What’s windows U value

what's windows U value

what's windows U value


What’s windows U value

The windows U value (or thermal conductivity) is a measure of how well a window or door prevents heat from escaping. The lower the U-value, the better the thermal insulation. Measuring and optimizing the U-value is essential to improving the windows and building overall energy efficiency.

Factors affect windows U value

The building windows belong to the transparent outer building cladding, and the heat transfer coefficient U value is a key indicator for evaluating the thermal insulation performance of doors and windows. How to reduce the heat transfer coefficient U value of the entire window can be analyzed step by step according to the following U value calculation formula.

Windows U value is decided by glass,frame and glass spacer
Windows U value is decided by glass,frame and glass spacer

Uw=(UgAg+UfAf+ΨgLg)/(Ag+Af)

The indicators in the formula are:

Uw is the heat transfer coefficient of the whole window, the unit is W/㎡·K

Ug is the heat transfer coefficient of the glass, the unit is W/㎡·K

Uf is the heat transfer coefficient of the window frame, the unit is W/㎡·K

Ag is the visible area of the glass, the unit is ㎡

Af is the visible area of the window frame, the unit is ㎡

Ψg is the linear thermal conductivity coefficient related to the glass spacer, the unit is W/㎡·K

Lg is the length of the edge of the visible surface of the glass, the unit is m

So the windows overall U value is decided by below factors:

  • Windows frame area and its thermal conductivity
  • Glass area and its thermal conductivity
  • Spacer type and glass perimeter

Windows Frame

Windows frame Profiles are an important part of exterior windows for energy saving and condensation prevention. The German passive building standard require that the heat transfer coefficient of profiles be U≤1.3W/㎡·K. At present, the main profile types include UPVC profiles, high-performance thermal insulation aluminum profiles, aluminum-wood composite profiles, etc.

Aluminum-wood composite profiles are widely used in passive buildings. Wood is a natural thermal insulation material. Wood has inherent advantages as a passive window: high mechanical strength, low thermal conductivity, inherent environmental protection performance and low carbon characteristics. In order to better improve the overall performance of doors and windows, wood can be compounded with aluminum alloys, polymer materials and other materials to further improve the overall performance of doors and windows.

Thermal break aluminium windows frame and alu-wood composite windows frame
Thermal break aluminium windows frame and alu-wood composite windows frame

High performance thermal break aluminium windows profile is another important option.Aluminium itself has high thermal conductivity,while by adding thermal bridge -a special nylon with 25% fiberglass reinforced thermal bridge,can block heat transfer effectively.The better thermal bridge,the lower thermal conductivity.

Aluminium profiles has high strength,long durability,multi color options,whether solid color or wood colors ,in addition to well designed structure to make it has much better structural performance,water tightness and air tightness,high performance thermal break aluminium profiles has account high market ratio.

Glass

Glass accounts for about 75% of the entire door and window area, so its thermal insulation performance is also very critical. The improvement of glass performance has decreased the windows over U value by about 80% compared with 40 years ago.r

Since the 1970s, most external windows use single glass with U value 5.5W/㎡·K. After 50 years of development, it has now evolved from simple float glass to now three-layer insulating glass. The thermal insulation performance of glass is determined by factors such as glass thickness, coating performance, insulating glass spacer and spacing, and internal inert gas.

While the technology development of every glass components has reached some limits and difficult to decrease overall glass U value further more, the best U value of existing triple glazed units is about 0.6W/M2.K,glass is becoming heavier and thicker and forced windows frame to be thicker and heavier,in addition to the increased hardware to hold the heavy sash.

Vacuum insulating glass is innovative glass ,it’s slim ,light and much better U value compared with triple glazed units,taking advantage of 0.15-0.3mm vacuum space between the glass panels, heat conductivity and heat convection are almost eliminated, and high performance low-E coating reflect most solar radiation,which makes vacuum glass units U value reach 0.5W/M2.K, hybrid vacuum glass units can reach 0.15W/M2.K.

As glass performance contribute about 70% of overall windows U value,the excellent VIG has helps VIG windows U value to 0.14W/M2.K .

Glass Spacers

In order to obtain the best thermal performance of doors and windows,  it is also necessary to reduce the linear thermal conductivity coefficient Ψg of the glass edge. It is found that the proportion of heat loss through the glass edge can be as high as 20% of the total window loss, it is necessary to use warm edge spacer materials to replace cold edge spacer materials to minimize the linear heat transfer loss of the glass edge.

warm edge spacer can effectively decrease the heat transfer through glass edge
warm edge spacer can effectively decrease the heat transfer through glass edge

Usually the edge of the glass is the lowest point in temperature and is also the most prone to condensation and mold. Therefore, the use of warm edges can effectively solve this problem and prevent condensation on the entire window edge to achieve the best living comfort.

By combing the best materials,high performance thermal break aluminium windows frame,high performance hybrid vacuum insulating glass,VIG windows has being the most energy efficient aluminium windows in the market,contact us now for your next energy efficient project.

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