The Bandar Sunway Ground-Floor Master Bedroom Got Carpet Laid Directly on the Screed. Nobody Tested for Moisture First. Six Months Later, the Carpet Smelled Musty and Had Mould Patches Underneath.

The master bedroom in Lai Ching's Bandar Sunway landed house was on the ground floor, directly above the concrete slab. She had always wanted carpet in the bedroom — warm underfoot, quieter than timber or tile, and more comfortable for her children who used the room as a play area. Her contractor laid a mid-grade commercial-grade carpet directly onto the concrete screed, with a foam underlay beneath the carpet for cushioning. The installation was clean and the carpet looked excellent.

By month four, the room had developed a faint musty smell that Lai Ching initially attributed to the carpet fibres off-gassing after installation. By month six, the smell was persistent and noticeable to anyone entering the room. When the carpet was lifted in one corner to investigate, the foam underlay had visible dark patches of mould growth across its full surface where it had been in contact with the screed. The underside of the carpet itself was damp to the touch.

The cause: the ground-floor concrete slab had rising damp — capillary moisture from the soil below the slab migrating upward through the concrete. This is a common condition in older landed properties and even in some newer ones where the damp-proof membrane beneath the slab has failed or was not installed. The concrete screed above the slab was carrying this moisture continuously. When a carpet and foam underlay were laid on top of the screed, they trapped the moisture against the foam surface — a warm, dark, damp environment ideal for mould growth.

Remediation: full carpet removal, foam disposal, cleaning and treating the screed surface for mould contamination, applying a penetrating damp-proof sealant to the screed, installing a physical vapour barrier (polyethylene sheet) on top of the sealed screed, and re-laying new carpet with a new breathable underlay. Total cost: RM7,800 — more than twice the original carpet installation cost. A moisture test on the screed before installation would have identified the rising damp condition and allowed a DPM to be included from the start at a cost of approximately RM800.

Why Ground-Floor Slabs Need Moisture Testing Before Carpet Installation

Concrete is not a moisture barrier. A ground-floor concrete slab and its screed finishing layer are in continuous contact with the ground below, and in the absence of an effective damp-proof membrane beneath the slab, moisture will migrate upward through the concrete by capillary action. The rate varies with soil moisture content and season, but the process is ongoing.

Most flooring coverings — vinyl, timber, carpet — are affected by sustained moisture at their base. Carpet and foam underlay are particularly susceptible because they absorb and retain moisture without any visible sign until mould has already established. The visible and olfactory symptoms appear months after the mould has been growing.

Before laying any soft or moisture-sensitive flooring on a ground-floor slab, a moisture reading should be taken of the screed surface — either with a calibrated moisture meter or, more accurately, with a plastic sheet taped to the floor and left for 24-48 hours. If the screed reads above acceptable moisture levels for the flooring being installed, a damp-proof membrane must be installed before the floor covering. This adds cost upfront and avoids the much larger cost of remediation after mould damage.

Renovating in Bandar Sunway, Subang Jaya, or USJ?

We test ground-floor slab moisture before any floor covering goes down — because carpet over rising damp is not a renovation, it's a delay.

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Studio D3 — Interior Design & Renovation, Kuala Lumpur and Selangor.