The most common or found in low-slope membrane roof systems in North America:

RIGID ROOF INSULATION

Cellular glass

Expanded Polystyrene (EPS)

Extruded Polystyrene (XPS)

Fibrous glass

Gypsum board

Mineral fiber

Perlite board

Polyiso

Wood fiberboard

Composite board

 Reduce Thermal Bridging

Mechanically fastners are good conduits for thermal bridging. With two or more layers of insulation, the potential for thermal bridging at fastners in the first layer is mitigated.

 Reduce Thermal Loss

A minimum of two layers reduces the thermal loss that occurs through the joints between single layer insulation boards.

Increase Thermal Performance

With double layer insulation roof systems, it is preferable that only the bottom layer of insulation be mechanically fastened that are appropriate for mechanical attachment.