When an engineered hardwood floor is installed using a glue-down method, a full, consistent transfer of adhesive to the subfloor is required for a solid, quiet installation. A hollow sound suggests that there is a void between the flooring and the subfloor. This can happen if the subfloor was not properly leveled or flattened according to NWFA guidelines prior to installation. If there is a dip in the subfloor and the installer does not use an adequate amount of troweled adhesive, the wood will span the void instead of being supported, creating an air pocket. Over time, the adhesive can also become brittle or fail to bond if the subfloor was contaminated with dust, debris, or moisture during the installation process. In some instances, these hollow sounds are a sign of "debonding," where the adhesive releases from either the subfloor or the wood backing. This is often linked to high MVER (Moisture Vapor Emission Rate) levels in concrete slabs. If moisture from the slab attacks the adhesive, the bond strength degrades, and the floor may begin to sound loose. If you detect hollow spots, it is important to have a professional test for both slab moisture and adhesive integrity. Ignoring these hollow sounds can lead to more serious issues like floor movement, excessive gapping, or physical damage to the tongues and grooves of the boards due to the lack of support underneath.
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