Thermal insulation has long been associated primarily with reducing heating demand, but it also plays a decisive role in controlling summer overheating. With heatwaves becoming more frequent and more intense, summer comfort is now a major design consideration for both new build and refurbishment projects.
The building envelope is the first line of defence against external heat gains. An effective strategy combines several levers: thermal insulation, thermal mass in the building fabric, solar shading, and appropriate ventilation.
Limiting overheating through a high-performing envelope
Under solar radiation, external building elements absorb thermal energy that gradually transfers inward. Roofs are particularly exposed and their surface temperatures can climb well above 60–70°C in summer conditions.
The quality of thermal insulation has a direct bearing on the heat flow passing through the envelope. A well-optimised specification slows this transfer and limits the rate at which internal spaces heat up, reducing reliance on active cooling systems.
Summer comfort requirements under Approved Document O
In England, overheating in new residential buildings is now regulated under Part O of the Building Regulations, supported by Approved Document O, which has applied to new dwellings since 15 June 2022.
Compliance can be demonstrated through one of two routes:
- The Simplified Method – a prescriptive approach that sets maximum glazed areas by orientation and minimum openable window areas, taking into account whether a location is classed as moderate or high risk, and whether the dwelling benefits from cross-ventilation.
- Dynamic Thermal Modelling – based on CIBSE TM59, which offers greater design flexibility and is required for buildings with very high levels of insulation and airtightness, unusual site conditions, or heavy shading from neighbouring structures.
The objective is the same as it has always been: to design buildings that sustainably limit temperature peaks while reducing the energy demand associated with cooling.
Low lambda and high thermal resistance: assets for controlling heat flow
Recticel Insulation's PIR boards are distinguished by their very low thermal conductivity (λ). The lower this value, the higher the thermal resistance achieved for a given thickness.
This performance allows:
- required insulation levels to be reached with reduced thicknesses;
- heat transfer through the envelope to be limited;
- greater stability of internal temperatures throughout the year;
- a contribution to reduced energy consumption for both heating and cooling.
That said, managing summer comfort doesn't rest on insulation alone. The building's constructive characteristics - particularly the thermal mass of load-bearing elements such as concrete, masonry or stone also contribute to the fabric's capacity to store heat and release it later, once conditions have cooled.
Managing solar gains and encouraging passive cooling
Incorporating effective solar shading
Solar shading is an essential complement to thermal insulation. Its purpose is to intercept solar gains before they enter the building.
Commonly used solutions include:
- fixed or adjustable brise-soleil / external louvres;
- external blinds;
- roller shutters;
- solar control glazing;
- planting and other architectural shading devices.
Under Approved Document O, residential buildings in higher-risk locations are also expected to provide shading to east, south and west-facing glazing, for example through solar control glass with a maximum G-value of 0.4, or overhangs sized to the sun's angle on south-facing façades. Effectiveness always depends on façade orientation and on shading being considered from the earliest design stage, rather than added on afterwards.
Ventilation and passive cooling
Managing summer comfort also depends on how air is exchanged through the building.
Daytime ventilation can help cool spaces when the external air temperature is lower than the internal temperature, though its effectiveness is limited by prevailing weather conditions.
Night-time purge ventilation is one of the most effective passive cooling strategies available. By drawing in cooler night air, it removes heat that has built up in the fabric and resets the building's thermal capacity ahead of the following day's peak temperatures.
Combined with high-performing insulation and a well-considered envelope, this approach helps deliver lasting occupant comfort while keeping energy consumption in check.