Building Materials Cooling & Ventilation

Window performance for summer comfort

Do you have big windows in your home? If you do, and especially if these windows are placed on the south side of your home you must have noticed how hot it gets just beside the window in summer. This is because not only daylight is transferred to the interior of your home via the window glass but also solar heat. Nobody can deny that it is amazing to have big windows, which provide you with great views out and proper natural light. However, big windows in small rooms can cause overheating problems specially if there is no shading or means to correctly ventilate the space. Several aspects have to work in harmony to provide a pleasant space with minimal to no energy consumption for air conditioning.

Morning Window
Big windows in small rooms can cause overheating problems

In addition to using shading devices and providing the space with proper natural ventilation it is important to select windows with adequate performance. Doing so when you are building a new home or renovating will allow you to have good size windows while minimizing the risks of overheating. But how can you check that?

Windows are composed of various elements including the frame, one, two or three glass panes of variable thicknesses and a cavity in between the panes. The cavity can be filled with air or special gases that improve overall window performance; the more resistant the gas, the less heat will be allowed to pass. Some commonly used gases in window cavities are air, argon and krypton, with the latter being the most resistant one. Special solar control films and coatings like low-e can also be applied to the glass of new and existing windows. The coating applied to the glass will be responsible for reflecting solar radiation that would otherwise enter the home by adding reflectivity or opacity to the glass.

sectional window profile
Window section showing its components

All these elements together will reflect some of the solar heat back to the exterior and provide some resistance to the passage of solar heat to the interior of the home contributing to the overall measured g-value of the window. G-value is the measure of the performance of the window in terms of solar heat transfer and varies between 0 (no solar heat transfer) and 1 (100% of solar heat hitting the window enters the space).

There are some trade-offs to consider when choosing the right window for your home. You want to protect your home from excessive solar heat gain in summer but do not want to loose on daylight or beneficial solar heat gain in winter. A good practice g-value for residential buildings, if you do not have any external shading, is around 0.5 to 0.6 which means 50%-60% of the solar heat hitting the window will be transferred into the room. This will protect your home from the hot summer sun but allow a good amount of solar heat in winter.

For large areas of south facing floor-to-ceiling glazing it might be necessary to choose windows with g-value around 0.4. However, the use of windows with g-values below 0.5 is not generally advised for residential buildings as there will be reduced daylight entering the space and not enough solar heat to passively heat your home in winter. This could increase your heating and electricity bills, as more energy will be required for heating and artificial lighting. Information about g-value is often not included on the overall description of off-the-shelf windows but you can ask the manufacturer to provide you with this.

bfrc_rating_window_energy_efficiency_comfort_giynowThe British Fenestration Rating Council (BFRC) has developed a rating system that considers some of these trade-offs and combines factors such as conduction heat loss (U-value) and heat loss through air infiltration with g-value in a single equation which results on the BFRC rating on an A to G scale. This is similar to the scale used to rate other domestic goods such as washing machines, fridges etc., so A-rated windows are energy efficient and G-rated ones aren’t. This is a good starting point for understanding window performance and choosing the right product for your needs.

Make sure to read window documentation and check if windows have appropriate performance and are BFRC certified. Look for the highest rating possible to guarantee that your home will be comfortable throughout the year with minimal energy use. Care for your comfort, pocket and the environment when shopping for windows and Green it yourself…Now!


Having difficulties choosing the right windows for your project? Have windows that work amazingly well in summer? Send us your doubts and share your experience with us on the comments below!


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