Building Regulations and Planning Permissions

Overheating in New Dwellings in Wales

modern architectural design in wales

Welsh Building Regulations Approved Document O

Approved Document O is a section of Welsh Building Regulations dictating the requirements of new build dwellings to combat overheating within the design.  

The approved document was issued in 2022 and came into force in the same year.  

This new Approved Document only applies to new build dwellings which are defined as;  

  • new build houses or flats  
  • living accommodations for the care of older or disabled people, or people under the age of 5.  
  • residential accommodation for college students, students and children over the age of 5.  
  • Live-work units (where a space is designed for both living accommodation and commercial purposes.  
  • For mixed use buildings the regulations only apply to the areas of the build that are for residential purposes.  

The approved document aims to improve the health and welfare of occupants of the building, and the overarching aims are as below.  

 an extract of a table from welsh building regulations approved document o

Extract from Requirement O1(1)  

In summary, what you are trying to achieve with adherence with this new regulation is to  

  • Limit unwanted solar gains in the summer  
  • Ensure there are adequate means to remove excess heat from the indoor spaces 

 Forty Farm featured in build it

How to ensure your glazing design adheres to Welsh Approved Document O  

There are two ways in which you can prove your design adheres to the Welsh Approved Document O.  

  1. The Simplified Method 
  1. The Dynamic Thermal Modelling Method 

Due to the type of work that IQ undertake in Wales, most of the projects we are involved in use the Dynamic Thermal Modelling Method to show compliance with the regulation.  

The dynamic method is more flexible and allows designers greater freedom to choose the layout and design of a building whilst reducing overheating.  

The building must be modelled using the CIBSE TM59 Design methodology for the assessment of overheating risk in homes (2017). This must be done by a competent person.  

 Silver House

Requirements of the Dynamic Method for Overheating in Welsh Building Regs  

The key requirements for complying with the dynamic method for overheating are to pass the thermal modelling exercise. There are various calculation requirements detailed in the regulation document.  

The regulation also details a list of acceptable strategies for reducing the overheating risk which are split into strategies for 'limiting unwanted solar gains’ (ie shading) and 'removing excess heat’ (ie cooling or ventilation).  

Below provides an overview of how these requirements can be reached with the glazing and associated design. 


Acceptable Strategies for 'Limiting Unwanted Solar Gains’ under Approved Document O  

The below strategies are acceptable ways to shade your building or glazing design under Approved Document O.  

Fixed Shading Devices for Overheating in Wales 

This includes 

  • Shutters 
  • External blinds 
  • Overhangs 
  • Awnings  

Within the glazing design, shutters and external blinds / louvres can be designed as part of the glazing system to ensure a minimal and cohesive design.  

external horizontal timber louvres on the outside of a gable window

Options for design include a mixture of timber solar shading designs or aluminium shading devices. Just some of the options available from IQ to provide fixed shading are below:  

  • External automated blinds 
  • Timber louvres or brise soleil  
  • External horizontal louvres  

For timber elements to builds - including external shading - we work with the range from Avino to bring the glass and shading design into one. 

You can read more about this in our technical article: Fixed Shading Devices for Glazing Designs .  


Glazing Design for Overheating in Wales 

This includes 

  • Size of the windows 
  • Orientation of the glazing 
  • g-value of the glass 
  • depth of window reveal (which creates shading to the window) 

The g-value of the glass is the key way in which IQ can help improve the glazing design to address overheating.  

The g-value (or g factor) of a glass unit is the measurement of how much solar radiation travels through a glass unit. You can read more about the G factor and what this here in our technical article Controlling Solar Gain & Overheating in Highly Glazed Spaces. 

The easiest way to change the g-value of a glass unit is to include a solar control coating within the glass specification. Solar control coatings are invisible coatings within a glass unit that reflect radiant solar energy away, therefore reducing the amount that comes into a building.  

Find out more about solar control coatings here:  


Unsuitable Shading Strategies for Overheating in Wales 

Internal blinds and surrounding trees or other foliage are not suitable methods for shading glazing under Welsh Building Regulations. Although they can provide shading to buildings, they cannot be used as part of the thermal modelling calculation needed to show compliance with Approved Document O.  


Acceptable Strategies for 'Removing Excess Heat’ under Approved Document O  

As well as reducing the amount of solar radiation that enters a building in order to reduce overheating, designers must also include ways to remove excess heat from internal spaces as well.  

Passive means of cooling must always be prioritized over mechanical cooling wherever possible. Technologies such as air conditioning should only be used when all possible passive means of mitigating overheating have been applied. 

Slim framed triple glazing to luxury newbuild

The most acceptable forms of removing excess heat are:  

  • Opening windows (using cross ventilation where possible) 
  • Venting louvres in the wall  
  • Mechanical ventilation systems 
  • Mechanical cooling systems  

By specifying windows and doors that open, you allow fresh air to enter a space and excess heat to escape. This is an important factor in Welsh Approved Document O.  

When designing this type of air movement through a building, you must also ensure that the building design is also compliant with all other areas of Welsh Building Regulations including the requirements for ventilation (Approved Document F).  

What is Cross Ventilation?  

Cross ventilation is when you include opening windows or doors on opposing sides of a building. This creates a ‘flow’ of fresh air through an internal space from one side of the building to the other.  

The windows and doors on either elevation don’t have to be the same, but they must be openable to allow air in.  

 Rooflight glazing solutions

Usability Considerations for Overheating Designs  

When designing buildings, the space's full use and usability must be looked at. With these wider building elements in mind, you may have to make changes or mitigation to the Approved Document O requirements for overheating.  

These could include;  

  • Noise at night. If it is noisy outside then it is unlikely that the occupant will open windows for cross ventilation. Therefore this might not be a suitable method for removing excess heat and alternative methods might be needed.  
  • Pollution. A similar consideration might be needed of areas where there are high levels of external pollution.  
  • Security. It might not be safe to leave windows open at night for ventilation during sleeping hours so openable windows might not be the best for removing excess heat.  
  • Protection from falling. Only windows with a very low risk of someone falling from height should be used as part of the thermal model calculation. All opening elements of a build must also comply with Approved Document K for safety and Approved Document B for fire safety.  
  • Protection from entrapment. Certain shading systems – like horizontal louvres – must be designed in a way that does not allow a body part to become trapped.  


Providing Information on Approved Document O 

When a builder completes a build, they have a requirement under Welsh Approved Document O, to provide the occupant with instructions on how to use all the technologies used for overheating mitigation.  

This is so that the building will perform as intended with the occupants in place.  

Instructions and directions must be provided for any opening or moving element, including rotating louvres, automated blinds or opening windows. The guide should include all aspects of the build that were designed for reducing overheating including the methods for removing excess heat.  

IQ can assist with this through the submission of our operation and maintenance manuals which we provide to each completed project in Wales, explaining how each aspect of our contract works.  


Ask Us Your Questions 

If you have a current project in Wales, or want further information about how to adhere to the Approved Document O in Wales for overheating, just contact the team at IQ.  

Our dedicated Midlands team provide local service and support for projects across the middle of the UK, including Wales.