|S.I.||S.I. R||U.S.||U.S. R||U.S. Customary Units are approximate|
|mm||K·m²/W||in||°F·ft²·h/Btu||Also non-SI 1 m²·K/W or 1 °C·m²/W = SI 1 K·m²/W|
|60||1.6||2.4||9.2||Joists are left exposed1,3|
|80||2.2||3.1||12.3||Will have significant thermal bridging1,3|
|110||3.0||4.3||16.9||Design limit of AS 3000 partially surrounded wiring2,3,4|
|150||4.1||5.9||23.0||AS 3000 allows for 150 mm of lower performance insulation|
|160||4.3||6.3||24.6||Values used in cold climates where heating is usual|
Higher values can be supplied, however we recommend that you seek advice from an engineer, electrical contractor or the supply authority before you specify values higher than R 3.0. For new buildings you can arrange to have wiring that is not surrounded by placing the wiring 200 mm or more above the ceiling.
An economical R value can be calculated using a basic return on investment model, however the improvement is reduced  as more insulation is used. There are also practical considerations including the actual performance  of various products.
Thermal insulation is also important for passive solar design and works with other thermal properties such as latent heat and the air tightness required to retain it. Thermal reistance is also required along with "thermal mass" or heat capacity. See window shading calcs with all important source code.
Note that the R value of insulation is not simply added to the existing R value of your building  and that the achieved system R value must work with the air sealed within the building  to regulate temperature.
R-for-R all insulation is NOT created equal! [CIMA]
Cellulose - Naturally Better Insulation