The building envelope plays a crucial role in reducing operational energy demand. In particular, the two main properties of the building envelope to look at in this perspective are thermal transmittance (U, W/m2K1) and thermal inertia, which is often expressed by a metric called periodic thermal transmittance (Yie, W/m2K1). These two properties are also traditionally connected to two different energy demands: while thermal transmittance is crucial to reduce heating energy demand, thermal inertia has an impact on energy demand for cooling. However, a question may rise about the impact of each property on the other demand – i.e. the impact of thermal insulation on the cooling energy demand and the impact of thermal inertia on the heating demand.
A parametric analysis on the influence of the thermal inertia on the energy performance of a single family house in a Nordic climate has been carried out to find an answer to this question. “Ideal envelopes” have been modelled and simulated, meaning that used thermophysical properties do not represent any configuration, but the entire spectrum of technological configurations.
The results show that the influence of the thermal inertia on the heating energy need is very limited. Even a relatively high value of Yie, which means no or little thermal inertia, does not determine a significant increase in energy need. Parallel to this, solutions characterized by very high thermal inertia do not allow heating energy demand to be sensibly decreased. Periodic thermal transmittance has instead an impact on the heating load. The impact of the thermal inertia is also assessed in the warmer season, and the results show that this parameter does not significantly contribute to a better behavior (especially when the upper limit of the indoor air temperature is controlled). Limitations to value of thermal transmittance are also pointed out to avoid non-energy effective conditions when the total (heating plus cooling) annual performance is considered.