In our previous article about energy efficient windows we mentioned, that window frames are available in a variety of materials including vinyl, aluminum, wood, and fiberglass, or in combination of these different frame materials.
Now, we are going to focus on aluminium window frames and their advantages and disadvantages related to energy efficiency and savings. Aluminum replacement windows are rarely used in residential buildings anymore because if it is very cold or hot outside that cold or heat will be transferred through the aluminum into the building.
Windows lose and gain heat not only by air leakage but also by convection, conduction or radiation. Windows U-values express the heat transfer. U-value is the mathematical inverse of R-value. Usually, window U-values range from 1.1 to 0.3 and R-values range from 0.9 to 3.0. To an R-value of 2 equals a U-value of 0.5.
Aluminum windows with no thermal break U-values range from 1.9 to 2.2, aluminum windows with thermal break from 1.0 to 1.3, Wood and vinyl from 0.3 to 0.5 and Insulated vinyl or fiberglass windows U-values range from 0.2 to 0.3. So as we can see, aluminum frames are usually poor energy performers.
But, to counteract heat loss, energy-efficient aluminum windows feature thermal breaks – strips of nonaluminum and nonmetal materials (for example wood or thermoplastics), dividing the aluminum elements into interior and exterior faces. To keep wind and water out, aluminum windows includes other features such as a double weatherstripping and interlocking meeting rails.