What is the difference between Simplified and Dynamic Thermal Modelling?

What is the difference between Simplified and Dynamic Thermal Modelling?

The new requirement O1: Overheating mitigation introduced into the Building Regulations aims to protect the health and welfare of occupants of the building by reducing the occurrence of high indoor temperatures. Requirement O1 can be met by designing and constructing the building to limit unwanted solar gains in the summer, and to provide an adequate means of removing excess heat from the indoor environment.

To that end, the new Approved Document O: Overheating provides two methods that can be used to demonstrate compliance with requirement O1, those being: 

  • Simplified method
  • Dynamic thermal modelling method

Let’s take a look at these two methods.

Simplified method

Broadly, the simplified method categorises the building in order to apply slightly different criteria to determine if there is a risk of overheating.

When using the simplified method, the strategy to reduce overheating risk should be selected according to the location of the new residential building and whether it has cross-ventilation (defined as the ability to ventilate using openings on opposite facades of a dwelling). The simplified method determines two risk categories depending on its location, with high-risk locations being urban and some suburban parts of London, and moderate-risk being everywhere else. It also suggests that areas in Central Manchester may have elevated night time temperatures, and that they should consider following the guidance for higher risk locations for buildings in these postcodes (this is left to the discretion of the BCB/local authority)

Once these factors have been determined, the simplified method looks at two areas, limiting solar gains and removing excess heat

Limiting solar gains

To limit solar gains, buildings (or parts of buildings) should not exceed the maximum glazing areas (as a percentage of floor area of both the residential unit and floor area of the most glazed room), which vary depending on whether cross-ventilation is present, the largest glazed façade orientation and whether the building is in a high or moderate risk location. These are outlined in Approved Document O.

Residential buildings in a high risk location also need to provide shading for glazed areas between compass points north-east and north-west via the south. Shading should be provided by one of the following means:

  • External shutters with means of ventilation
  • Glazing with a maximum g-value of 0.4 and a minimum light transmittance of 0.7
  • Overhangs with 50 degrees altitude cut-off on due south-facing façades only

Removing excess heat

In order to effectively remove excess heat, buildings (or parts of buildings) should equal or exceed the minimum free areas outlined in Approved Document O, including total minimum free area and bedroom minimum free area. Free area is defined as ‘the geometric open area of a ventilation opening…this area assumes a clear sharp-edged orifice that would have a coefficient of discharge (Cd) of 0.62. The total minimum free area is defined as the greater of either a set percentage of the floor area or of the glazing area, whichever is greater, whilst the bedroom minimum free area is defined based on a set percentage of the floor area of the room. Approved Document O offers no guidance on how to calculate the free areas, but does give guidance on how to calculate the equivalent area. The equivalent area is different to the free area, defined as a measure of the aerodynamic performance of an opening. It is the area of a sharp-edged circular orifice through which air would pass at the same volume flow rate, under an identical applied pressure difference, as through the opening under consideration. Approved Document O was updated to include a point noting that the equivalent area of the opening should meet or exceed the free area of the opening, intended to clarify the use of equivalent area values to compare to the removing excess heat targets.

The simplified method is not suitable for buildings with more than one residential unit which use a communal heating or hot water system with significant amounts of horizontal heating or hot water distribution pipework. There are also considerations that may mean the standards of the simplified method cannot be used, including:

  • Noise
  • Pollution
  • Security
  • Protection from falling
  • Protection from entrapment

In such cases, dynamic thermal modelling should be used

Dynamic thermal modelling

This method is suitable for all residential buildings, and may offer the designer more flexibility over the solutions presented in Section 1, particularly in the cases of:

  • Residential buildings with very high levels of insulation and airtightness
  • Residential buildings with specific site conditions that mean the building is not well represented by the two locations (i.e. high/moderate risk)
  • Residential buildings that are highly shaded by neighbouring properties, structures or landscape

To demonstrate compliance using this method, there are a number of bits of guidance that must be followed, including:

  1. CIBSE’s TM59 methodology for predicting overheating risk
  2. The limits on the use of CIBSE’s TM59 methodology (set out in AD:O)
  3. The acceptable strategies for reducing overheating risk (set out in AD:O)

Dynamic thermal modelling takes into account a lot more information than the simplified method, such as local shading (i.e. other buildings), thermal mass, occupancy and internal gains profiles, building materials, and local weather data (although not at a local micro-climate level). Compliance using this method is shown by limiting the internal temperatures (defined in TM59), which varies depending on whether the home is naturally or mechanically ventilated.

Given the much greater level of detail used, the dynamic thermal modelling method offers greater design flexibility than the simplified method. Further to this, buildings that fail the simplified method may pass the dynamic thermal modelling with no alterations to the design, although this does not mean that the use of dynamic thermal modelling will ensure all designs pass Part O requirements.


Clearly, the two methods differ in the routes they take to demonstrate compliance with requirement O1, with the simplified method looking at glazing and free areas, whilst the dynamic thermal modelling method taking a more detailed and holistic view of the building. In our view, the simplified method will be the more widely adopted method to achieve compliance with Part O, given that the dynamic thermal modelling method requires greater expenditure due to the costs of software, training and modelling time. However, achieving compliance with the simplified method will come with changes to the design of buildings, and how these interact with other requirements of the Building Regulations, and the costs they bring, must be considered as well. In some cases the dynamic method may be the more practicable solution overall, even with the greater initial modelling costs in mind. 

Want to find out more?

Find out more about our Overheating Risk Assessment service. 

Article published 08/06/22

Elmhurst Energy Services Ltd. 16, St Johns Business Park, Lutterworth, Leicestershire, LE17 4HB

© Elmhurst Energy Services Ltd 2022. All rights reserved. Elmhurst Energy Services Limited is registered in England, Company Number: 06087105