TGD L 2011 – A2.1 – SIMPLE STRUCTURE WITHOUT THERMAL BRIDGING

A2.1 To calculate the U-value of a building element (wall or roof) using I.S. EN ISO 6946: 2007, the thermal resistance of each component is calculated, and these thermal resistances, together with surface resistances as appropriate, are then combined to yield the total thermal resistance and U-value. The result is corrected to account for mechanical fixings (e.g. wall ties) or air gaps if required. For an element consisting of homogenous layers with no thermal bridging, the total resistance is simply the sum of individual thermal resistances and surface resistances. I.S. EN 6946: 2007 provides for corrections to the calculated U-value. In the case of Example A1 (see Diagram A1), corrections for air gaps in the insulated layer and for mechanical fasteners may apply. However, if the total correction is less than 3% of the calculated value, the correction may be ignored. In this case no correction for air gaps applies as it is assumed that the insulation boards meet the dimensional standards set out in I.S. EN ISO 6946: 2007and that they are installed without gaps greater than 5 mm. The construction involves the use of wall ties that penetrate fully through the insulation layer. A potential correction factor applies which, assuming the use of 4 mm diameter stainless steel ties at 5 ties per m2, is calculated as 0.006 W/m2K. This is equal to 3% of the calculated U-value and the corrected U-value for this structure would be 0.21. It should be noted that, if galvanised steel wall ties were used, a correction of 0.02 W/m2K would apply, and the corrected U-value for this construction would be 0.22 W/m2K.

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