What is solar gain and issues surrounding solar heat gain in secondary glazing

What is solar gain and why is it important to factor into a window specification? Solar gain is the increase in thermal energy of a space, object or structure as it absorbs radiant light and heat from the sun. Transparent construction materials like glass allow visible light to pass through, yet once that light is converted to long wave infrared radiation by other materials indoors, it is unable to revert back through the window, as glass is opaque to those longer wavelengths. This trapped heat causes solar gain, also known as the ‘greenhouse effect’, where it can cause overheating within a space. However, when carefully designed, it can be used as a passive heating strategy. In the UK, southernly facing windows benefit, in most cases all day, by solar energy, with peak absorption in the summer months. Those facing east and west will benefit from maximum heat gain in the mornings and afternoons during the summer. Solar gain is most frequently addressed in the choice and specification of windows and doors, so the most common metrics for quantifying solar gain are used as a way of reporting on the thermal properties of window assemblies. What glass does with solar energy diagram Solar gain through glass includes energy transmitted directly through the glass and energy absorbed by the glass and then re-radiated (Image credit: Vitro Architectural Glass) The issue with solar heat gain to consider when specifying secondary glazing Glass is susceptible to various types of breakage. In the case of solar gain, it can be subject to thermal stress breakage, which is caused by an uneven distribution of heat across the surface of a pane of glass. As a glazing system is exposed to sunlight is absorbs the heat which causes the glass to expand within the frame. If the heat is distributed unevenly, a section of the glass may seek to expand quicker than the rest, which can result in a crack or a completely broken pane due to the surface pressure. In the instance of a refurbishment of a Listed office block, for example, high performance thermal glass could be used in the secondary glazing, such as low-E or even a sealed double glazed secondary glazing unit, if trying to attain extremely low U-values. When secondary glazing is installed, it creates a cavity between the primary window. This gap will retain the solar gain and can cause a vast difference in temperatures. Then, depending on the location of the building, how it is in relation to neighbouring buildings, which may cast a shadow as the sun moves round during the day and its orientation to the sun, could cause a thermal stress break. There are many calculations which can be carried out to try and minimise the ‘pop’ occurring, however, the rule of thumb is to keep solar gain below 50%. In the case of listed buildings, whereby the façade can have no adaptations such as canopies or louvres, it can be made very tricky minimising solar gain at peak times. Not all is lost on projects where high levels of sustainability and self-sufficiency in terms of natural energy use is key, as there is the option of solar film which can be used to help deflect some of the sunlight away to stop the build-up of heat. If solar gain is something you are trying to harness to achieve a certain build rating – PassivHaus/EnerPhit or a high BREEM rating; then it is best to speak to us as soon as possible to discuss suitable options if working on the refurbishment of a Listed building.

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