Glossary
Polyfoam (polyurethane foam)
The most common mattress foam, made from polyurethane. Quality varies enormously and is best judged by density: low-density polyfoam (under ~1.5 lb/ft³) softens and sags quickly, while high-density polyfoam (1.8 lb/ft³ and up) holds its shape for years. Most 'mystery foam' in cheap mattresses is low-density polyfoam.
Related: foam density · memory foam