Sun Shade Sail for Patio or Pool 16.5' Triangle in Multiple Colors Very effective at blocking harmful UV rays. The fabric is a speacially knitted high-density polyethylene fabric that allows air circulation. The temperature below the sail can be lowered by at least 6.5 degrees. The sail resists mildew and mold making it perfect for all seasons. Has a rust proof fixing system made with stainless steel rings.