Understanding solar heat gain coefficient

One of the metrics building code and rating scheme energy efficiency provisions consider is the solar heat gain coefficient (SHGC) of glazing in windows and external doors such as sliding glass doors to a balcony. The SHGC measures how much heat from the sun the glazing allows into a building or space, and it can affect how comfortable a space is, and how much money is spent on heating and cooling.

A high SHGC will allow a high level of heat transference into the space, which is great for winter to keep warm, however, will be intolerable during summer and require air conditioning to cool.

Similarly, a low SHGC will not let in much solar heat which assists during summer to keep spaces cooler but might mean more active heating is required during winter to stay warm.

Is a low Solar Heat Gain Coefficient (SHGC) indicative of better glazing performance?

While it's commonly believed that a low Solar Heat Gain Coefficient (SHGC) signifies better glazing performance, this isn't always the case. While a low SHGC effectively prevents excessive heat gain in the summer, it also reduces the passive heat gains a space can receive during the winter.

This consideration is particularly significant in the southern regions of Australia and during the colder months when maintaining warmth with minimal supplementary heating usage is important. Where reverse cycle air conditioning is used for heating, there is a major cost implication, and in states like Victoria or New South Wales where gas heating is still common, there is also a Scope 1 carbon emissions impact.

Striking the right balance with SHGC not only impacts energy consumption and emissions significantly, and also has implications for comfort and natural daylighting.

The orientation and shading of a glazed facade has a substantial influence on its exposure to sunlight and, consequently, the SHGC requirements. South-facing windows may benefit from higher SHGC values to optimise passive solar heating, whereas east and west-facing windows may require lower SHGC to minimise heat gain throughout the day in summer. Similarly, a well-designed fixed or operable shading system tailored to the orientation can effectively ease the stringency of SHGC requirements for windows and this is reflected in rating systems and building code provisions.

Is SHGC the same for all types of windows?

While SHGC values serve as a starting point for energy efficiency assessments, they must be adjusted once specific window types are confirmed. It ultimately depends on the ratio of glass area to frame area. Typically, awning, casement, or louvre windows exhibit total system SHGC values 10-15% lower than fixed or sliding windows due to differences in glass and frame size. This variation also applies to different window types, such as single-glazing or double-glazing.

Conversely, the framing area influences the total system U-value of a window, contingent on the material chosen for the frame as the U-value is the thermal conductivity of the entire assembly including both glazing and framing. For instance, an awning window equipped with an aluminium frame, typically results in a higher U-value due to the expanded frame area, which can lead to greater heat gain or cooling losses. But fixed windows cannot be applied everywhere, especially in areas that seek natural ventilation. Hence, again a right balance between the various parameters of the windows, user needs and design is necessary.

A practical example

The graph below represents the heating and cooling demand of a typical home in Perth for different orientations and varying solar heat gain coefficients.

1. Several analyses can be derived from this data, here’s some to start with:
   
   We are a heating-dominant climate.
   Minimal energy demands are observed for the northern orientation, while the western facade requires higher energy consumption.
   The southern façade shows increased heating demand due to the absence of passive heat gains more specifically on glazing with SHGC 0.2. SHGC 0.6 allowing passive heat gains in the south works well to reduce heating demand.
   When using low SHGC glass, typically with a rating of 0.2 and likely darker in colour, heating demand tends to be high across all orientations as it blocks passive solar heat gain. However, it may prove beneficial on the western façade for the same reason.
   A high SHGC 0.6, clear glass, will most likely result in high solar heat gains, especially on east and west orientation.
   
   
   
   The key takeaway
   
   
   Ultimately, the specification of glazing is driven by various factors including aesthetics, safety, privacy, views and climate. However, with more building designs incorporating extensive glazing as one of the key elements of the architectural design – in some cases comprising almost 90% of the façade, it is important to consider solar heat gain in the early phases of the design to provide an effective, comfortable, low-energy space.