CIBSE TM59 assessment of overheating risk in homes
The problem of overheating within homes presents a health risk for residents. These range from the mild effects of exposure to high temperatures, such as dehydration, prickly heat, and fainting, to more significant effects such as heat exhaustion, heat stroke and mental health consequences, including increased suicide risk.
There is growing evidence of overheating in homes becoming more of a problem. Why this is the case relates to a number of factors, including increasing average temperatures (more of an issue in the summer months), a design preference for high levels of glazing, better insulated homes and low air permeability rates.
The most vulnerable in society, such as the elderly or the sick, are particularly at risk as these are the people who are more likely to be in their homes during daytime, when the heat is most intense.
To tackle this problem, the Chartered Institution of Building Services Engineers (CIBSE) have developed CIBSE TM59, an approved methodology for the assessment of overheating risk in homes. This assessment should be undertaken at the design stage via Dynamic Thermal Modelling.
Approved Document O: Overheating mitigation
Approved Document O came into force on 15 June 2022 and applies to all new residential buildings. Compliance with Approved Document O can be demonstrated either by the simplified method or by dynamic thermal modelling (CIBSE TM59). For the simplified method each dwelling must be designed with glazing areas that are below the prescribed maximum and ventilation "free-areas" that are above the prescribed minimum.
Our consultants are on hand to guide you through the entire process. Along with providing an analysis of your building set against the CIBSE TM59 metric, we also offer support and advice to make sure that each dwelling achieves a pass.
Naturally Ventilated Homes
For homes with natural ventilation (for example, with high levels of openable windows), CIBSE TM59 sets two criteria for compliance: Criterion A: Hours of Exceedance (applying to living rooms, kitchens and bedrooms) and Criterion B: Night hours above 26°C (applying to bedrooms only). Should a dwelling fail either criterion then the risk of overheating is regarded as being unacceptable.
Criterion A
During the non-heating season (1 May to 30 September), the temperature within living rooms, kitchens and bedrooms must not exceed the threshold comfort temperature, Tmax, by 1K or greater for more than 3 per cent of the occupied hours.
Criterion B
To ensure comfort while asleep, the operative temperature in bedrooms from 10 pm to 7 am must not exceed 26°C for more than 1 per cent of annual hours.
Mechanically Ventilated Homes
For homes where temperature control is largely provided via mechanical means, the CIBSE fixed temperature test must be followed. Under the CIBSE fixed temperature test, all occupied rooms should not exceed an operative temperature of 26˚C for more than 3 per cent of the annual occupied hours.
Community Heating
Some buildings will have community heating systems with hot water pipes running through communal areas such as corridors. These communal spaces need to have the risk of overheating assessed. The overheating test for corridors is based on the number of annual hours for which an operative temperature of 28°C is exceeded. No mandatory target exists within community heating. Although, despite this, should an operative temperature of 28°C be exceeded for more than 3 per cent of total annual hours, the area in question is deemed to represent a risk.
How to reduce the risk of overheating:
- Manage heat by incorporating areas of exposed thermal mass into the specification.
- Limit solar gains through the implementation of fixed shading devices, such as blinds, shutters or overhangs.
- Specify low g-value glazing.
- Limit the size of openings, particularly on South and West facing elevations.
- Remove excess heat from the dwelling by specifying sufficient areas of openable windows (the effectiveness of this method is improved by cross-ventilation).
- Specify natural ventilation louvres in external walls.
- Specify a mechanical ventilation system with sufficient ventilation rates.
If all of the above options have been exhausted, and a risk of overheating is still present, then a mechanical cooling system should be specified.
Why choose Achieve Green?
Fast, competitive and accurate CIBSE TM59 overheating risk analysis.
Achieve Building Regulations compliance for Approved Document O.
A complete advice and support service, comprehensive enough to make sure that you achieve compliance in the most cost-effective and practical way possible.
CIBSE TM59 reports can be used to assist building design, demonstrate compliance with Building Regulations, support Planning applications, and adhere to industry requirements.
New dwelling Building Regulation compliance pack
- Part L: SAP calculations and Energy Performance Certificates.
- Part L: air leakage testing.
- Part E: sound insulation testing.
- Part F: extractor fan flow-rate testing.
- Part G: water calculations.
- Part O: dynamic thermal modelling.
COMPETITIVE
Fast and accurate assessments at a competitive price.
EXPERIENCED
Comprehensive advice and support to ensure you arrive at the most economic and feasible solution to achieve compliance.
QUALIFIED
You can take comfort that the certificates we provide will be accepted as evidence for Building Regulations sign-off.
VALUE
Best quality advice, at a price that offers real value for money.
Testimonials
We are an RIBA Chartered Architectural Practice based on the South Coast, and have used the services of the Achieve Green team in Tunbridge Wells on countless projects over many years, mainly for the preparation of SAP calculations but also energy statements and air pressure testing across a broad range of projects. They have offered us invaluable practical advice on more complex projects to ensure compliance with the ever evolving statutory standards and we have always found them to provide a prompt, efficient, friendly and cost effective service and would not hesitate to recommend them.
Lightbox Architecture Limited
We have found the Achieve Green team to be highly experienced, specialised and competent. Advice is constructive and their approach to projects is strategic, ensuring the most effective solutions are taken forward. The team has a comprehensive understanding of planning requirements and appreciates how these translate into technical and on-site delivery. We highly recommend the team and look forward to working with them on forthcoming projects.
Kitewood Estates Ltd
As a business we have been involved with Achieve Green for a number of years. Their proactive involvement in our projects from design through to completion has enabled us to continually improve the energy efficiency of all our homes. The prompt and efficient service has meant they have become an integral part of our design team.
Panfile Ltd
I have been working with Jonathan and his Achieve Green team in Tunbridge Wells since 2011 on a range of projects from Schools, new build housing to straightforward extensions. They are very knowledgeable and efficient and have provided invaluable input to projects over the years ensuring that as a designer I can put forward the most cost effective solutions to complying with Part L.I always recommend Jonathan if ever anybody is in need of SAP and compliance testing services.
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