Differences Between g-Value and g-Window in terms of SAP and TM59 Assessments
- Darren
- Apr 4
- 3 min read
Understanding g-Value and g-Window in SAP and TM59 Assessments and why so many find it difficult.
When it comes to SAP & TM59 assessments for new home construction in the UK, two key metrics help house builders and architects navigate energy performance and comfort levels: the g-value and the g-window. Understanding the distinctions between these terms is essential for ensuring compliance with building regulations and creating comfortable living environments. As energy assessors it's important we are provided with the correct glazing values, find out why below.
What is g-Value?
The g-value or solar factor measures how much solar radiation is transmitted through the glazing into a building. Here’s what you need to know:
Definition: Reflects the total solar energy transmittance of a glazing unit.
Importance in SAP Assessments: The g-value directly influences the energy needed to heat a building. Lower g-values limit solar gain, which can help reduce overheating risks but may increase heating demands in colder months.
Relevance to TM59: Although TM59 focuses more specifically on thermal and comfort criteria, the g-value’s effect on solar gain is critical for assessing potential overheating in residential buildings.
What is g-Window?
The g-window is a broader metric that includes not only the g-value but also other aspects of window design:
Definition: Encompasses the entire window's design, including frame, spacer bars, and glazing.
Context: While not required for TM59 assessments, the g-Window value can be used with the SAP calculations if provided using a BFRC datasheet.
Significance in SAP and TM59 Assessments
Both the g-value and g-window play significant roles in the assessments conducted for energy performance and overheating risks:
SAP (Standard Assessment Procedure): Focuses on energy efficiency and CO2 emissions, where the g-value is crucial for energy calculations. The SAP calculation methodology allows for either of the g-Value or g-Window value to be used.
TM59 (Overheating Assessment): Aims to predict potential discomfort caused by overheating during warmer months by considering internal heat gains and solar radiation. Only the g-Value can be used within the Dynamic Thermal Modelling (TM59) simulations.
Common Challenges Faced by House Builders
Several issues may arise from misunderstandings of g-value and g-window metrics, these typical include only being provided with the BFRC certificate. The BFRC certificate only confirms the g-Window: Without the accompanying datasheets and calculation pages, the g-Value in unknown. As energy assessors we see customers struggling to obtain the correct paperwork from the supplier and/or manufacturer.
Misunderstanding Metrics: Confusion between g-value and g-window can lead to inaccurate assessments, potentially resulting in non-compliance with regulations.
Overheating Risks: Inadequate assessment of solar gain may exacerbate overheating issues, particularly in high solar exposure areas or dwellings with large amounts of South and West facing glazing.
Solutions to Improve Assessment Accuracy
To mitigate common pitfalls, consider the following strategies:
Clear Guidance To Suppliers: Providing clear guidelines on the differences between g-value and g-window to suppliers and manufacturers ensure the correct information is obtained. We have a guidance document about g-Values and the g-Window values. You can download it below.
Integrative Design Approach: Encourage a holistic view from your energy assessor of the energy performance that considers both metrics during the design phase.
Conclusion
Understanding the differences between g-value and g-window is crucial for UK house builders and architects aiming to comply with SAP and TM59 assessments. Ensure your window and door suppliers know the difference. Share our guidance document below with your design team, suppliers to ensure everyone knows the importance of obtaining the right information when it comes to glazing performance and energy assessments. Proper knowledge can lead to better design decisions that enhance energy efficiency and occupant comfort. Need support with Part L or Part O compliance? Contact us here.
Comments