Overheating in New Dwellings in Wales

On 24^th May 2022, the Welsh government issued new and updated Approved Documents in support of the Building Regulations for residential buildings in Wales. These changes are expected to have a significant impact on glass and glazing products in new and existing dwellings.

Taking effect on 23^rd November 2022, the new Approved Document O introduces requirements to mitigate the risk of overheating in new dwellings in Wales.

Applying only to new dwellings, Approved Document O requires 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 provided:

• Simplified method, related to glazing and floor areas
• 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 and an approach for heat removal from the dwelling (see Table 1 below).

Table 1 - Controlling overheating by minimising summer solar gains and heat removal

Mitigating approach	Minimising summer solar gains	Heat removal (openable windows and ventilation louvres)
Single aspect residential buildings
S1	Maximum glazed area of 15% of floor area	Minimum free area of 12% of floor area
S2	Maximum glazed area of 20% of floor area Glazing with maximum g value of 0.40	Minimum free area of 10% of floor area
Dual aspect residential buildings
D1	Maximum glazed area of 15% of floor area	Minimum free area of 12% of floor area
D2	Maximum glazed area of 35% of floor area Glazing with maximum g value of 0.40	Minimum free area of 12% of floor area
D3	Maximum glazing area of 35% of floor area. External shutters with means of ventilation on all facades. Overhangs with 50o altitude cut-off can be used instead of external shutters on South facades (i.e. walls with South East to South West facing orientation)	Minimum free area of 12% 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.

To access the new Approved Documents on the Welsh government's website, please click 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.