Lay an Engineered Oak Floor With Sika AcouBond in 9 Steps for 40% Less Noise

Sika AcouBond is a bonded acoustic floor system made from SikaLayer fleece and an elastic polyurethane adhesive. The fleece is rolled across the prepared subfloor, then SikaBond AT-Universal or SikaBond-52 is extruded through the AcouBond notched comb trowel in parallel beads. Those beads remain flexible for the life of the installation. That permanent elasticity is the decoupling layer between engineered oak and slab. A rigid adhesive or a missing fleece layer removes the mechanism that gives the system its acoustic value.

Slab flatness controls the beads

Manufacturers commonly give a flatness tolerance of about 3 mm under a 2 metre straightedge for glue-down timber. Existing screeds often miss that limit. A power-floated concrete slab poured during a development can be 5 mm or more out across a room. Hand-applied self-levelling compound can also leave ridges when it has been spread without a pin rake.

Those ridges matter because the comb trowel follows them. If the trowel rides over a high area and then crosses a dip, the adhesive bead may fail to touch the underside of the board through the low section.

The result is mechanical. A board spanning a dip bears on the raised areas either side, then drums when stepped on. The fleece below remains present, yet the timber has become a small sounding surface because the load path is concentrated at high spots.

Before adhesive is opened, check the slab with a 2 metre alloy straightedge and feeler gauges. Any gap above 3 mm needs correction with a latex-modified levelling compound such as Mapei Ultraplan Renovation or Ardex K 15. Mix to the stated water ratio and pour the compound so it can flow properly across the low areas. After that, wait for the manufacturer’s foot-traffic time, commonly 3 to 4 hours, before walking the surface to place the fleece.

Flatness also affects adhesive use. The AcouBond trowel is designed to leave a particular bead height and spacing. On a wavy floor the tool may still be held correctly, while the actual bead contact changes with every hollow and ridge.

Moisture checks before timber enters the sequence

A concrete slab can feel dry at the surface while carrying residual moisture deeper down. The relevant limit is around 75 percent relative humidity measured with an in-situ probe to BS 8203, or roughly 2 percent moisture by the carbide, or CM, method on a sand-cement screed. A surface dampness meter pressed to the floor gives little information about conditions 40 mm below the surface.

If the slab exceeds the limit, the polyurethane bond can be undermined at the interface. The visible failure may arrive months later as patch lifting, long after the floor looked complete. The remedy in that condition is a two-coat epoxy moisture barrier such as Sika Primer MB before the fleece. It seals residual moisture and gives the SikaBond adhesive a stable substrate. On a ground-floor slab lacking a functioning damp-proof membrane, that primer belongs in the build-up.

Engineered oak also has to be managed for moisture. It is more dimensionally stable than solid timber, although a 14 mm or 20 mm plank still settles toward the room’s equilibrium moisture content. Bring the packs into the installation room and leave them flat and banded for at least 48 hours. Open the packs while keeping them stacked. The goal is for timber and room air to reach the same humidity while the boards remain controlled in the stack.

The 9-step sequence

1. Confirm subfloor flatness to 3 mm over 2 m and level areas that fail the check.
2. Test slab moisture with an in-situ probe; if readings are above 75 percent RH, apply Sika Primer MB in two coats.
3. Acclimatise the engineered oak flat in the room for at least 48 hours.
4. Roll SikaLayer fleece across the whole floor, butting the edges and avoiding overlaps. The fleece is the continuous decoupling layer.
5. Load SikaBond AT-Universal into a bulk gun, or use the foil packs, then extrude beads through the AcouBond comb trowel. Keep the trowel angle consistent so bead height stays even.
6. Lay boards into the wet adhesive within the open time, around 40 minutes at 20 degrees C. Press each board into the beads so the polyurethane wets the underside.
7. Keep a 10 to 15 mm expansion gap at walls and fixed upstands. Use spacers instead of judging the gap by eye.
8. Stagger end joints by at least 300 mm from board to board so seams do not line up across adjacent rows.
9. Keep foot traffic off the floor for 24 hours. Leave the expansion gap clear until the floor has settled, then cover it with skirting or scotia.

Sequence matters here. Laying fleece after adhesive work has begun creates breaks in the layer. Filling the perimeter gap immediately can restrain the floor before it has had time to settle.

What the 40 percent claim covers

The headline reduction refers to impact sound transmission: the footfall thud measured in the room below. In testing language it is an improvement in weighted impact level, delta Lw, under ISO 10140. Airborne sound sits outside that claim, so conversations through a floor remain a separate issue. The click heard while standing on the finished oak surface is also a separate in-room acoustic effect.

The figure assumes that the complete system remains intact. That means continuous fleece, correctly spaced adhesive beads, a flat slab, and freedom from rigid bridges. A heavy bookcase screwed through the timber into the slab creates a flanking path through the decoupled layer. The same problem occurs if levelling compound rises between fleece seams and cures as a hard post. Acoustic decoupling depends on the absence of hard connections through the soft layer.

A worked figure shows the scale. If a bare slab transmits footfall at a weighted impact level of 78 dB, a system delivering a delta Lw of about 18 to 20 dB moves the result toward the high 50s. That is the range where building regulations for separating floors in flats typically sit. The acoustic result comes from the installed construction as a whole, with the adhesive name only one part of the chain.

Tools that decide the outcome

Use the specified AcouBond comb trowel. Its notch profile sets the volume, height, and spacing of the adhesive beads. A generic 6 mm tiling trowel lays the wrong amount of polyurethane, which can starve the bond or flood the fleece.

A bulk dispensing gun for SikaBond foil packs reduces strain and helps maintain bead height across a larger room. A hand cartridge running dry mid-row makes consistency harder to hold.

The rest of the kit is plain but important: a 2 metre straightedge, feeler gauges, a moisture probe instead of a surface meter, a pull bar, a tapping block, and solid spacers for the perimeter gap. A tapping block faced with the same oak profile helps close joints without crushing the board edge. A bare mallet against an exposed oak edge leaves marks that remain visible after the adhesive has cured.

The perimeter gap

The unfinished edge tempts people to close the gap too early. Leave the 10 to 15 mm space clear until the floor has settled. When room humidity changes, a restrained glued-down engineered board can cup or tent.

The downstairs result

The structural deck underneath sets the starting condition for the neighbour below. A 150 mm cast concrete slab and a thin timber-joist floor with a plasterboard ceiling behave as different floors before any oak, fleece, or adhesive is added.

The AcouBond delta Lw stacks onto each starting structure differently. A lightweight deck will usually give away more sound than dense concrete, so the same acoustic layer can produce a weaker downstairs result over that construction. Between those two starting structures, the same AcouBond build-up can leave very different noise below.