Glossary: Flooring Underlayment
Flooring underlayment is a thin protective barrier installed between the subfloor (concrete or wood) and the finished floor covering (laminate, vinyl planks, hardwood, or carpet). It acts as a moisture vapor retarder, provides sound attenuation, insulates thermally, and cushions minor subfloor imperfections.
Damp-Proofing and Material Types
Depending on the finished floor covering and subfloor conditions, different underlayments are specified:
- Standard Foam (Polyethylene): Standard, affordable cushion layer for laminate or LVP over plywood subfloors. Typically comes in rolls covering 100 sq ft (or 10 sq m in metric markets).
- Vapor Barrier (6-mil Polyethylene): Mandatory over concrete subfloors to prevent ground moisture from rising and warping wood or delaminating adhesives, in accordance with ASTM F2170 relative humidity testing.
- Cork & Rubber: Premium underlayment options that offer high Sound Transmission Class (STC) ratings, ideal for multi-family residential zoning requirements.
- Carpet Padding (Polyurethane Foam): Thick, elastic cushion layer that absorbs impact load and extends carpet wear life.