Overheating in new dwellings

Overheating in new dwellings (England)

Part O
A new Building Regulation for overheating in new dwellings in England is expected to have a significant impact on glass and glazing products in residential buildings.

Published on 15th December 2021 and coming into force on 15th June 2022, the government has set out its new requirements for Part O (overheating) of the Building Regulations and the supporting Approved Document for new dwellings in England.

Applying only to new residential buildings, new residential buildings must be designed and constructed in such a way as to limit unwanted solar gains in summer and provide an adequate means to remove excess heat from inside. Two methods of assessment are given in Approved Document O:
  • Simplified method, related to location, dwelling type, glazing area and presence of cross-ventilation
  • Detailed thermal analysis method, involving calculation of the risk of overheating
For the simplified method, England is split into two:
  • Moderate risk locations (England excluding higher risk parts of London
  • High risk locations (urban and some suburban parts of London)
As buildings located in some Central Manchester postcodes may also experience elevated night-time temperatures, they should be considered high risk.

Using the simplified method, new residential buildings should meet the maximum glazing area and shading standards in the tables below.

Limiting solar gains for buildings with cross-ventilation
Largest glazed façade orientation
High risk location
Moderate risk location
Maximum area of glazing (% floor area)
Maximum area of glazing in most glazed room (% floor area of room)
Maximum area of glazing (% floor area)
Maximum area of glazing in most glazed room (% floor area of room)
North 15 37 18 37
East 18 37 18 37
South 15 22 15 30
West 18 37 11 22

Limiting solar gains for buildings without cross-ventilation

Largest glazed façade orientation
High risk location
Moderate risk location
Maximum area of glazing (% floor area)
Maximum area of glazing in most glazed room (% floor area of room)
Maximum area of glazing (% floor area)
Maximum area of glazing in most glazed room (% floor area of room)
North 15 26 15 26
East 11 18 18 26
South 11 11 15 15
West 11 18 18 11

Cross-ventilation is defined as the ability to ventilate using openings on opposite facades of a dwelling. Having openings on facades that are not opposite is not allowing cross-ventilation, e.g. in a corner flat.

In addition to the above, residential buildings in high risk locations should also provide shading to glazed areas between compass points north-east and north-west via the south. Where shading is required, glazing with a maximum g value of 0.40 and a high light transmittance of 0.70 can be used.

In terms of removing excess heat, the proposals set a minimum free area (geometric open area of a ventilator) dependent upon floor and glazing areas and whether the building has cross-ventilation.

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 the simplified method.

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.

The 2021 edition of Approved Document O can be downloaded from here.

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