Effect of thermal diffusivity of insulating materials on room free-float temperature with façade external insulation

External insulation of building façade is widely used to reduce heating energy demand in buildings. Usually, its design concerns only thermal transmittance, while transient thermal behaviour is commonly addressed only as dumping factor or time lag of outdoor heat wave. During summer, in many mild climates, outdoor daily mean temperature is close to comfort temperature. Yet, even though mean heat transfer through building envelope is null, heating during daytime may lead to positive cooling loads or discomfort temperatures in non-conditioned rooms. In residential buildings internal loads are usually very low, so the most relevant loads are heat transfer though outer facades. Moreover, where there is no cooling, wall dumping factor is not meaningful to evaluate the thermal performance of wall insulation, as it is referred to constant indoor temperature. In this framework, a model of a room with a single outer wall has been developed to study the effect of insulating material on free-float temperature. Transient heat transfer through the envelope as well as through inner walls is considered to model indoor air temperature. Different localities in Italy and commonly used insulating materials are considered.