into the walls
• Too soft a build-up under a heavy covering: Without a load-spreading layer the insulation gradually compresses and the covering cracks at the joints
• A damp subfloor without protection: Wood fibre absorbs moisture, warps and loses its properties
• Joints in line: Without staggered joints you get long lines where the base gives less support
• Forgetting the height: Doors then catch, and you end up with a step between rooms

When renewing a floor in a flat, check your lease and the building's rules, especially on hours for noisy work and on hard flooring, which many leases restrict, and speak to the building management or owners' association before larger jobs. If you also intend to alter the structure of the floor itself, that counts as structural work. Check with your local building authority what approval it needs under local building regulations.

Before laying the floor covering, walk over the whole area and check:

• That no board flexes or moves underfoot
• That the boards butt tightly with no larger gaps between them
• That joints in neighbouring rows are staggered
• That the surface is flat with no steps between boards
• That there is an expansion gap with an edge strip all the way round
• That the membrane, where specified, is intact and turned up the walls
• That door thresholds and the floor height against the door frames are sorted out