How much insulation should I use?

What are the 'R' values in U.S. literature?

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
100 2.7 3.9 15.3 Covers joists2,3
110 3.0 4.3 16.9 Design limit of AS 3000 partially surrounded wiring2,3,4
120 3.2 4.7 18.4
130 3.5 5.1 20.0
140 3.8 5.5 21.5
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
180 4.9 7.1 27.6
200 5.4 7.9 30.7

Notes: -

  1. Values less than R 2.5 do not properly cover 85 mm joists and thus experience excessive thermal bridging [2][3].
  1. Insulation that sufficiently covers the joists will save a significant amount of energy, furthermore this saving can be readily calculated.
  1. Ceiling joists or battens must not be confused with other roof beams. These are roof members that are intended solely for ceiling fixing. These members are not capable of holding persons weight therefore covering them with insulation poses no risk.
  1. Though AS 3000 allows for 150 mm of insulation, it is an equivalent of 150 mm of fibreglass insulation which was taken to be R 3.0. Exceeding this value may pose a risk with any partially surrounded circuit that is fully utilised.

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 [1] as more insulation is used. There are also practical considerations including the actual performance [2][3] 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 [2][3] and that the achieved system R value must work with the air sealed within the building [4][5] to regulate temperature.

R-for-R all insulation is NOT created equal! [CIMA]

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