To illustrate the method, Fig. 3 compares the NER of different
insulation thicknesses of an energy intensive material (polystyrene)
in its application in a highly efficient house in a maritime
climate. The studied house is a passive design with no cooling
demand and a very low heating demand of 15 kWh/m2 as the base
case. Details of the house and calculations can be found in
Hernandez and Kenny [53]. This house employs an average of
115 mm of insulation for walls, 85 mm for floors, and 235 mm for
roofs. Four different polystyrene upgrades are tested, increasing
insulation by 50 mmeach time. It can be observed in Fig. 3 that the
slope of the lines in between upgrade options, which is equal to the
NER, diminishes as the insulation level increases. The NER drops
below 1 in the last upgrade from UP3 to UP4, which corresponds to
the increase of insulation from an average of 250 mmto an average
of 300 mm. This NER