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A measure of thermal transmittance, U-values express the rate of heat transfer through any element of a building, such as a wall, roof or window. Because the construction of these elements can vary so much with design and the choice of materials available, the U-values can vary too – hence they need to be calculated specifically for each element.
In practice, the manufacturers of insulation (for instance) will give guides to the U-values achieved when their products are used in a variety of constructions and some have U-value calculators on their websites.
The units used to express U-values are watts per m² Kelvin (W/m²K). This means that if a wall, for example, had a U-value of 1.0 W/m²K, for every degree of temperature difference between the air on the surface inside the wall and the air on the surface outside, 1 watt of heat would pass through any m² of walling.
Can you add up the U values for several different layers?
An element (normally a wall, floor or roof) will be constructed of several layers. For example, a wall could be 100 mm brick, 100 mm Celotex & 100 mm Blocks. if we know the thermal resistance value for each layer (this information is readily available online on the manufacture’s websites), we can accurately calculate the overall U-Value for the element using specialist software.
Thanks, great article.