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

J1.5a relates to the calculation of the U-value and solar admittance of building elements, which are important factors in determining the overall energy efficiency of a building. It specifies the methods that should be used to calculate the U-value and solar admittance of building elements. It provides tables of default values for different types of materials and constructions. The calculations consider factors such as the thickness and thermal conductivity of the material, as well as any air gaps or reflective surfaces that may affect the overall performance.

J1.5a has been reproduced without amendment, as per NCC 2019 - Vol 1 - Amendment 1 1.

1. Scope

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

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

(ii) Specification J1.5b for spandrel panels.

(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

(ii) Specification J1.5b for spandrel panels.

(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:

$$\text{SA} = \frac{\text{AW1} \times \text{SW1} \times \text{SHGCW1}}{\text{AWall}} + \frac{\text{AW2} \times \text{SW2} \times \text{SHGCW2}}{\text{AWall}} + \dots$$

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:

$$\begin{aligned} & \text{Er} = \text{A}_\text{N} \times \alpha_\text{N} \times \text{SA}_\text{N} + \text{A}_\text{E} \times \alpha_\text{E} \times \text{SA}_\text{E} \\ & + \text{A}_\text{S} \times \alpha_\text{SA} \times \text{S}_\text{S} + \text{A}_\text{W} \times \alpha_\text{W} \times \text{SA}_\text{W} \end{aligned}$$

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	0.97	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