Typical high tunnel covering is greenhouse grade 6 mil polyethylene rated for 4-6 years. Using two layers, separated by air blown between the layers, reduces heat loss during cold season production and provides stability under windy conditions, reducing damage to the plastic. Solid plastic “spacers” are available to separate two layers of plastic in locations without electricity. Some greenhouse plastics have additives to enhance durability and performance. UV stabilizers are essential to slow degradation of plastic. Anti-fog and anti-drip surfactants make water condense and run down to the sides of the structure, rather than bead and drop on the plants below. IR radiation-blocking additives reduce heat loss at night. UV-absorbing films have the potential to suppress certain insects and diseases. In summer, plastic may be covered with shade cloth to reduce light intensity and temperature. Shade cloth is rated by the percent of light blocked. Whitewash is also used by some growers to help keep tunnels cool in summer.