This is a paper-backed reinforced foil material that is typically used in joisted floor applications to reflect the radiant waves upward towards the subfloor. It comes in a roll that is 50” wide and you use a sharp hacksaw or band saw to cut it into 3 equal sections of 16 2/3 inches (assuming that your floor joists are 16” on center). This product is then stapled to the inner side of each joist about 2” down from the tubing and plates. Other insulation is then installed below this.
It is common to order about 10% more than the square footage it will be installed in to account for waste. Order in increments of 500 square feet.