NCC 2016 Volume One
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Specification J1.3 Roof and Ceiling Construction

Specification J1.3 Roof and Ceiling Construction

1. Scope

This Specification describes the thermal performance of some common forms of roof and ceiling construction.

Intent

To clarify that outlines the thermal performance of some of the more common forms of roof and ceiling construction.

2. Construction Deemed-to-Satisfy

Figure 2 details the R-Values considered to be achieved by some common forms of roof and ceiling construction.

Figure 2 TYPICAL R-VALUES FOR ROOF AND CEILING CONSTRUCTION

Roof construction description Item Item description R-Value Unventilated R-Value Ventilated
Up Down Up Down
(a) Roof 15° to 45° pitch
– Horizontal ceiling
– Metal cladding
v1_SpecJ13a_2007.svg
 1. Outdoor air film (7 m/s) 0.04 0.04 0.04 0.04
2. Metal cladding 0.00 0.00 0.00 0.00
3. Roof airspace (non-reflective) 0.18 0.28 0.00 0.46
4. Plasterboard, gypsum (10 mm, 880 kg/m3) 0.06 0.06 0.06 0.06
5. Indoor air film (still air) 0.11 0.16 0.11 0.16
Total R-Value 0.39 0.54 0.21 0.72

(b) Roof 15° to 45° pitch
– Horizontal ceiling
– Clay tiles 19 mm

v1_SpecJ13b_2007.svg

 1. Outdoor air film (7 m/s) 0.04 0.04 0.04 0.04
2. Roof tile, clay or concrete (1922 kg/m3) 0.02 0.02 0.02 0.02
3. Roof airspace (non-reflective) 0.18 0.28 0.00 0.46
4. Plasterboard, gypsum (10 mm, 880 kg/m3) 0.06 0.06 0.06 0.06
5. Indoor air film (still air) 0.11 0.16 0.11 0.16
Total R-Value 0.41 0.56 0.23 0.74

Figure 2 TYPICAL R-VALUES FOR ROOF AND CEILING CONSTRUCTION

Roof construction description Item Item description R-Value Unventilated
Up Down
(c) Cathedral ceiling 15° to 45° pitch
– 10 mm plaster on top of rafters
– Metal external cladding
v1_SpecJ13c_2007.svg
 1. Outdoor air film (7 m/s) 0.04 0.04
2. Metal cladding 0.00 0.00
3. Roof airspace (30 mm to 100 mm, non-reflective) 0.15 0.18
4. Plasterboard, gypsum (10 mm, 880 kg/m3) 0.06 0.06
5. Indoor air film (still air) 0.11 0.14
Total R-Value 0.36 0.42
(d) Cathedral ceiling 15° to 45° pitch
– 10 mm plaster on top of rafters
– Tiles external cladding
v1_SpecJ13d_2007.svg
 1. Outdoor air film (7 m/s) 0.04 0.04
2. Roof tile, clay or concrete (1922 kg/m3) 0.02 0.02
3. Roof airspace (30 mm to 100 mm, non-reflective) 0.15 0.18
4. Plasterboard, gypsum (10 mm, 880 kg/m3) 0.06 0.06
5. Indoor air film (still air) 0.11 0.14
Total R-Value 0.38 0.44
(e) Skillion roof less than 5° pitch
– 10 mm plaster below rafters
– Metal external cladding
v1_SpecJ13e_2007.svg
 1. Outdoor air film (7 m/s) 0.04 0.04
2. Metal cladding 0.00 0.00
3. Roof airspace (100 mm to 300 mm, non-reflective) 0.15 0.22
4. Plasterboard, gypsum (10 mm, 880 kg/m3) 0.06 0.06
5. Indoor air film (still air) 0.11 0.16
Total R-Value 0.36 0.48
(f) Skillion roof 5° to 15° pitch
– 10 mm plaster on top of rafters
– Metal external cladding
v1_SpecJ13f_2007.svg
 1. Outdoor air film (7 m/s) 0.04 0.04
2. Metal cladding 0.00 0.00
3. Roof airspace (30 mm to 100 mm non-reflective) 0.15 0.21
4. Plasterboard, gypsum (10 mm, 880 kg/m3) 0.06 0.06
5. Indoor air film (still air) 0.11 0.16
Total R-Value 0.36 0.47
(g) 100 mm solid concrete roof to 5°
– 10 mm plaster, suspended ceiling
– Applied external waterproof membrane
v1_SpecJ13g_2007.svg
 1. Outdoor air film (7 m/s) 0.04 0.04
2. Waterproof membrane, rubber synthetic (4 mm, 961 kg/m3) 0.03 0.03
3. Solid concrete, (100 mm, 2400 kg/m3) 0.07 0.07
4. Ceiling airspace (100 mm to 300 mm, non-reflective) 0.15 0.22
5. Plasterboard, gypsum (10 mm, 880 kg/m3) 0.06 0.06
6. Indoor air film (still air) 0.11 0.16
Total R-Value 0.46 0.58

Notes:

  1. The R-Value of an item, other than an airspace, air film or air cavity, may be increased in proportion to the increased thickness of the item.
  2. The Total R-Value of a form of construction may be increased by the amount that the R-Value of an individual item is increased.
  3. Where an airspace is filled, the R-Value listed for the airspace must be deducted from the Total R-Value of the roof construction.
  4. For information on a roof space that is considered to be ventilated, see Specification J1.2, Clause 2(d).
Intent

To detail the Total R-Values considered to be achieved by common forms of roof construction.

Typical construction

The Total R-Value of the basic roof and ceiling has been determined by adding together the R-Values of the outdoor air film, roof cladding, roof airspace, ceiling sheet lining and internal air film.

The total R-Value of the roof and ceiling materials may need to be adjusted if other building elements, such as sarking, are to also be installed.

Insulation can be installed in the roof, the ceiling, or a combination of both, provided the required thermal performance is achieved and other aspects of the building’s integrity are not compromised. It should be noted that the thermal performance of the roof may vary depending on the position of the insulation, the climatic conditions, the design of the building and the way in which it is operated. For example, although not recognised in the values, insulation installed under the roof, rather than on the ceiling, in a building with a large roof space in a cold climate, or when a room is being air-conditioned, may be less effective because of the additional volume of roof airspace that would need to be heated or cooled.

For a material that is not listed as an item in , other than air, the R-Value may be determined by dividing the thickness of the item in metres by the thermal conductivity in W/m.K (typical values are described in ).

Irrespective of the framing material used, the minimum added R-Value specified in is considered to include an allowance for the effect of thermal bridging created by framing members.

There are a number of different insulation products that may be used to achieve the minimum added R-Value. Care should be taken to ensure that the choice made is appropriate for the construction and climate conditions. For instance, in some climate zones, an impermeable insulation sheet needs to be installed with due consideration of condensation and associated interaction with adjoining building materials.