With recent efforts focusing on improvements to insulation, British homes face the threat of overheating, especially during the warmer months. To tackle this risk, the Government published Approved Document O in December 2021. This accompanied the publication of the 2021 edition of Part L and Part F of the Building Regulations for non-domestic buildings and dwellings.
Part O (Overheating) applies to new build homes and buildings where people sleep overnight. This means pre-existing buildings and commercial spaces will be unaffected by these changes.
Approved Document O is set to take effect in England on 15 June 2022. It will not apply to work subject to a building notice, full plans application or initial notice submitted before that date.
Demonstrating Compliance
Domestic dwellings subject to Part O will have to demonstrate compliance via one of two proposed methods outlined in Approved Document O. These methods are summarised below:
The Simplified Method: this assessment considers the size and orientation of glazing and compares the glazing area to the floor area. Window opening areas are also considered, to deliver suitable purge ventilation.
Dynamic Thermal Analysis (CIBSE TM59): this can be used to ensure dwellings comply with the document by modelling the building to predict the risk of overheating and offers a range of strategies for reducing this risk.
The Simplified Method
The simplified method aims to limit unwanted solar gains in summer and to provide an appropriate means of removing excess heat from the indoor environment through openable windows.
It is a risk-based method of assessment which adjusts the window to floor area ratio based on if cross ventilation is present, and if the building is in a ‘high risk area as outlined below:
- ‘Moderate risk’ location – England, excluding high risk parts of London
- ‘High risk’ location – urban and some suburban parts of London
Drawbacks of the Simplified Method
There are a number of cases in which the simplified method will not suffice:
- Most single aspect apartments will fail the simplified method
- Schemes with communal heating system contain significant amounts of horizontal heating or water distribution pipework cannot be assessed via the simplified method
- Any site with acoustic constraints will fail the simplified method
- Any site with air quality constraints will fail the simplified method
Dynamic Thermal Modelling
Approved Document O outlines an alternative method of assessment to demonstrates overheating complaince. Alternatively, the dynamic thermal modelling method provides a standardised approach to predicting overheating risk for residential buildings using dynamic thermal modelling.
To demonstrate compliance using the dynamic thermal modelling method, Approved Document O sets out clear guidelines which will need to be followed rigorously. You can read about these in more detail in the Government’s online publication of Approved Document O.
The building control body will require a report that demonstrates the residential building passes CIBSE’s TM59 assessment of overheating.
Key Findings
To sum up the key takeaways from Approved Document O: there are two methods of assessment which demonstrate compliance with Part O.
These are the simplified method and thermal dynamic modelling. The simplified method takes into account location, orientation, window sizes and cross-ventilation of domestic dwellings. However, there are a number of significant cases in which the simplified method cannot be used and TM59 overheating assessments will be needed to comply with Approved Document O when it comes into effect.
