The invention includes a method of making and using a flowable thermal backfill. The flowable thermal backfill has a low thermal resistivity, useful for filling trenches containing electrical cables and other lines that generate heat. The method includes process steps of blending ingredients of stone, sand and fly ash, making a flowable thermal fill by mixing the blended stone, sand and fly ash with Portland cement and water, and pouring the flowable thermal fill in an excavation containing heat generating lines, immediately after mixing the flowable thermal backfill.