Why address overheating risk in homes?
Climate change: The UK’s climate is changing and heatwaves will become more frequent and intense. A summer such as 2024 is likely to occur every other year by 2050 and cities like London will have a climate similar to that of Barcelona today.
Health: Prolonged exposure to high temperatures can lead to heat stress; where the body’s means of controlling internal temperature starts to fail. There are currently around 2,000 heat-related deaths per year and this could treble without climate adaptation measures.
Vulnerable people such as the elderly, less mobile and pregnant women are at the highest risk of heat-related illness.
Wellbeing: Overheating impairs sleep quality and has been shown to impact concentration, alertness, cognitive performance and mental health. The rate of unintentional injury and accidents in children is also shown to increase during hot weather.
Productivity: Following the Covid pandemic, homes are now commonly used for hybrid working and educational study. The economic cost of productivity losses due to sleep disruption in new-build homes in England has been estimated at £8.4m per year. Studies in schools have also demonstrated poorer exam performance and academic learning in hot weather.
Occupant satisfaction: Householder surveys suggest that one fifth of UK homes already overheat in the current climate. In competitive housing markets, cool and comfortable homes might become more desirable and at lower risk of becoming climate-related stranded assets.
Cost: A ‘passive-first’ approach of reducing solar heat gains through good building design reduces the need for active cooling systems. This reduces the space needed for plant and equipment and allows developers and occupants to save on construction and operating costs. At a national scale, a passive-first approach is also shown to cost less than active cooling in meeting projected UK cooling demands.
Urban heat island: Cities experience higher temperatures than surrounding areas due to heat released from buildings and transport. Cities such as Manchester and Birmingham can have a heat island effect of 5°C, with up to 7°C measured in London. Passive-first measures reduce active cooling and help maintain lower temperatures in the community.
Compliance: The Building Regulations Part-O now require all new homes to manage overheating risk through passive-first approach.
Building Regulations Part-O
Part-O applies to all new homes from 2022 and builds on previous industry standards and guidance. Part-O now provides the most onerous thermal comfort requirements for new homes and effectively supersedes the Part-L Criterion 3 (solar heat gain check), CIBSE TM59
and the Passivhaus summertime overheating check.
Compliance methods
Compliance can be demonstrated via a simplified method or a dynamic thermal model.
Simple method
The simple method sets maximum glazing areas and minimum free ventilation areas for new dwellings. The limits vary based on a number of factors including:
- Different façade orientations;
- Medium-risk and high-risk locations (e.g. London);
- Whether cross ventilation is possible.
Additionally, homes in high-risk locations (e.g. London) must provide shading for NE to NW facades (i.e. all except due North facing):
- External shutters with ventilation;
- Maximum glazing solar g-value of 0.4;
- Overhangs with 50° altitude cut-off (e.g. roughly as deep as the height of the window it shades).
In practice, it is often difficult to achieve compliance by means of the simple method as the minimum free areas given in Part O are quite high/ conservative.
Dynamic method
Alternatively, homes can be assessed using a dynamic simulation method. This uses a modified CIBSE TM59 methodology and offers the highest levels of design flexibility.
Occupancy patterns to be modelled are set out in the Part O document and it is assumed that people will be in the house throughout the full day/ night cycle. In practice, this typically means that windows need to be openable during both day and night to achieve compliance through passive
means alone. In more urban locations, the need for windows to be openable at night can potentially conflict with limits on night-time noise levels set out within Part O, making some form of mechanical cooling a requisite for compliance.
Usability
The overheating risk strategy should reflect real-world conditions of the building and should take into account any restrictions to the use of windows arising from:
• Acoustics
• Safety and security
• Air quality
How?
Designs should be reviewed by a suitable consultant early in the design-stage (e.g. pre-planning) to establish a passive-first approach to avoiding overheating and plan in an appropriate servicing strategy.
Recommendations
- Remember that London in 2050 will feel like Barcelona today and passive-first approach to overheating will reduce construction costs, operational costs and environmental impacts!
- Limit the glazing ratio to 11%-18% of floor area.
- Avoid full-height windows – they are difficult to shade, lose excess heat in winter, are more expensive than the equivalent area of wall and provide little daylight benefit below around 800mm.
- Provide horizontal shading (e.g. overhangs) and/or external movable shutters to southerly facades.
- Provide external movable shutters to westerly living spaces and bedrooms which tend to overheat late in the day. Horizontal projections are not effective on westerly or easterly facades.
- Ensure that windows are safely openable for extended periods of the day and are not restricted.
- Inward opening windows offer the largest free area and will not interfere with external shading.
- Provide lockable louvred shutters, grilles or railings to ground floor and easily accessible bedrooms.
- Provide guarding to a height of 1100mm on all above-ground windows (e.g. where the change in internal to external floor level is greater than 600mm).
- In noisy locations, appoint an acoustician to confirm if the bedroom noise limits of <40dB LAeq,T (8hr average) and <55dB LAF max (<10 times per night) can be met with the proposed overheating strategy.
- In polluted locations, appoint an air quality consultant to confirm if natural ventilation is appropriate.
- Don’t rely on internal blinds or green/blue infrastructure in the overheating strategy. Although these measures can provide some benefits, their effectiveness can’t be guaranteed over the lifetime of the building and are therefore discounted for Part-O.
