Flute Fill Insulation for Metal Decks InsulFoam FL (Flute-Fill) is a custom-made, engineered insulation consisting of a superior closed-cell, lightweight expanded polystyrene (EPS). InsulFoam FL is cut from the same high-quality EPS as our flat Insulfoam roof products and, upon request, will be manufactured to meet or exceed the requirements of ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. InsulFoam FL offers a long-term, stable R-Value and has excellent dimensional stability, compressive strength and water resistance properties.