Part J2 Glazing

See comments for Deemed-to-Satisfy Provisions of .

The Deemed-to-Satisfy Provisions of do not apply to a sole-occupancy unit of a Class 2 building or Class 4 part of a building. This is a consequence of sole-occupancy units in a Class 2 building or Class 4 part of a building being covered by .

The Deemed-to-Satisfy Provisions of do not apply to a Class 8 electricity network substation as these buildings are not required to be air-conditioned for the purposes of . See the definition for air-conditioning. The air-conditioning systems of these buildings are instead designed to maintain the efficient operation of sensitive electrical equipment.

See comments for Deemed-to-Satisfy Provisions of J1.1.

The content of , which existed in BCA 2009, has been removed. The provision number has been retained without text so as not to change the numbering of the current BCA from that of BCA 2009.

The content of , which existed in BCA 2009, has been removed. The provision number has been retained without text so as not to change the numbering of the current BCA from that of BCA 2009.

By referring to glazing elements, requires Total System U-Values and Total System SHGCs to be assessed 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 provided they measure combined glass and frame performance according to AFRC requirements.

Total System U-Values and Total System SHGCs are shown for some simple types of residential glazing elements in the table below. (Smaller numbers indicate better glazing element performance). The table gives worst case assessments of glazing elements, which can be improved by obtaining custom product assessments from suppliers, manufacturers, industry associations (including their online resources) and from competent assessors. Custom assessments consider glazing element components in more detail and return the highest levels of assessed performance for a given type of glazing element.

Typical ranges of generic ratings are set out in the table below to illustrate the levels of performance available through such assessments. Numbers from this table should not be used in compliance calculations.

This calculation method is based on an annual dynamic simulation of energy consumption. It is intended for buildings which are fully air-conditioned and have a high day time use. It is about controlling the annual air-conditioning energy consumption, rather than reducing the impact of seasonal peak heating and cooling loads. As the main consideration is to reduce the energy consumed by air-conditioning plant over the year, the factors in the formula also make provision for the various efficiencies of the likely heating and cooling plant.

The calculation lends itself to being expressed in a spreadsheet and is a single formula that covers the total air-conditioning energy consumption for the full year (although the heating and cooling components can be recognised within the formula).

As the relationships are approximately linear, they are expressed arithmetically with a series of constants.

There are different indices in for Class 3 buildings, Class 9c buildings, other classes of buildings and display glazing. There are also two stringency options; Option A for most applications and Option B for a wall with less insulation than would otherwise be required (see ).

The energy indices in have been developed on the basis of a facade height equal to the depth of the air-conditioning system's perimeter zone; nominally 2.4 metres. If this is not the situation with the proposed design, some of the solar load may be handled by an internal zone possibly leading to poor temperature control in the internal zone. The facade area is based on the occupied floor facade and includes the glazing area and any part of the facade, typically a wall, which is exposed to a conditioned space (see for definition of "conditioned space"). Therefore, the facade area would not include parts of the wall forming a parapet or reveal, or parts of the wall that are not exposed to a conditioned space such as an unconditioned plant room or unconditioned interstitial floors in a laboratory or hospital.

Glazing in the internal fabric of a building, such as between a conditioned office and an un-conditioned factory or bulk store, is to be assessed using energy index Option A and the appropriate value for the south orientation sector in and the shading multipliers in . This provides values equivalent to maximum shading as would be expected for glazing in the internal fabric. is intended only for internal glazing with virtually no exposure to direct solar radiation and it cannot be used for glazing in external walls. Despite using the values for the south orientation sector, glazing in the internal fabric must be assessed separately to external glazing in the south orientation.

Example

Example

The following is an example of how these provisions are applied in a north wall of a multi-storey office building in climate zone 4. The proposed facade is 150 m² in area. The facade contains 1 window 2.8 m high and 3.0 m wide (8.4 m² area) with a 1 m projection 200 mm above the window and 8 windows each of 3 m² with no projection. All the glazing is proposed as single glass toned [Total System U-Value of glazing (glass and frame) is 6.9 and Total System Solar Heat Gain Coefficient (SHGC) of the glazing (glass and frame) is 0.6].

NORTH FAÇADE ELEVATION

The allowance for the aggregated air-conditioning energy value = 150 x 0.142 (from Energy index Option A) = 21.3

The air-conditioning energy value for each glazing element is calculated using the formula:

A₁ [ SHGC₁ ( C_A x S_H1 + C_B x S_C1 ) + C_C x U₁ ]

where—

A1	= the area of glazing element 1
CA B,& C	= the energy constants A, B and C for the specific orientation from Table J2.4b
SHGC1	= the Total System SHGC of glazing element 1
SH1 & SC1	= the heating shading multiplier and cooling shading multiplier for the glazing element 1 obtained from Table J2.4c and Table J2.4d respectively
U1	= the Total System U-Value of glazing element 1
For window 1:	P/H = 1.0/3.0 i.e. 0.33
Hence	SH1 = 0.98 and SC1 = 0.82 (interpolating values between P/H = 0.2 and 0.8)
Thus	A1 [SHGC1 (CA x SH1 + CB x SC1 ) + CC x U1]
	= 8.4 [0.6 (-0.16 x 0.98 + 1.25 x 0.82 )+ 0.0 x 6.9]
	= 8.4 [ 0.6 (-0.157 + 1.025)]
	= 8.4 x 0.6 x 0.868
	= 4.37
For window 2:	A2 [SHGC2 ( CA x SH2 + CB x SC2 ) + CC x U2]
	= 3.0 [0.6( -0.16 x 1.0 + 1.25 x 1.0 )+ 0.0 x 6.9]
	= 3.0 [0.6( -0.16 + 1.25)]
	3.0 x 0.654
	= 1.96
For all windows:
	= 4.37 + (8 x 1.96)
	= 20.1

As the calculated aggregate air-conditioning energy value for the glazing, at 20.1, is less than the aggregated air-conditioning energy value allowance of 21.3, the glazing complies.

Note that the ABCB has produced a glazing calculator to assist with the above calculations.

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

Permanent projections, such as verandahs, balconies, fixed canopies, eaves or shading hoods, must have a means of restricting the amount of solar radiation that reaches the glazing from the sides of the feature. Hence, the requirements in for the feature to extend either side of the glazing or to have a reveal or similar shading element at the side of the shading feature.

External shading devices, such as shutters, blinds, vertical or horizontal building screens are required by 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 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.