(Includes Effects of Aging and Settling).
| Insulation type | R-value per inch of thickness |
| Fiber glass blanket or batt | 2.9 to 3.8 (use 3.2) |
| High performance fiber glass blanket or batt | 3.7 to 4.3 (use 3.8) |
| Loose-fill fiber glass | 2.3 to 2.7 (use 2.5) |
| Loose-fill rock wool | 2.7 to 3.0 (use 2.8) |
| Loose-fill cellulose | 3.4 to 3.7 (use 3.5) |
| Perlite or vermiculite | 2.4 to 3.7 (use 2.7) |
| Expanded polystyrene board | 3.6 to 4 (use 3.8) |
| Extruded polystyrene board | 4.5 to 5 (use 4.8) |
| Polyisocyanurate board, unfaced | 5.6 to 6.3 (use 5.8) |
| Polyisocyanurate board, foil-faced | 7 |
| Spray polyurethane foam | 5.6 to 6.3 (use 5.9) |
Use this formula to determine the R-value of your existing insulation:
 |
× | |
= | |
| Thickness (inches) | × | R-value per inch | = | Total R-value |
Use this formula to determine how much insulation you need to add :
| |
- | |
= | |
| Recommended R-value | - | Existing insulation R-value | = | R-value needed |
Do you want to know if you have the space available to add the insulation you need? Then use this formula to determine the approximate thickness you need to add:
| |
÷ | |
= | |
| R-value needed | ÷ | R-value per inch | = | Approximate thickness needed |
However, remember to use the product information on the insulation packaging to determine the actual thickness for any new insulation.
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