Thinnest R38 Insulation . The fiberglass comes in rolls, also known as batts, which vary in size. Also know, what is the best insulation to use in a tiny house? Johns Manville R38 Fiberglass Batt Insulation with Sound Barrier (16 from www.lowes.com The fiberglass comes in rolls, also known as batts, which vary in size. The higher the value of r, the greater the insulation will be at slowing down heat transfer. R38 insulation thickness for different materials.