Fixed Shading Devices for Glazing Designs

Glass Designs for Overheating: Approved Document O

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 Approved Document 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.

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;

The simplified method

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.

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;

Fixed shading devices

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.

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

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.

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 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.

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 hello@iqglassuk.com.

Overheating in New Dwellings in Wales

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.

Extract from

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

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.

The Simplified Method

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.

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).

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.

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

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 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:

Project Examples with Solar Control Coatings

Solar Control Options for Structural Glass

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.

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.