Is solar gain good or bad?

One of the key inputs into the energy balance of a Passive House project is solar gain, but is it helpful to the designers or a hindrance?

The Passive House Planning Package (PHPP) is the designer’s tool to manage the energy balance of the building. There are inputs and outputs; these include heat generated internally by people and appliances (input), heat lost through the fabric (output) and heat lost through ventilation (output). Solar gain is an input of energy and can provide 2/3rds of the total heating demand in a Passive House, completely free.

So, for the most part, solar gain is helpful, promoting the use of large south facing windows to bring in daylight and solar radiation…. until the point where it becomes unhelpful. At the height of summer, this solar energy provides too much free heat unless used in combination with shading devices or heaven forbid, cooling. East and west facing elevations can also suffer from too much low sun throughout the year, so can also contribute to overheating.

Window design can help mitigate this unhelpful solar gain. Deep reveals can provide shading. Window selection and glass g-value limits the amount of radiation entering. Full height glazing can add aesthetic value and enhance feelings of spaciousness but the lowest portion (say under desk height) does nothing to contribute to daylighting so is only contributing to heat losses and solar heat gain. Window size and placement is therefore key to managing this effectively.

There's no one size fits all solution to designing south facing windows and shading, as it depends on your geographical location, the sun path, external shading sources (e.g. neighbouring buildings) and climate. Luckily PHPP has much of this data built in, so once your location is set, you can run accurate scenarios on many of these design parameters to avoid summer overheating.