Everyone's dream is a cozy, warm house, in which we will spend mornings, afternoons and evenings with pleasure. We don't want to worry about runaway heat and, as a result, ever-increasing heating bills. Therefore, the key aspect when choosing windows will be the coefficient Uw, that is, the coefficient of heat transfer. What is the Uw factor? How does its height affect the insulation performance of windows? Why is the heat transfer coefficient so important when ordering windows? You will read about this in the article below.
What is Uw, or heat transfer coefficient?
The coefficient of thermal conductivity (Uw) is one of The most important parameters in the opening joinery. It is a measure that determines amount of thermal energy, penetrating through the window in a unit of time and a unit of area, when the temperature outside and inside the building is different. A low coefficient of heat transfer means that the better the window insulates our house from heat loss. Thus, the lower the parameter of thermal permeability, the more heated air remains in the building.
Why is the heat transfer coefficient so important?
Choosing windows with a low coefficient of thermal conductivity is key to maintaining the right temperature inside the house and for reducing heating bills. Correct thermal insulation of windows allows us to enjoy a comfortable atmosphere in the house all year round. It protects us from the cold in cold weather and from excessive heating of the premises in the hot ones.
How is the heat transfer coefficient calculated?
To calculate the heat transfer coefficient for windows, measurements and calculations according to PN-EN ISO 10077-2 are used. Any reputable window supplier will present thermal calculations for the thermal conductivity of each structure when presenting a price offer.
The formula for the heat transfer coefficient looks like this:
Uw = (Ug * Ag) + (Uf * Af) + (Ψg * lg) /A
The value of the parameter Uw consists of the following elements:
Ug — heat transfer coefficient of glass
Ag — glass surface
Ugh — heat permeability of the window frame
Off — the surface of the window frame
Ψg — heat transfer coefficient for the combined elements of the glass, spacer frame and frame
lg — the length of the thermal bridge at the junction of the glass and the frame
IB — total window area = (Ag + Af).
Thermal permeability of windows - what affects its value?
The heat transfer coefficient is affected by almost every component of the window. These are:
Shaft packages — 3- or 4-glass packages have the best parameters, which are additionally equipped with low-emission coatings and warm spacers made of stainless steel or plastic.
The size of the window - thermal permeability also largely depends on the overall size of the window. Glass, as the warmest element, is largely responsible for the value of heat permeability. The larger the glazing area, the lower the Uw parameter.
Window profiles — in their case, the depth of their construction and the type of reinforcement are of great importance.
Fittings — in windows with a high degree of energy efficiency, it is recommended to use fittings completely hidden in the compartment between the sash and the window frame. The absence of hinges on the outer surface prevents the formation of thermal bridges, and the continuity of the seal is preserved.
Mounting Technique — although it does not directly affect the Uw value, it has a significant effect on increasing the tightness of the window seat. The most energy-efficient method of installation is the so-called warm installation in the insulation layer of the building. Thanks to this, the thermal bridge along the perimeter of the frame is minimized to almost zero. An alternative solution combining high thermal insulation with lower costs is the so-called three-layer installation with the use of vapor-permeable and vapor-insulating layers.
Window frames - the insulating properties of the window frame are influenced, among others, by the use of additional insulating material, the number of chambers inside and the thickness of the profile.
Heat transfer coefficient of energy-saving windows and passive windows
Each manufacturer is obliged to provide the value of the heat transfer coefficient for all models of manufactured windows to customers. By default, the value of the coefficient Uw is calculated for reference window, that is, one with dimensions of 1230x1480 mm. If the coefficient values for this window are below 1W/m2K, according to the regulations, they are classified as Energy-saving windows. If, on the other hand, the heat transfer coefficient of the window is below 0.8W/m2K, this is it Passive Window. This is due to the fact that windows for passive houses have to meet much more stringent requirements, and nowadays building passive houses has become one of the most popular choices.
Uw window heat transfer coefficient - summary
The choice of windows with a low permeability coefficient is a key aspect in the design and construction of energy-efficient homes. Properly selected parameters translate into living comfort, financial savings and a positive impact on the environment. Therefore, it is worth taking the time to carefully study the different options and consult with professionals to choose the best solution adapted to our needs and expectations. We will be happy to tell you which windows will be best for your home.
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