FABRIC CONSIDERATIONS FOR


ENERGY EFFICIENT DESIGN Heating


The thermal transmittance of a construction is given by the U-value. This is the heat in watts (W) passing through a m2


of construction per degree temperature difference, from inside to outside.


Maximum allowable U-values are given in ADL2. Increasing insulation thickness to lower U-values will reduce fabric losses from the building, but the benefits become proportionately fewer as thickness is increased. U-values are already low, so the advantage of adding more insulation is limited.


Figure 3. U-value for different insulation thickness


1.0 0.8 0.6 0.4 0.2 0.0


20 40 60 80 100 120 140 160 180 200 220 240 260 Thickness mm


As the graph demonstrates, the curve is starting to flatten out significantly as it moves beyond the current roof and wall backstop values. Specifying thicker, more highly insulated constructions will raise a number of issues, including:


Significant increase in cost Additional loads that the building structure must be designed to accommodate Sharper focus on local thermal bridging through spacer systems and through fasteners Greater weight of composite panels and associated handling issues Increased structural requirements on spacer bar systems.


Reducing U-values will have a greater impact on the total heat loss and CO2 emissions of smaller buildings than larger ones. This is due to the ratio of building volume to surface area. So, for


typical industrial, warehouse or retail buildings with relatively high volume, increasing insulation has much less effect than in small buildings.


Key


■ Wall backstop value 0.35 ■ Mineral wool ■ Roof backstop value 0.25 ■ Polyurethane foam


56


U-values (W/m2


K)


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