Making the most of passive façade design

Passive solar heating from large, south-facing windows has the potential to provide a significant amount of carbon-free heating and is an important environmental design strategy in the race to zero carbon.  

Large windows, however, lead to higher heat losses when the sun doesn’t shine, as the U-value of a window is typically 10 times larger than that of a wall. Glazing also has higher embodied carbon than a typical wall build-up.

Therefore, the key design question is: what is the optimum level of glazing?

This optimum level should minimise whole life carbon emissions whilst factoring in other aspects such as overheating risk, daylight and views/aesthetics.

Glazing optimisation case study

This early-stage design study was carried out by Integration to help determine the optimum amount of glazing for passive solar heating for a bespoke residential dwelling in the North of England. We varied the glazing area between 5% and 90% of the south façade to find the optimum value. We ran these cases over four scenarios which varied the glazing type (double vs. triple) and floor type (lightweight timber vs. heavyweight stone). 

Study results – double glazing vs triple glazing  

Double glazing typically has a passive solar potential (g-value) around 25% higher than triple glazing, allowing more passive solar heat to enter the space.  

However, double glazing has higher heat losses when the sun isn’t shining compared to triple glazing (typically 40% more), while triple glazing has more embodied carbon than double glazing (over 50% more). 

Accounting for all these aspects, double glazing performed best in terms of whole life carbon (see graph below), with its reduction in embodied carbon outweighing its increase in operational carbon.  

Study results – heavyweight vs lightweight floor  

The stone floor resulted in lower whole life carbon than the timber floor. This is due to the stone floor providing exposed thermal mass which can store passive solar heat in winter and reduce heating requirements. Through heating the thermal mass at night, the home is also able to benefit from flexible energy use, which means lower energy bills and lower carbon emissions.

During the summer, the thermal mass can be used in conjunction with a night-time ventilation strategy to store coolth and reduce overheating.

Study results – optimum levels of glazing  

The optimum design for whole life carbon consisted of double glazing with a low carbon stone floor, with a glazed area of 35% of the south façade. 

The glazing design can then be adjusted to optimise daylight distribution within the space.

How does this study compare with Building Regulation requirements?

The study shows that prescriptive limitations, such as the maximum glazing areas in Part O or the push for triple glazing in Part L, are not necessarily aligned with best design.

Mitigating overheating  

A horizontal overhang was introduced to the project as a key part of the design. This allows for passive solar heating in winter when the sun angle is low, whilst providing shading in summer when the sun angle is high. Using this approach, the project was confirmed to surpass the TM59 (Part O) standards via an overheating simulation.

Final thoughts

By designing buildings using a holistic approach – incorporating passive solar strategies, low carbon materials, heavyweight thermal mass and whole life carbon modelling – we can achieve excellent carbon and overheating performance whilst still prioritising bright and beautiful designs.

Please contact us if you would like environmental design support for your next project! hello@integrationuk.com

Article by Jennifer Williamson