As touched upon within some of our recent blog posts, with more people than ever working from home during the last few months, having a comfortable living and working environment has become essential. One of the key issues affecting occupant comfort is excessive internal temperatures. This has come to the forefront of our attention particularly with extended heat waves over the last few years!

The technical manual TM59: Design methodology for the assessment of overheating risk in homes introduced by CIBSE aims to address specific criteria from which overheating risk in homes can be assessed. With this post we aim to briefly describe the criteria of TM59 and some of the important factors to consider when designing a building to reduce the risk of overheating.

How do we assess overheating risk?

The assessment method looks at the amount of time a set temperature has been exceed. This is done through Dynamic Simulation Modelling (DSM) where a 3D copy of the building is created within and inputs around the construction, windows and heat gains imputed.  Areas which are only occupied infrequently such as bathrooms and hallways are also included in the model to give a rounded picture of air movement within the home. To show compliance within a naturally ventilated building, both criteria listed below will need to be met:


Overheating Assessment Criteria
Predominantly Naturally Ventilated Homes
Criteria A: Hours of Exceedance

For living rooms, kitchens and bedrooms: the number of hours during which DT is greater than or equal to one degree (K) during the period May to September inclusive shall not be more than 3 percent of occupied hours. (CIBSE TM52 Criterion 1:Hours of exceedance).

Criteria B: Night-time Comfort

For bedrooms only: to guarantee comfort during the sleeping hours the operative temperature in the bedroom from 10 pm to 7 am shall not exceed 26°C for more than 1% of annual hours. (Note: 1% of the annual hours between 22:00 and 07:00 for bedrooms is 32 hours, so 33 or more hours above 26 °C will be

recorded as a fail).

What should we consider in the building design?

In essence, preventing overheating comes down to a balance between reducing unavoidable heat gains and providing a way for these can be dissipated. As with all aspects of building design, the earlier overheating is considered, the more the design can be influenced to limit this risk.


Window design and opening area


Clearly there is some benefit to large windows from the natural daylight that they provide however fixed glazing should be avoided where possible and large opening sections would be much more desirable allowing for more natural ventilation through window opening to purge warm internal air.


Important aspects to consider with window openings are safety and security concerns. Window restrictors may be required, in which case the opening portions of the windows should be enlarged to compensate. Other points to consider would be how window opening would interact with shading devices such as blinds and shutters.



Night-time ventilation

With the standard set for night-time temperatures within bedroom spaces, keeping windows open to some degree overnight is crucial. For ground floor and other accessible windows this may bring safety concerns therefore including security devises will be necessary here.  Where noise is an issue night-time ventilation will not be desirable so other mitigation measures will need to be maximised.


Internal heat gains

Although less of a factor, internal gains occur from lighting, equipment, cooking and from the building occupants themselves do also have an impact. Heat gains of lighting can be limited by installing efficient LED lighting which emit much less heat than older halogen bulbs.


Glazing properties

One of the key glazing properties is the ‘g-value’ which indicates the levels of solar radiation that can pass through the window. The benefit of free solar heat gains is an integral part within Part L

SAP Calculations so it’s important that overheating assessments and energy efficiency are assessed as a balanced approach with occupant comfort at the fore. Lower g-values are desirable for reducing the risk of overheating.


Controllable Shading Devices

As mentioned above, reducing the levels of solar heat gains is an important concept of the mitigation strategies. Although internal blinds can be used, by the time that the blinds are hit by incoming sunlight the heat is already inside and the blinds will act as a radiator directing some of the heat into the room.  Fixing external shading devices are much more efficient as the sunlight is stopped from entering the home in the first place.

You should consider how the shading device will affect window openings, for example if louvered external shutters are proposed, the window may need to open inwards to allow for natural ventilation while the shutters are in use.


How can Greenbuild Help?


Greenbuild have a wealth of experience with overheating assessments and façade analysis working on projects ranging from modern apartment buildings to care homes and hospital research facilities. We can advise on the best course of action to reduce the heating demand and give bespoke recommendations for your project.


With the impacts of climate change, higher temperatures and regular heat waves are predicted to become a more frequent feature of the UK climate. Therefore, it is vital that today’s buildings are designed with this future climate in mind, ensuring a comfortable living and working environment.

Get in touch to find out more.


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Written by Owain Morgan | Sustainability Consultant