Overheating in New Dwellings in Wales

Overheating in New Dwellings in Wales

Overheating image
Building on an earlier consultation, the Welsh government has initiated a new consultation (Stage 2B) concerning the introduction of a new Welsh Building Regulation for overheating in new dwellings. These changes are expected to have a significant impact on glass and glazing products in residential buildings.

Ending on 17th January 2022, the consultation sets out proposals for a new Approved Document S covering the mitigation of overheating risk in new dwellings in Wales.

Applying only to new residential buildings, Approved Document S would require residential buildings to be designed and constructed in such a way as to provide reasonable mitigation from the risk of summertime overheating. In addition, any mitigation measures would need to be safe, secure and reasonably practical for occupants. Two methods of assessment are foreseen:
  • Simplified method, related to glazing and floor area
  • Detailed thermal analysis method, involving calculation of the risk of overheating
The simplified method involves adopting one of several mitigation approaches to limit solar gains (see Table 1 below) and an approach for heat removal from the dwelling (see Table 2 below).

Table 1. Controlling of overheating by minimising summer solar gains:

Mitigating approach Single aspect residential building Dual aspect residential building
1 Maximum glazing area of 15% of floor area
Maximum glazing area of 15% of floor area
2 Maximum glazing area of 20% of floor area.
Low g value glazing (maximum 0.40)
Maximum glazing area of 35% of floor area.
Low g value glazing (maximum 0.40)
3 N/A Maximum glazing area of 35% of floor area.
External shutters with means of ventilation on all facades. Overhangs with 50 altitude cut-off can be used on south façades (SE to SW) instead of external shutters on that façade.

Table 2. Controlling for overheating through heat removal:

Heat removal Single aspect residential building Dual aspect residential building
Openable windows and ventilation louvres
Minimum free area of 18% of floor area, except 15% for mitigation approach 2 Minimum free area of 18% of floor area (all approaches)

The glazing area in Table 1 should be calculated as the net glazing area excluding the window frame. When selecting the preferred approach from Table 1, the ability to deliver adequate daylighting and maximise winter solar gains should be considered. Glazed areas should be distributed across all facades to ensure adequate daylight levels.

The proposed Approved Document defines a single aspect residential building as a self-contained unit with windows in one direction only, whereas a dual aspect residential building has windows facing two or more directions / orientations.

Dwelling houses with two or more parallel aspects to facilitate cross-ventilation are considered to adequately mitigate the risk of summer overheating, unless ventilation is restricted due to noise, pollution, safety or security concerns.

To satisfy the above requirements, Pilkington provides a range of solar control glass that can provide solar control and / or thermal insulation. These include products that can meet the low g value glazing requirement of a maximum 0.40 as stated in mitigation approach 2 in Table 1 above.

If using the dynamic thermal analysis method to predict overheating risk, the methodology set out in CIBSE TM59 'Design methodology for the assessment of overheating risk in homes' should be followed.


Documents related to the consultation on proposed requirements for mitigating the risk of overheating in new residential buildings can be found here.

If you have any questions on Part L 2022 in Wales and what the implications might be for glass, please complete our online contact form, selecting 'Architect' or 'Trade' as the Nature of Enquiry and 'Part L' as the Enquiry Type.

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