NCC 2019 Volume One Amendment 1
Specification J1.5a Calculation of U-Value and solar admittance

Specification J1.5a Calculation of U-Value and solar admittance

1. Scope

This specification describes the methods of calculating the U-Value and solar admittance of a wall-glazing construction.
Intent

This Specification describes the two calculation methods for determining if wall-glazing construction complies with the U-Value and solar admittance requirements in J1.5.

Wall-glazing calculations

Two Deemed-to-Satisfy compliance pathways for both Total System U-Value and solar admittance are detailed in Specification J1.5 a:

  • Method 1: Each aspect of wall-glazing construction is required to meet the applicable Total System U-Value and solar admittance for the building classification and climate zone.
  • Method 2: The wall-glazing construction facing multiple aspects are assessed together to determine if it collectively achieves the applicable Total System U-Value and solar admittance. Method 2 effectively allows for trading of thermal performance values between different aspects and is intended to make it easier for a building to have the same glazing system in all directions.
Glazing

In applying these methods, the Total System U-Value and Total System SHGC of glazing must account for the combined effect of the glass and frame. The measurement of these Total System U-Values and Total System SHGCs is specified in the Technical Protocols and Procedures Manual for Energy Rating of Fenestration Products of the Australian Fenestration Rating Council (AFRC).

Various assessors using AFRC procedures might refer to their published performance values by slightly different terms (including "U-factor" or "Uw" for Total System U-Value or "SHGC" for Total System SHGC). Such values can be used under Specification J1.5 a provided they measure combined glass and frame performance according to AFRC requirements.

Shading

The presence of shading projections and devices will reduce the level of thermal performance that is required of glazing, as detailed in the calculation methods for solar admittance. However, to be effective, shading projections and devices must restrict a significant proportion of solar radiation.

External shading devices, such as shutters, blinds, vertical or horizontal building screens are required to be capable of restricting the amount of summer solar radiation that reaches the glazing by at least 80%. This is the sum of the amount of hour-by-hour summer (December, January, February) solar radiation that does not reach the glazing as a percentage of what would have reached the glazing if the shading device was not fitted.

The amount of summer solar radiation that reaches the window may be measured cumulatively over the three month period of December to February if a fixed device is used, alternatively if the device adjusts automatically in response to the sun, the worst case scenario during the period of December to February can be used.

The 80% figure acknowledges that while a device may be capable of providing 100% shade during summer, some leakage of solar radiation may occur at the sides of the device. For example, although adjustable blinds are capable of providing 100% shade when they are fully closed or lowered, it is accepted that they may allow some summer solar radiation to reach the glazing at the sides of the blinds. Similarly, while a horizontal building screen may have slats which have been designed to provide 100% shade in summer, it is accepted that there may be some leakage of solar radiation at the sides of the slats.

A degree of judgement is required to determine whether the amount of summer solar radiation that reaches the glazing at the sides of a device exceeds that permitted. Generally, a close fitting blind should sufficiently restrict the amount of summer solar radiation that reaches the glazing at the sides of the device. A horizontal building screen that extends either side of the glazing by the same projection distance (P) should also restrict a sufficient amount of solar radiation at the sides of the slats.

Adjustable shading devices can only be recognised in the calculations if they are automatically operated. This is based on the premise that occupants are less likely to operate the devices as those in the best position to operate the devices are less inclined to do so because they are not paying the energy bills.

For complying with the shading requirement, note that the shading projection for walls is measured from the wall face whereas for glazing the projection is measured from the glass face.

Gutters can only be considered as providing shading if attached to a building projection such as a veranda, fixed canopy, eaves, shading hood, balcony or the like. On their own they are likely to be well above the head of the window and so not likely to produce any significant shading.

Wall construction

The figure below provides examples of typical insulation locations in various types of wall construction. The Total R-Value required is achieved by adding the material R-Value of the basic wall and the material R-Value of any additional insulation incorporated. The Total R-Value of the typical wall construction has been produced by adding together the material R-Values for outdoor air film, wall cladding, wall airspace, internal lining and internal air film.

Note it should not be assumed that these figures are representative of all construction scenarios. For example the spacing of framing members, the number of windows or the specific type of frame could all effect the actual Total R-Value by creating thermal bridging between elements or by compressing insulation. If following a Deemed-to-Satisfy compliance pathway, Total R-Value must be calculated using the methods prescribed in AS/NZS 4859.2 to properly account for these effects.

The most common forms of construction for low-rise buildings are represented. It has not been possible to cover other forms of construction, particularly those used for high-rise construction, because of the wide range and the greater influence of winds, cyclones and earthquakes on the elements of the building. The Total R-Value of other forms of construction can be determined by adding the individual R-Values together.

For a material that is not listed as an item below, 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 Specification J1.5 a).

Reflective insulation that has just one reflective surface is considered to achieve the R-Values when used in conjunction with the Total R-Value of the common wall construction stated in the figure below. The actual R-Value added by reflective insulation should be determined for each product in accordance with the standards prescribed in the BCA, which take into consideration factors such as the number of adjacent airspaces, dimensions of the adjacent airspace, whether the space is ventilated and the presence of an anti-glare coating.

The width for any reflective airspaces adjacent to reflective insulation will not override other requirements such as minimum cavity requirements for masonry waterproofing.

Where a diagram shows reflective insulation or other insulation, these are indicative only. In some climates and using certain materials, neither may be necessary. In other cases, reflective insulation or other insulation may be provided separately or in combination to give the required R-Value.

A minimum thickness of 70 mm is stated for framing. In some cases, the frame thickness may need to be increased to avoid compressing the bulk insulation and thus reducing its R-Value.

External wall construction description

Item

Item description

R-Value

  1. Masonry veneer - 25 mm to 50 mm cavity, 10 mm internal plaster on 90 mm stud frame

1.

Outdoor air film (7 m/s)

0.04

2.

Masonry (See Notes 3 and 4)

0.09

3.

Cavity and airspace (115 to 140 mm, made up of 90 mm stud + 25 mm to 50 mm airspace non-reflective and unventilated

0.17

4.

Plasterboard, gypsum (10 mm, 880 kg/m3)

0.06

5.

Indoor air film (still air)

0.12

Total R-Value: 0.48

guide_SpecJ15a_2019.svg

External wall construction description

Item

Item description

R-Value

  1. Cavity masonry - 20 mm to 50 mm cavity, 10 mm internal plaster on battens or furring channels

1.

Outdoor air film (7 m/s)

0.04

2.

Masonry (See Notes 3 and 4)

0.09

3.

Masonry cavity (20 mm to 50 mm, non-reflective and unventilated)

0.17

4.

Masonry (See Note 4)

0.09

5.

Airspace (20 mm to 35 mm, non-reflective and unventilated)

0.17

6.

Plasterboard, gypsum (10 mm, 880 kg/m3)

0.06

7.

Indoor air film (still air)

0.12

Total R-Value: 0.74

guide_SpecJ15b_2019.svg

External wall construction description

Item

Item description

R-Value

  1. Dense weight hollow concrete block with internal plaster on battens or furring channels

1.

Outdoor air film (7 m/s)

0.04

2.

Dense weight hollow concrete block (See Notes 3 and 4)

0.15

3.

Airspace (20 mm to 40 mm non-reflective and unventilated)

0.17

4.

Plasterboard, gypsum (10 mm, 880 kg/m3)

0.06

5.

Indoor air film (still air)

0.12

Total R-Value: 0.54

guide_SpecJ15c_2019.svg

External wall construction description

Item

Item description

R-Value

  1. 125 mm solid reinforced concrete (dense weight) - 10 mm internal plaster on battens or furring channels

1.

Outdoor air film (7 m/s)

0.04

2.

125 mm minimum solid reinforced concrete (See Note 3)

0.09

3.

Airspace (20 mm to 40 mm non-reflective and unventilated)

0.17

4.

Plasterboard, gypsum (10 mm, 880 kg/m3)

0.06

5.

Indoor air film (still air)

0.12

Total R-Value: 0.48

guide_SpecJ15d_2019.svg

External wall construction description

Item

Item description

R-Value

  1. Timber wall - external 6 mm cement sheet cladding, 90 mm stud frame, 10 mm plaster

1.

Outdoor air film (7 m/s)

0.04

2.

Fibre-cement (6 mm, 1360 kg/m3)

0.03

3.

Airspace (90 mm non-reflective and unventilated)

0.17

4.

Plasterboard, gypsum (10 mm, 880 kg/m3)

0.06

5.

Indoor air film (still air)

0.12

Total R-Value: 0.42

guide_SpecJ15e_2019.svg

External wall construction description

Item

Item description

R-Value

  1. 200 mm autoclaved aerated concrete block - 10 mm internal plaster on battens or furring channels

1.

Outdoor air film (7 m/s)

0.04

2.

Autoclaved aerated concrete block (200 mm, 350 kg/m3)

2.00

3.

Airspace (20 mm to 40 mm non-reflective and unventilated)

0.17

4.

Plasterboard, gypsum (10 mm, 880 kg/m3)

0.06

5.

Indoor air film (still air)

0.12

Total R-Value: 2.39

guide_SpecJ15f_2019.svg

External wall construction description

Item

Item description

R-Value

  1. 150 mm hollow-core concrete panels - 10 mm internal plaster on battens or furring channels

1.

Outdoor air film (7 m/s)

0.04

2.

Prestressed hollow-core concrete panels (150 mm, 1680 kg/m3, 30% cores)

0.14

3.

Airspace (20 mm to 40 mm non-reflective and unventilated)

0.17

4.

Plasterboard, gypsum (10 mm, 880 kg/m3)

0.06

5.

Indoor air film (still air)

0.12

Total R-Value: 0.53

guide_SpecJ15g_2019.svg

External wall construction description

Item

Item description

R-Value

  1. Dense weight hollow concrete block with external 6 mm cement sheet cladding on battens or furring channels

1.

Outdoor air film (7 m/s)

0.04

2.

Fibre-cement (6 mm, 1360 kg/m3)

0.03

3.

Airspace (20 mm to 40 mm non-reflective and unventilated)

0.17

4.

Dense weight hollow concrete block (See Note 4)

0.15

5.

10 mm render

0.02

6.

Indoor air film (still air)

0.12

Total R-Value: 0.53

guide_SpecJ15h_2019.svg

External wall construction description

Item

Item description

R-Value

  1. Dense weight hollow concrete block with external 6 mm cement sheet cladding on battens or furring channels

1.

Outdoor air film (7 m/s)

0.04

2.

Clear or opaque glass (10 mm, 25600 kg/m3)

0.01

3.

Airspace (20 mm to 40 mm non-reflective and unventilated)

0.17

4.

Steel sheet (1 mm to 3 mm, 7850 kg/m3)

0.00

5.

Airspace (2 mm to 100 mm non-reflective and unventilated)

0.17

6.

Plasterboard, gypsum (10 mm, 880 kg/m3)

0.06

7.

Indoor air film (still air)

0.12

Total R-Value: 0.57

guide_SpecJ15i_2019.svg

Notes:

2. General

For determining the aspect of a wall-glazing construction
(a)

the northern aspect is at or within 45° of true north; and

(b)

the southern aspect is at or within 45° of true south; and

(c)

the eastern aspect is within 45° of true east; and

(d)

the western aspect is within 45° of true west.

3. U-Value — Method 1 (Single Aspect)

(a)

For the purposes of this method, a wall-glazing construction only includes the walls and glazing facing a single aspect.

(b)

The Total system U-Value of the wall component of a wall-glazing construction must be calculated as the inverse of the Total R-Value, including allowance for thermal bridging, in accordance with—

(i)

AS/NZS 4859.2; or

(c)

The Total system U-Value of a wall-glazing construction must be calculated as the area-weighted average of the Total system U-Value of each of the components of the wall-glazing construction.

(d)

The Total system U-Value must not exceed the applicable value in J1.5(a).

4. U-Value — Method 2 (Multiple Aspects)

(a)

For the purposes of this method, a wall-glazing construction only includes the walls and glazing facing multiple aspects.

(b)

The Total system U-Value of the wall component of a wall-glazing construction must be calculated as the inverse of the Total R-Value, including allowance for thermal bridging, in accordance with—

(i)

AS/NZS 4859.2; or

(c)

The Total system U-Value of a wall-glazing construction must be calculated as the area-weighted average of the Total system U-Value of each of the components of the wall-glazing construction.

(d)

The Total system U-Value must not exceed the applicable value in J1.5(a).

5. Solar admittance — Method 1 (Single Aspect)

(a)

The solar admittance of a wall-glazing construction must be calculated in accordance with the following formula:

v1_SpecJ15a1_2019.svg

where—
SA = the wall-glazing construction solar admittance; and
AW1, AW2, etc = the area of each glazing element; and
SW1, SW2, etc = the shading multiplier for each glazing element in accordance with Clause 7; and
SHGCW1, W2, etc = the Total system SHGC of each glazing element; and
AWall = the total wall-glazing construction area.

(b)

The solar admittance of the wall-glazing construction must not exceed the applicable value in J1.5(e).

6. Solar admittance — Method 2 (Multiple Aspects)

The solar admittance of wall-glazing construction must achieve a representative air-conditioning energy value less than that achieved by the reference solar admittance, when using the following formula:

v1_SpecJ15a2_2019.svg

where—
ER = the representative air-conditioning energy value; and
AN, E, S, W = the area of the wall-glazing construction facing each aspect; and
αN, E, S, W = the solar admittance weighting coefficient of each aspect equal to—

(a)

where the glazing area on an aspect is less than 20% of the wall-glazing construction area, 0; and

(b)

the values in Table 6a and Table 6b; and

SAN, E, S, W = the wall-glazing construction solar admittance of each aspect—

(a)

equal to the applicable value in J1.5(b) in the reference case; and

(b)

calculated in accordance with Clause 5(a) in the proposed case.

Table 6a Solar admittance weighting coefficient — Class 2 common area, Class 5, 6, 7, 8 or 9b building or Class 9a building other than a ward area
Climate zone 1 Climate zone 2 Climate zone 3 Climate zone 4 Climate zone 5 Climate zone 6 Climate zone 7 Climate zone 8
Northern aspect 1.47 1.95 1.95 2.05 2.28 2.12 2.40 1.88
Southern aspect 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
Eastern aspect 1.39 1.58 1.63 1.72 1.72 1.62 1.84 1.92
Western aspect 1.41 1.68 1.65 1.69 1.75 1.67 1.92 1.25
Table 6b Solar admittance weighting coefficient — Class 3 or 9c building or Class 9a ward area
Climate zone 1 Climate zone 2 Climate zone 3 Climate zone 4 Climate zone 5 Climate zone 6 Climate zone 7 Climate zone 8
Northern aspect 1.42 1.77 1.72 1.55 1.88 1.52 1.60 1.24
Southern aspect 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
Eastern aspect 1.30 1.49 1.48 1.37 1.48 1.28 1.35 1.26
Western aspect 1.37 1.54 1.50 1.36 1.52 1.33 1.40 1.05

7. Shading

For the purpose of calculating solar admittance, the shading multiplier is—
(a)

for shading provided by an external permanent projection that extends horizontally on both sides of the glazing for the same projection distance P, as shown in Figure 7

(i)

the value in Table 7a for shading on the northern, eastern or western aspects; or

(ii)

the value in Table 7b for shading on the southern aspect; or

(b)

0.35 for shading that is provided by an external shading device such as a shutter, blind, vertical or horizontal building screen with blades, battens or slats, which—

(i)

is capable of restricting at least 80% of summer solar radiation; and

(ii)

if adjustable, will operate automatically in response to the level of solar radiation.

Table 7a Shading multipliers — Northern, eastern and western aspects
G/H P/H = 0 P/H = 0.1 P/H = 0.2 P/H = 0.3 P/H = 0.4 P/H = 0.5 P/H = 0.6 P/H = 0.7 P/H = 0.8 P/H = 0.9 P/H = 1
0 1.00 0.90 0.80 0.72 0.64 0.57 0.51 0.46 0.41 0.38 0.35
0.1 1.00 0.95 0.89 0.81 0.74 0.66 0.59 0.52 0.47 0.42 0.40
0.2 1.00 0.98 0.94 0.89 0.82 0.75 0.68 0.62 0.56 0.51 0.47
0.3 1.00 1.00 0.97 0.94 0.89 0.84 0.78 0.72 0.66 0.61 0.57
0.4 1.00 1.00 0.99 0.97 0.94 0.90 0.86 0.82 0.77 0.73 0.68
0.5 1.00 1.00 1.00 0.99 097 0.95 0.92 0.90 0.86 0.83 0.79
Table 7b Shading multipliers — Southern aspect
G/H P/H = 0 P/H = 0.1 P/H = 0.2 P/H = 0.3 P/H = 0.4 P/H = 0.5 P/H = 0.6 P/H = 0.7 P/H = 0.8 P/H = 0.9 P/H = 1
0 1.00 0.93 0.87 0.82 0.77 0.73 0.69 0.65 0.62 0.60 0.58
0.1 1.00 0.97 0.93 0.88 0.84 0.79 0.75 0.71 0.67 0.64 0.62
0.2 1.00 0.98 0.96 0.93 0.89 0.85 0.81 0.77 0.73 0.70 0.68
0.3 1.00 0.99 0.98 0.96 0.93 0.90 0.87 0.83 0.80 0.77 0.74
0.4 1.00 1.00 0.99 0.98 0.96 0.94 0.91 0.89 0.86 0.84 0.81
0.5 1.00 1.00 0.99 0.99 0.98 0.96 0.95 0.93 0.91 0.90 0.88
Figure 7 Permanent external shading - Measurement of P, G and H
v1_SpecJ15a7_2019.svg