Types Of Underlay

Apart from their fire retardance and ability to self-support, underlays are classified according to their absorbency and permeability.

Permeable and Absorbent:

• Kraft paper-based — bitumen impregnated paper
• Synthetic — 2 or 3 layers, using permeable synthetic film strengthened by sandwiched non-woven fabric.

Permeable, non-absorbent:

• Synthetic permeable non-absorbent underlays are mainly used as wall wrap inside a drained cavity or with direct fixed absorbent claddings.

Non-Permeable, non-absorbent:

• foil — Reflective aluminium foil over a flexible substrate. These can be either double-sided or white-faced.