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Free heat from the sun. Capturing sunlight

Updated: Dec 29, 2021

Modern buildings with good levels of airtightness and insulation can benefit more from solar gains than older buildings that are less energy efficient. This is because any 'free heat' gained is stored for longer, with the energy takes less time to escape. This in turn reduced heating demand for the property.


Care needs to be taken to avoid the property does not overheat for the same reasons, in particular if the building incorporates high levels of airtightness and insulation.

Good practice states the internal temperature in buildings should not exceed 25 degrees for more than 10% of the occupied year (Source PHPP illustrated. A Designer's Companion to the Passive House Planning Package: pg. 18).

Window costs.

Consideration of window cost should also be taken into account. it is widely accepted in Passive House circles that 'free solar heat' costs about 10-20 times more than gas heating. (Source PHPP illustrated. A Designer's Companion to the Passive House Planning Package: pg. 16). In addition, large South facing windows may in fact lose more energy than they gain during the winter months; contributing to net losses. the graph below Identifies different window types and performances.

Simply providing large South-facing windows are not a solution by themselves; windows will not provide all the Winter heat requirement and will likely contribute to overheating in the Summer.

Solar gains-South facing windows.

We look to incorporate free solar gains into our designs wherever possible, using daylight analysis software to model heat gains within a property. The software provides an indication of where we can improve the design to maximise solar heat gains during the heating season, whilst avoiding summer overheating.

Solar analysis of 3d model showing gains through South facing glass using De Luminae Software. Copyright Carvell Associates.

The above model demonstrates how the roof overhang over the upper windows contributes significantly to summer shading whilst allowing the sun to heat the space during the winter heating season. The lower windows demonstrate how a lack of shading increases solar gains. The model informs us that it is likely the large window to the right will contribute to unwanted gains during the warmer summer months, leading to overheating. We are now proposing to provide additional external shading.

solar shading analysis
Analysis of model showing solar gains in summer. De Luminae Software. Copyright Carvell Associates.

For this project a 3 storey South facing window was a fundamental part of the design, allowing daylight to penetrate deep into the building interior. The main staircase and circulation route is directly off the feature window which faces due South).

Analysis of model showing solar gains in Winter. De Luminae Software. Copyright Carvell Associates.

The same model showing winter analysis. Because Winter sun is lower in the sky, direct sunlight penetrates deeper into the building, providing a natural heat source.

Development model showing horizontal shading externally.

carvell Associates are increasing using 3d models and shading analysis as a quick and informative way to assess how the design of an extension can benefit from free solar heat, We use teh same model to investigate the risk of overheating during Summer months.

Detail of Roof overhang provides additional shading to reduce Summer overheating.

Photo of the completed project showing the full-height feature window detail, with shading to reduce summer overheating. Whilst not a Passive House project the design looks to incorporate some fundamental principles of Passive House design.

The insulation and airtightness levels are significantly lower for this extension (the extension was built to Building Regulations standards for extensions, which is less onerous than Passive House requirements).

As a failsafe measure we incorporated high level roof lights. These can be opened in the summer to allow for purge ventilation of warm air from the the stairwell by way of the stack effect.

Further reading.

The following link is to an excellent article on overheating by Ellrond Burrell. To quote the article: 'Solar gain should not be maximised in a Passivhaus building, it should be optimised – that is, carefully balanced so that it is beneficial'

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