R-values are usually expressed per inch of thickness, so you can multiply the R-value by the thickness of the insulation to get the total R-value of the layer. Fiberglass insulation is made of ...

This means you can have a combined R-value ranging from R-23 to R-31 with only 5-1/2 inches of combined fiberglass and foam insulation. To effectively ventilate your roof, create a 1-inch airspace ...