Calculated as the internal surface temperature minus the external temperature, divided by the internal temperature minus the external temperature. It runs between 0 and 1, with better insulated junctions scoring higher. UK regulations require a value of not less than 0.75 for dwellings. Defined in BS EN ISO 13788, with further guidance in BRE IP 1/06.
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The free diagnostic works out the mechanism from what you can see and measure, and tells you what it cannot conclude.