Thermal Performance for Glazing in Buildings Other Than Dwellings
In June 2022 a series of updates to Building Regulations came into effect. Approved Document L (Conservation of fuel and power) considers all aspects of thermal efficiency of a building or renovation, aiming to reduce carbon emissions and the carbon footprint of a building including the thermal performance of glazing.
A big part of this regulation is to improve the insulation of a building, including the glazing, in order to create a highly efficient building envelope that minimises heat losses but also minimises power requirements for the building (reducing overheating to reduce air conditioning and strategic use of natural light to reduce energy costs for lighting).
This article details the updates to Building Regulations Approved Document L and how they affect the specification of glazing in a non-residential (non-dwelling) setting. If you are specifying glass for a non-residential project please do contact us to speak to the team. We would be happy to discuss what is possible from the architectural glazing.
What Buildings are Covered by Volume 2?
Volume 2 of Approved Document L covers the thermal performance of glazing in new or existing buildings that are not dwellings. The thermal performance of glazing in dwellings is covered by Approved Document L Volume 1 – you can find out more about this at the IQ Glass website here.
In previous iterations of Approved Document L, the thermal requirements for glazing were split into two documents that covered new builds and work to existing buildings.
These have now been combined into one document that applies to the glazing specification in any non-residential building.
There are certain exceptions and caveats throughout the document to allow for flexibility to the requirements in special circumstances. As an example:
– places of worship are exempt from energy efficiency standards.
– non-dwelling buildings that are stand alone with less than 50m2 of useful floor space are exempt from energy efficiency standards.
– there are certain exemptions for listed buildings, buildings in conservation areas or protected buildings. You can read more about this below.
– conservatories or porches have certain exemptions depending on the size of the structure. Read more about these below.
Thermal Performance for Glazing on Non-Residential Builds
A key part of Approved Document L and our target for minimising energy usage is to limit heat gains and losses through the external building envelope.
To limit unwanted heat losses Requirement L1(a) explains that you should meet the standards for;
– the building fabric and all external glazing
– air permeability through any external windows or glazed doors
The limiting standards for new or replacement glazing in new and existing buildings are detailed in Table 4.1 in Approved Document L, Volume 2:
Extract from Table 4.1
All new buildings must ensure that the glazing they install has a maximum air permeability of 8.0 m³/(hm²) @ 50 Pa.
Specific Advice for Glazing to Existing Buildings
Section 10 of Approved Document L2 details some specific advice for glazing energy efficiency for existing buildings.
All new thermally insulated glazing on an existing building should achieve the thermal performance requirements set out in Table 4.1 (above), they
should be draught proof and installed with cavity closers where appropriate. This includes:
– new windows or glazed doors in an existing building
– replacement windows or glass doors in an existing building
– glazing in an extension to an existing building
If you are working on a renovation project to an existing building with a need to maintain the character of the building, then there are exemptions or flexibilities to the limiting standards set out in Table 4.1. You have more flexibility in the specification of the glazing However, the Ug value of the glazing (centre pane) should not exceed 1.2 W/m2K or single glazing can be used with secondary glazing added which has a low e coating.
Where a glazed opening is enlarged or created (including rooflights, windows and glass doors) then the area of the glazing either needs to comply with the limiting sizes on Table 10.1 (below) or compensating methods can be taken to improve the overall energy efficiency of the building. This could include achieving a better Uw value than stated in Table 4.1 or improving the building performance elsewhere.
Thermal Performance Requirements for Glazing to Building Extensions
The specific thermal requirements for glazing to an extension will depend on its size and its size in relation to the existing building.
If the extension has a total useful floor area > 1000m2 then ‘consequential improvements’ should be taken (see below).
If the extension has a total useful floor area of > 100m2 OR more than 25% of the existing building, then the extension should be treated as a new building and the relevant thermal requirements for a new building followed.
If the extension is classed as a conservatory or a porch, then different exemptions and performances apply (see below for more information about the exemptions for conservatories or porches).
All other building extensions should follow the glazing thermal performance requirements in Table 4.1. This includes all glazing that becomes a thermal element thanks to a changing building layout or demolition works.
Maximum Sizes of Glazing Elements to Building Extensions or Renovations
Building Regulations Approved Document L, Volume 2 states the maximum area of a glazed opening in a renovation depending on the building type. This has been created under the understanding that glazing has a lower thermal insulation value than a solid construction such as an insulated wall.
That being said, there are certain exemptions to these rules and you can disregard these maximum sizes if you can provide improved thermal insulation or efficiency in other areas of the build. This can be used to provide an alternative approach to compliance as detailed in paragraph 10.10 (below).
Extract from Table 10.1
Display windows, vehicle access doors and smoke vents are excluded from these maximum size requirements. These elements can be as large as required for their purpose.
Alternative Approaches to Compliance for Extensions
The glazing limitations in table 4.1 and 10.1 can provide issues for architects looking to achieve a bold or interesting architectural design for an extension.
With this in mind, Approved Document L Volume 2 provides some alternative routes to compliance, whilst ensuring that the overall building and extension limit energy losses and maintain an efficient level of energy use.
Paragraph 10.10 in Building Regulations Approved Document L Volume 2 details an alternative method to compliance which provides architects and designers with a more flexible approach to extension design. If you can show that the area-weighted U value of the proposed extension elements (including all the glazing) does not exceed the area-weighted U value of an extension of the same size and shape that does comply, then you can design the extension with no limitations on size of glazing or design.
Another alternative approach is detailed in paragraph 10.11 using approved calculations tools and a notional extension. If you can show that the building primary energy rate and building emission rate for the building AND proposed extension do not exceed those for the existing building plus notional extension, then this will also be accepted as compliant. The notional extension should be the same size and shape as the proposed extension and should comply with the listed requirements for glazing performance.
Thermal Performance for Glazed Conservatories or Porches
Firstly, an extension is considered a conservatory or a porch if the space has a thermal separation from the existing building and the heating system for the existing building does not extend into the space.
If there is no thermal separation or the heating does extend into the space, it should be treated as an extension and the relevant thermal requirements apply.
A conservatory or porch is exempt from all energy efficiency requirements if all the below apply:
- It is at ground level
- The floor area does not exceed 30m2
- The glazing complies with part K of building regulations
- There is a thermal separation between the conservatory/porch and the building. If this element is being replaced, then it should meet the thermal requirements for glazing in an existing building.
- The heating system of the building does not extend into the space and the space does not have its own fixed heating.
If the conservatory or porch is not exempt based on the requirements above, then the glazing must comply with the thermal performance requirements in Table 4.1. Although, the limitation on the size of the glazing from Table 10.1 does not apply.
Thermal Performance of Glazing to a Listed or Protected Building
Where a building is protected or listed there are certain exemptions from the thermal performance requirement for glazing. These have been listed
under the understanding that the character of the existing building is important and should not be harmed due to required building works or renovations.
The below building types are exempt from the glazing thermal performance requirements:
- Buildings listed in accordance with section 1 of the Planning (Listed Buildings and Conservation Areas) Act 1990.
- Buildings in a conservation area designated in accordance with section 69 of the Planning (Listed Buildings and Conservation Areas) Act 1990.
- Buildings included in the Schedule of Monuments maintained under Section 1 of the Ancient Monuments and Archaeological Areas Act 1979.
Glazing to these buildings should comply with the energy efficiency requirements wherever possible as long as it does not “unacceptably alter the building’s character or appearance”.
The glazing in new extensions to historical or traditional buildings should comply with Approved Document L2 unless there is a need to match the extension appearance or character of the existing building.
In these circumstances, the advice of the local authority’s conservation officer is taken as to whether full energy efficiency improvements should be made.
Thermal Performance of Glazing to Specific Building Types
Approved Document L Volume 2 covers any building that is not a private house. The types of buildings covered by this document are varied and broad therefore certain elements of specific information has been included to cover all building types.
Live/work units (a building that contains both living accommodation and space for commercial purposes like a workshop or an office) should be treated as a dwelling if the commercial part can be reverted to domestic use. Therefore, the glazing should comply with the thermal performance requirements in Approved Document L Volume 1.
Paragraph 0.21 lists the rules that are used to determine whether a commercial unit can be reverted to domestic. They are:
- There is direct access between the commercial space and living space.
- The commercial and living space are within the same thermal envelope.
- The living accommodation is a ‘substantial proportion’ of the total area of the unit. This is determined by building control on a project-by-project basis.
A small flat for a building manager within a large non-domestic build is not treated as a dwelling whereas, a small office within a house build is still treated as a dwelling.
When creating a mixed-use development, the architectural glazing for the individual dwelling units should be created in compliance with Approved Document L Volume 1 for dwellings. The glazing to the non-dwelling parts of the building (like heated common areas and any commercial/retail space) should be created in line with Volume 2.
Shell and core developments (where the internal fit out work will be made by the incoming occupier) should be designed showing how the building could reasonably meet the energy efficiency requirements after fit out.
The glazing on the actual base building should be constructed in line with the requirements in Volume 2. For the purposes of the calculations for energy rates and emissions rates, the areas to be fitted out should be assumed to be conditioned to the temperatures appropriate to their designated use with no associated energy demands.
Rooms for residential purposes and buildings that contain only rooms for residential purposes (ie apartment blocks) are not dwellings and are covered by the requirements within Volume 2 of Part L.
The common areas of buildings that contain more than one dwelling should follow the guidance in Volume 2 if they are heated. If they are not heated the glazing should comply with the minimum Uw values set out in Volume 1, Section 4.
Thermal Performance Requirements for a Display Window
A display window is defined by building regulations as ‘an area of glazing, including glazed doors, to display products or services on offer to the public within a building’. For a window to be defined as a display window it must be positioned as in all of the following.
– at the external perimeter of the building
– at an access level
– immediately adjacent to a pedestrian thoroughfare.
If a window or glazed façade is determined to be a display window it is exempt from the thermal performance requirements of glazing and also the maximum size limitations as set out in table 10.1.
However, if there is permanent workspace within one glazing height of the window, this cannot be considered to be a display window. Glazing more than 3m above than access level should not be considered part of a display window, expect where either of the following applies.
– the products on display require a greater height of glazing.
– building work involves changes to the façade and glazing that requirement planning consent, and planning requirements mean that a great height of the glazing is necessary. Ie to fit with the surrounding buildings or to march the character of the existing façade.
Glazing Requirements for Consequential Improvements
Where an extensive extension is being added to an existing building, consequential improvements may have to be made to the thermal performance of glazing in the existing building.
This is to ensure that the entire building complies with Approved Document L. If the existing building already complies with Part L then no consequential improvements are needed.
If a building with a total useable floor area of over 1000m2 is being extended or habitable areas are being increased, then consequential improvements to the glazing thermal performance should be conducted. The overall cost of all consequential improvements (including those to the glazing) should not be less than 10% of the value of the main building works. If you need to conduct a cost exercise for consequential improvements that team at IQ would be happy to work with your QS (or other qualified person) to determine the cost of the glazing works to the build and improvements to the glazing. Contact the team to speak to us.
Any consequential improvements should be ‘technically, functionally and economically feasible’. Typically, this means that the improvement works meet the feasibility criterion of a simple payback of 15 years. Table D1 in Appendix D of Approved Document L Volume 2 details out works that have already been pre-determined to be feasible in most normal circumstances.
Extract from table D1
Routes to Achieving the Required Thermal Performance for Glazing
The exact Uw value for any glazing installation must be calculated per project in order to showcase a true thermal insulation value. Section 4 of Approved Document L Volume 2 gives all the ways that U-value compliance can be demonstrated. Calculating the U-value based on the actual size and configuration of the architectural glazing is the first way this can be done.
However, some general guidance in order to achieve these values is to always use thermally broken framing and always use a double glazed unit as a minimum with a low e coating.
All frameless structural glazing should be used with a thermally broken fixing detail (such as the Invisio structural glass fixing system from IQ) with a high performing IGU.
If steel windows and doors are being used on the external envelope then specifiers should ensure that these have a full thermal break in order to comply.
Additional elements to the glazing – such as solar control coatings – can be used to improve the overall Uw value or Ug value of a glass unit.
Triple glazing can be used to achieve performance that improves upon the requirements from Table 4.1 however, consideration must be taken on the impact of triple glazing on other areas of the building. Ie triple glazing is heavier, thicker and requires a deeper rebate for installation. More thought needs to be put into the technical detailing of triple glazing, especially on renovation projects.
Speak to IQ if you would like to consider triple glazing for your project and we would be happy to assist with the detailing and specification works.
What Next
If you are looking for advice or guidance for the specification of glazing to your build, or have any questions about the elements of this article in regard to glazing, please contact the team at IQ.
Our specification and technical teams will be happy to discuss your project requirements and offer solutions that help achieve your requirements for building regulations. As architectural glazing is bespoke, it can be tailored to suit most project requirements in terms of size, detailing and performance.
For further advice or guidance, contact the IQ team today at hello@iqprojectsuk.com or 01494 722880.
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