How to Improve Summer Thermal Comfort in Buildings

With rising temperatures and increasingly frequent summer overheating, reducing heat loss during winter is no longer enough. Building envelope systems must also be designed to control the dynamic behaviour of heat during the hottest hours of the day.

In this context, several parameters play a key role:

• thermal time lag;
• thermal attenuation;
• periodic thermal transmittance;
• thermal inertia of the building envelope.

Contrary to a common misconception, achieving high levels of summer thermal performance does not depend solely on the thermal mass of the building. Lightweight systems incorporating high-performance insulation materials, such as:

• polyurethane foam;
• extruded polystyrene (XPS),

can also provide excellent results in terms of summer thermal comfort and heat-flow reduction.

Alongside thermal time lag, it is essential to consider thermal attenuation, which describes the ability of a building structure to reduce the intensity of the heat flow passing through the building envelope over a 24-hour period.

A well-designed building envelope does not simply delay the transfer of heat: it also reduces the amount of heat that reaches the indoor environment.

This results in more stable indoor temperatures, reduced reliance on summer air conditioning and significantly lower energy consumption, providing benefits in terms of both operating costs and indoor thermal comfort.

The Italian Ministerial Decree of 26 June 2015 Changed the Approach to Summer Thermal Comfort

For many years, summer thermal comfort was considered to depend almost exclusively on the mass of building structures. Heavy walls were regarded as the primary solution for limiting overheating inside buildings.

With the introduction of the Italian Ministerial Decree of 26 June 2015 (DM 26 June 2015), this approach was partially superseded.

The regulation introduced a more comprehensive assessment of the dynamic thermal behaviour of the building envelope, recognising that lightweight structures can also provide excellent summer thermal performance when properly designed.

High thermal resistance provided by insulation, the use of ventilated cavities and the correct design of the overall building envelope can achieve results comparable to, or even better than, those obtained simply by increasing the mass of the wall.

The idea that a very heavy wall automatically provides better summer thermal performance is now considered outdated.

Modern dynamic thermal simulations show that the performance of the building envelope depends on the interaction of several factors, including:

• insulation quality;
• thermal resistance of the building envelope;
• ventilation;
• ability to attenuate heat flow;
• correct sizing of the individual layers.

For this reason, today it is more appropriate to refer to the dynamic thermal performance of the building envelope, rather than considering thermal inertia alone.

PIR polyurethane and XPS extruded polystyrene insulation panels are often associated with the winter thermal performance of buildings, but they also play an important role in the summer performance of the building envelope.

Thanks to their high thermal resistance, these materials help limit the heat flow passing through walls and roofs, contributing to improved thermal attenuation and reduced overheating of indoor spaces.

The effectiveness of the system naturally depends on the overall design of the building envelope, but the use of high-performance insulation materials makes it possible to achieve excellent thermal performance even with reduced insulation thicknesses.

One aspect that is often underestimated is the relationship between thermal resistance and heat flow.

Increasing the thermal resistance of the building envelope reduces the amount of heat energy that can pass through the structure.

The result is less overheating of indoor spaces and a reduced energy demand for cooling, without necessarily requiring particularly heavy building structures.

Ventilation Complements the Performance of Thermal Insulation

Thermal insulation is only one of the factors that determine the summer thermal performance of a building.

Ventilated roofs and ventilated façades help dissipate part of the heat generated by solar radiation, reducing the surface temperature of the building envelope and further improving indoor thermal comfort.

When insulation, ventilation and the design of the building envelope are developed as an integrated system, it is possible to achieve buildings with excellent dynamic thermal performance.