Measure what you have, then use the following chart to determine the R-value of your insulation. The higher the R-value, the more energy savings you can expect.
Insulation Type
Approx. R/Inch
Approximate Inches Needed:
Loose Fill Machine-Blown
R-11
R-19
R-22
R-30
R-38
R-49
Fiberglass
R2.25
5
8.5
10
13.5
17
22
Mineral Wool
R3.125
3.5
6
7
12.5
16
Cellulose
R3.7
3
5.5
10.5
Loose Fill Hand-poured
Vermiculite
R2.1
9
14.5
18
23.5
Batts or Blankets
R3.14
Rigid Board
Polystyrene Beadboard
R3.6
6.5
14
Extruded Polystyrene
R4-5.41
3-2
5-3.5
5.5-4
7.5-5.5
9.5-7
(Styrofoam) Urethane
R5.4-6.2
2
8
R4
7.5
9.5
*All numbers are approximate; please review the manufacturer's specifications on any insulation you consider to determine the exact thickness needed for the R-value you desire.