Overheating Assessments and UK Building Regulations

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Overheating Assessments need to become a compulsory part of UK Building Regulations.

With last week’s heat wave in the UK now behind us it has given us time to reflect on the need for overheating calculations to become a requirement in Part L Building Regulations or as a part of Local Authority Planning Requirements.

With temperatures reaching as high as 38°C here in the UK last week and with a potential to reach over 40°C, as shown in Paris and other European Cities, there is a clear need to deal with overheating experienced within our buildings.

overheating-assessments-and-uk-building-regulationsIs there a Solution?

At first glance the solution may seem like a simple one, ‘why don’t we just install air conditioning into more of our homes and offices’, and on the face of it that would work. But this would come at a cost of higher energy usage, carbon emissions, and expenditure. This would also be extremely detrimental to the UK’s plan to reduce carbon emissions to almost zero by 2050.

Current Methodology

Current methodology used to assess overheating in both new dwellings (Part L1A) and new commercial buildings (Part L2A) is rudimentary at best.

New dwellings fall under the SAP assessment overheating methodology which uses a static heat balance for the months of June, July and August, for which the average temperature is calculated. It is heavily dependant on the effective ventilation air change rate and therefore be easily made to comply by increasing ventilation rates. Not that it even needs to comply, as a building that is showing as having a risk of overheating could still pass the overall SAP assessment.

For new commercial buildings falling under Part L2 of the Building Regulations, there is no overheating assessment carried out as such, what it does do is make a limiting solar gain check. What that means is that the amount of solar gain experienced through a buildings glazing is compared to that of a notional building. If the proposed building is lower, then it passes this criterion.

Proposed Methodology

As you can, the two methods currently employed by Building Regulations to assess overheating in both new dwellings and commercial buildings is not the greatest. There are now more up to date and comprehensive overheating assessments that can be carried out for both domestic and commercial properties.


For domestic properties the new CIBSE TM59 Assessment of Overheating Risk in Homes provides a set of criteria that determines whether a dwelling will suffer from overheating.

Criterion 1: For Living Rooms, Kitchens and Bedrooms – The number of hours during which ∆T is greater than or equal to one degree (K) during the period May to September inclusive shall not be more than 3% of the occupied hours.

Criterion 2: For Bedrooms Only – The operative temperature from 10pm to 7am shall not exceed 26°C for more than 1% of annual hours (32.85 hours).

Bedrooms must pass both criteria in order to satisfy the CIBSE TM59 Overheating Assessment.


CIBSE TM 52 is the latest guidance for overheating in non-domestic buildings. The TM 52 calculation uses the CIBSE DSY (Design Summer Year) weather files.

There are 3 separate DSY weather files available which represent different types of hot summers.

  • DSY1: Moderately Warm Summer
  • DSY2: Short, Intense Warm Spell
  • DSY3: Long, Less Intense Warm Spell

The DSY1 weather data is used to show compliance with TM52, DSY2 and DSY3 are used to explore further scenarios and how the building would react to these.

CIBSE TM52 has 3 criteria to assess the level of overheating and it must pass 2 of the 3 to pass the guidance overall. The TM52 overheating compliance criteria is as follows;

Criterion 1: Hours of Exceedance – The number of hours during which ∆T is greater than or equal to one degree (K) during the period May to September inclusive shall not be more than 3% of the occupied hours.

Criterion 2: Daily Weighted Exceedance – To allow for the severity of overheating the weighted exceedance shall be less than or equal to 6 in any one day.

Criterion 3: Upper Limit Temperature – To set an absolute maximum value for the indoor operative temperature the value of ∆T shall not exceed 4K.

Moving Forward

It is clear to see that overheating within UK Building Regulations is behind with the times, and it needs to incorporate more extensive overheating assessments into its requirements within its next update.

At present only major developments within London require a detailed CIBSE TM 52 or CIBSE TM 59 overheating assessment as part of an overall London Plan Energy Assessment when applying for planning permission. This is a great step forward in designing against overheating is it allows changes to be incorporated at the very early design stage, and the cooling hierarchy of the London Plan ensures that passive cooling techniques are investigated first.

The new Building Bulletin 101 2018 – Guidelines on Ventilation, Thermal Comfort and Indoor Air Quality in Schools, also states that free-running (naturally ventilated) schools should carry out an overheating risk assessment in accordance with the CIBSE TM52 methodology but with slightly different compliance criteria.

So, there is progress being made into the design against overheating and we feel that is about time the UK Building Regulations requirements for both new dwellings and commercial buildings catches up.

How we can Help?

Here at EEABS we have years of experience in carrying out overheating assessments on a wide variety of building types. We use full dynamic simulation thermal modelling to produce the most accurate representation of your building and can investigate a number of different designs or constructions quickly and easily.

Overheating assessments can be run to any industry standard such as CIBSE TM52, CIBSE TM59, BB101 2018, and HTM 03-01, we can even let you know the peak temperatures occurring within any room at anytime of year.

An overheating assessment is more than just a box ticking exercise too, we have saved clients considerable amounts of money by using passive cooling techniques to reduce their cooling plant sizes and reduce their annual cooling energy costs.

Contact us today to see how an effective overheating assessment can reduce your construction and energy costs, whilst also providing a more pleasant internal environment for the building’s occupants.

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