What is the acceptable tolerance for window U-value and SHGC?
Once the windows achieve compliance, it means that the total system U-value and SHGC is in accordance with the minimum thermal performance requirements of the code. However, in reality, you can not always have what you wish for! meaning, not all window products have the exact thermal performance as you demand. Most of the time, SHGC is allowed to vary by plus-minus 5% and U-values should be less than the defined value for compliance. With VURB and JV3 report (alternative solutions), the tolerance should be investigated and declared in the report.
Pathways to achieve energy compliance for windows
According to the Australian Building Codes Board (ABCB), there are several pathways for residential and commercial buildings to achieve energy efficiency compliance.
For residential buildings, energy compliance could be achieved with Deemed to Satisfy (DTS) pathway or alternative solutions (e.g. VURB). According to NCC 2019 Vol 2.0, DTS pathway consists of two options:
- Elemental provisions: thermal performance of building fabric and windows are strictly in accordance with energy efficiency provisions.
- Energy Rating: using accredited software (FirstRate5, BERS Pro, AccuRate), the thermal performance of building fabric and windows are obtained.
For commercial buildings, energy compliance could be achieved with Deemed to Satisfy (DTS) pathway or alternative solutions (Green Star rating, JV3, NABERS, . . . ).
When you specify thermal performance values for windows using one of the pathways above, you may not find a window product matching your thermal performance values. In these cases, there are acceptable criteria for each pathway that you will have to check if the selected product still suits your needs. If not, you can either select another window product that matches closer to your specified properties.
Acceptable tolerances for U-value and SHGC with different pathways
For residential and commercial buildings, acceptable tolerance for thermal performance of windows is summarized below.
Residential buildings
BASIX
Must be less than the achieved value.
In the case of a new building, must be within ±10% of the achieved value.
In the case of alterations and additions, must be within -10% of the achieved value.
First Rate 5
In case you applied default windows to the building, must be less than the achieved value.
In case you applied custom windows, no tolerance is allowed and you must re-assess.
In case you applied default windows to the building, must be ±5% of the achieved value.
In case you applied custom windows, no tolerance is allowed and you must re-assess.
AccuRate
In case you applied default windows to the building, must be less than the achieved value.
In case you applied custom windows, no tolerance is allowed and you must re-assess.
In case you applied default windows to the building and you are an accredited rater, must be within ±10% of the specified value.
In case you applied default windows to the building and you are not an accredited rater, must be within ±5% of the specified value.
In case you applied custom windows, no tolerance is allowed and you must re-assess.
BERS Pro
In case you applied default windows to the building, must be less than the achieved value.
In case you applied custom windows, no tolerance is allowed and you must re-assess.
In case you applied default windows to the building, must be ±5% of the achieved value.
In case you applied custom windows, no tolerance is allowed and you must re-assess.
Alternative
Solutions
U-value of the windows must be implemented in the energy model and compliance must be re-assessed and addressed in the new report.
SHGC of the windows must be implemented in the energy model and compliance must be re-assessed and addressed in the new report.
Elemental
Provisions
Must be less than the achieved value.
Must be within ±10% of the achieved value.
Commercial buildings
Alternative
Solutions
U-value of the windows must be implemented in the energy model and compliance must be re-assessed and addressed in the new report.
SHGC of the windows must be implemented in the energy model and compliance must be re-assessed and addressed in the new report.
Elemental
Provisions
Must be less than the achieved value.
Must be within -10% of the achieved value.