NCC 2022 Volume One - Building Code of Australia Class 2 to 9 buildings
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39

Specification 39 Sub-floor and soil thermal performance

Specification 39 Sub-floor and soil thermal performance

This Specification describes the thermal performance of sub-floor spaces and soil in direct contact with a floor for the purposes of calculating the Total R-Value of a floor.

To detail the thermal performance achieved by sub-floor spaces and soil in direct contact with a floor.

(1) Table S39C2a details the R-Values considered to be achieved by enclosed sub-floor spaces that are—

  1. mechanically ventilated by not more than 1.5 air changes per hour; or
  2. provided with not more than 150% of the aggregate sub-floor ventilation area required by Part F1 and are not mechanically ventilated.

(2) Table S39C2b details the R-Values considered to be achieved by the soil for floors that are in direct contact with the ground.

Table S39C2a R-Value of sub-floor spaces
Ratio of floor area (m2) to floor perimeter (m) Sub-floor space R-Value
1.0 0.10
1.5 0.15
2.0 0.20
2.5 0.25
3.0 0.30
3.5 0.35
4.0 0.40
4.5 0.45
5.0 0.50
5.5 0.55
6.0 0.60
6.5 0.65
7.0 0.70
Table Notes

Where the ratio of floor area to floor perimeter is between the values stated, interpolation may be used to determine the sub-floor space R-Values.

Table S39C2b R-Value of soil in contact with a floor
Ratio of floor area (m2)  to floor perimeter (m) Wall thickness of 50 mm Wall thickness of 100 mm Wall thickness of 150 mm Wall thickness of 200 mm Wall thickness of 250 mm Wall thickness of 300 mm
1.0 0.4 0.5 0.5 0.6 0.7 0.8
1.5 0.6 0.7 0.7 0.8 0.9 1.0
2.0 0.7 0.8 0.9 1.0 1.1 1.3
2.5 0.9 1.0 1.1 1.2 1.3 1.5
3.0 1.0 1.2 1.3 1.4 1.5 1.7
3.5 1.2 1.3 1.5 1.6 1.7 1.9
4.0 1.3 1.5 1.6 1.7 1.9 2.2
4.5 1.5 1.7 1.8 1.9 2.1 2.4
5.0 1.6 1.8 2.0 2.1 2.3 2.6
5.5 1.8 2.0 2.1 2.2 2.4 2.8
6.0 1.9 2.1 2.3 2.4 2.6 2.9
6.5 2.0 2.3 2.4 2.6 2.8 3.1
7.0 2.2 2.4 2.6 2.7 3.0 3.3
Table Notes
  1. Where a wall thickness or ratio of floor area to floor perimeter is between the values stated, interpolation may be used to determine the soil R-Value.
  2. Wall thickness means the thickness of the envelope wall that sits on or around the slab.

To provide the R-Values considered to be achieved by sub-floor spaces and soil in direct contact with a floor.

The R-Values provided are intended to assist in determining whether and how much additional insulation is required to achieve the minimum Total R-Values in Table J4D7. This is provided as an alternative to carrying out a more detailed calculation using Section 3.5 of CIBSE Guide A.