Building Regulations and Planning Permissions

Glass Designs for Overheating: Approved Document O

Cost engineering for glazing package

Building Regulations (England) added its first new Approved Document in years in 2022 with the launch of Overheating: Approved Document O. In the same year, Wales also released its own Approved Document O, and the Scottish government released Scottish Building Standard 3.28.  

The new Approved Documents addressed key concerns within the built environment regarding buildings unsuited to the new warmer climate and undue reliance on cooling technology that increased carbon emissions.  

But how do these new areas of Building Regulations affect the glass design for overheating?  


What Buildings do the Approved Documents O Apply To?  

All of the updated Building Standards / Regulations in relation to overheating are for residential buildings, with the England and Wales Approved Documents only applicable to new build dwellings.  

In Scotland, any residential Building Warrant applications submitted after 1st February 2023 were required to adhere to the new Standard 3.28 of the Scottish Building Standards Domestic Technical Handbook.  

In Wales and England, compliance with Approved Document O is a requirement for any new dwellings.  

 an arched glass extension with solar control coatings

What is the purpose of Overheating: Approved Document O?  

Each of the overheating building regulations documents are designed to ensure buildings are designed to  

  • Limit overheating or unwanted solar gain into buildings 
  • Ensure there are proper measures for ventilation and cooling within the building design 

All of this has the underlying purpose of making buildings that are more comfortable and less reliant on carbon emitting methods of cooling (like air conditioning).  


How can my glass design comply with Overheating: Approved Document O?  

There are two ways to show compliance with Approved Document O;  

  1. The simplified method 
  1. The dynamic method 

Due to the type of glazing we undertaken and the subsequent buildings we work on, most projects we are involved in use the dynamic method to show compliance with approved document O.  

Under the dynamic method, the building must be assessed using the CIBSE TM59 Design methodology for assessing overheating risk in homes (2017) and showcase that it passes.

sliding patio doors with solar control glass

The approved documents detail just some of the ways in which the building can integrate shading and cooling methods into the build. Passive methods of cooling and ventilation are always preferred, and designers must prove that all reasonable efforts have been made to use passive methods before air conditioning can be used.  

The glazing design for overheating should use a combination of all the available shading methods in order to create a cohesive glazing design to limit overheating.  

These methods can be broken down into two categories;  

  1. Fixed shading devices  
  1. Glazing design 


Fixed Shading Devices for Approved Document O  

You can read more about the options available for fixed shading devices in our wider technical article.  

Fixed shading devices for approved document O must be installed on the outside of the building. Options include fixed structures like fixed external battens, louvre roofs, canopies and overhangs in structures.  

Other fixed external shading devices are also suitable such as external sliding or bifolding shutters, external louvre systems or external blinds.  

Internal blinds and shutters are not compliant as shading devices under approved document O and surrounding trees, or foliage can also not be used as part of the thermal modeling.  

 timber louvres over a set of sliding doors

Glazing Design for Approved Document O 

As well as providing some external shading to the glass design, you should also consider the specification, orientation and placement of the glazing itself as part of a build in order to comply with building regulations.  

When designing glazing and placing windows within a build you must consider;  

  • The glazing size 
  • The glazing orientation  
  • The g-value of the glazing  
  • The depth of the window reveal  

All of these considerations for glazing design must work together - along with fixed shading structures - to create a holistic design that reduces overheating. 

overhang over frameless windows creating building shading


How can I change the g-value of my glazing?  

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.  

A solar control coating is a coating applied within the glass unit that is specifically designed to reflect solar radiation.  

Historically, solar control coatings had a tint to them which helped to achieve the g-value needed for the build. Therefore, images of solar control glass that many hold in their minds are of very reflective or blue tinted glass designs.  

mirror glass elevation

Although you can still achieve this look if you want to as part of the design, the technology of solar control glass has come a long way. It is now possible to create solar control glass with a ‘neutral’ glass design that has no discernible tint or additional reflection.  

Below is a chart showing the typical light transmission vs g-value of neutral solar control coatings. You can see that even as the g-value decreases (the solar control gets stronger) that the light transmission stays the same. 

a graph of solar control vs light transmission

A great example of this is the front elevation of our architectural glazing showroom in Amersham. To the left-hand side of our front elevation, we have a 6.5m tall glass facade split into two. The left side is a 6.5m tall sliding glass door and the right side is a 6.5m tall vertical sash window.  

solar control glass on the front elevation of a showroom

One side of this elevation has a solar control coating, and one doesn’t... can you tell which? 

If you want to know the answer give is a call on +44 1494 722 880 or email