Derived from recycled glass and free of chemicals and hazardous materials, Glavel foam-glass gravel is a high-performance aggregate used for sub-slab insulation and green roof fill. The fire-, water-, and pest-resistant product provides thermal insulation of R-1.7 per compacted inch and can typically be used in place of both gravel and foam board beneath slabs, according to the manufacturer. The gravel’s closed cell structure will not absorb water, and pieces can be compacted into mounds and slopes up to 45 degrees. The gravel costs $65 per cubic yard plus ground transit.