Windows are great contributors in heat loss and gain in buildings and that’s why Part L regulations contain key requirements for windows, most specifically regarding their U-Values, solar gain control capability, and air tightness. Part L considers both energy efficiency and thermal comfort in the requirements. The standards set ensure that new and existing buildings maintain comfortable temperatures for occupants while at the same time reducing the need for mechanical cooling systems that require energy for usage.

Windows that have a lower U-value is an indication that a window is energy efficient. Part L indicates the maximum allowance U-Value for windows depending on the type of building and location, and it should be ensured that the windows used for the new or existing building ideally have lower U-Values. The main goals are to improve energy efficiency and reduce heat loss.

Solar gains should be managed effectively by choosing the appropriate windows located in the different areas of the building that block solar heat gain and also have protection against glares and ultraviolet exposure. The key is to have both balanced heat gain from the sunlight and better management for overheating. The metric used to measure how much solar heat is allowed to pass through a window is called the G-Value.

Poorly sealed windows may cause air leakage through gaps, cracks, or any openings around the windows. Causing heat loss due to ineffective sealing increases the demand of using heating and cooling systems that increase electricity and energy usage. The metric used to measure how airtight the window is and the heat lost through warm air escaping is called the L-factor. A lower L-factor means that it has better airtightness.

Windows should meet the standards set in Part L as they contribute in maintaining indoor comfort and reducing energy requirements and bills.