Overheating in New Dwellings in Wales

Overheating in New Dwellings in Wales

Overheating image
The Welsh government has initiated a consultation 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 February 2021, the consultation sets out proposals for a new Approved Document S covering the mitigation of overheating risk in new dwellings in Wales. This follows on from an earlier consultation on new dwellings, the responses of which are currently being reviewed.

Applying only to new dwellings, Approved Document S would require dwellings 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 dwelling Dual aspect dwelling
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 Maximum glazing area of 20% of floor area.
External shutters with louvres / retractable louvres on all facades. Overhangs can be used on south façade instead of external shutters on that façade. Overhangs should be designed to shade the windows from June – August.
Maximum glazing area of 35% of floor area.
External shutters with louvres / retractable louvres on all facades.  Overhangs can be used on south façade instead of external shutters on that façade.   Overhangs should be designed to shade the windows from June – August.

Table 2. Controlling for overheating through heat removal:

Heat removal Single aspect dwelling Dual aspect dwelling
Openable windows and ventilation louvres
Minimum free area of 18% of floor area Minimum free area of 18% of floor area

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 dwelling as one that has windows in one direction only, whereas a dual aspect dwelling has windows facing two or more directions.

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.

A further consultation covering non-domestic buildings in Wales will be held in 2021.

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

If you have any questions on Part L 2021 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.