The Future Build

The importance of the U-Value of windows



Knowing the u-Value of windows helps planning energy related renovations of buildings, as well as it supports the selection process for windows of a new building. Solar heat gain through windows into air-conditioned buildings in the Gulf region is an issue local glass distributors have to face. Due to the big temperature difference between inside and outside it is important to keep the cold inside not losing it through inefficient building materials. The thermal performance of a given window glass can vary greatly.

The U-value is the measurement unit used for assessing the heat loss of a building element. U-values describe how much heat (measured in watts) passes through 1 square meter of a given material when the temperature difference between the two sides of the element is 1°C.

The lower the U-value, the better the insulation quality of the building element. The U-value of window glass may range from 5 W/m²K for single glazed windows to 0.7 W/m²K for triple glazed windows. This means that a lot of energy and costs can be saved by replacing poorly insulated windows with windows of the latest standards or by choosing the right windows right from the start.

However, the exact thermal performance of the glass within a window is hard to examine which makes analyzing the cost-effectiveness of replacement difficult. In-situ measurement data can therefore be of great value. Reliable in-situ measurements of glass can be conducted using heat flux meters. It can be precisely calculated by measuring the heat flux along with the inside and outside temperature, based on a standardized formula according to ISO 9869. A temperature difference of 5°C is sufficient for a precise U-value measurement.

However, as the thermal behavior of glass differs from that of walls, a different measurement approach is required. For example measurements should be conducted during the night under stable conditions. Daytime measurements are very much influenced by (indirect) solar radiation and do not provide a reliable U-value. Measuring in-situ U-values of glass can be valuable as the range of thermal characteristics of glass can vary broadly while the exact characteristics of installed glass are not always known. Gaining exact data on U-values for glass allows a precise cost-benefit analysis for window replacing.


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