Building Regulations Part L1A for Glazing

Building Regulations are a set of governmental rules and guidelines to ensure all building works in the UK are safe, accessible and limit waste and environmental damage.

Part L of the Building Regulations deals with the Conservation of Fuel and Power, ensuring good thermal insulation to all external facades and dictating targets for CO2 emission, thermal efficiency, heating and waste management.

Approved Document L1A covers the building requirements regarding insulation and energy usage for new build private houses (or new dwellings as it is referred to in the document). This includes the thermal performance requirements of the glazing and considerations for design. Read below for an outline of the building regulations requirements for glazing on your new build house project.

For information about the glazing performance to an extension or renovation read our article Building Regulations Part L1B for Glazing.

Glossary of terms from L1A

Approved document L1A uses many acronyms and technical terms that can make trying to read it confusing. Here are a few of the most important terms that relate to the energy efficiency of the glazing on a new build house:

Dwelling = a self-contained unit designed to accommodate a single household

TER = Target CO2 Emissions Rate. This is measured as the mass of CO2 emitted in kg per square meter of floor area per year (kgCO2/(m2year).
The TER and TFEE are the minimum energy performance requirements for a new build house. They must be calculated using SAP 2012.

DER = Dwelling CO2 Emissions Rate (kgCO2(m2 year)

DFEE = Dwelling Fabric Energy Efficiency (kWh/(m2 year)
The DER and DFEE are the actual performances of the building once constructed.

A Notional Dwelling = an example building specification that achieves all building requirements of L1A.

Fixed Building Services = fixing systems for heating, mechanical ventilation, lighting including their associated controls


glazing on a new build home
steel doors to rear extension in London

Glazing Requirements for L1A

The performance of glazing on a new build house is not simply a statement of performance values.

Building Regulations Part L1A looks at the balance between the insulation of the building and the energy demands of the building. It strives to ensure the external building envelope is as insulting as possible, whilst balancing this against reducing the energy requirements of the building.

For example

  • Limiting the glazing on a project will increase the building's reliance on artificial lighting, increasing the houses energy demands.
  • Unconsidered specification of glass without sufficient solar control coatings will increase the buildings energy demands for cooling.
  • Overglazing might reduce the overall thermal insulation of a building.

Part L1A looks to achieve a balance between glazing and other building materials to satisfy the criteria.

The requirements of a new build house are into 5 criteria.

If the areas of glazing is much less than 20% of the total floor area, some parts of the dwelling may experience poor levels of daylight, resulting in increased use of electric lighting

From Criterion 3 of Approved Document L1A.

Thermal Requirements of a New Build House

In order to understand the thermal performance requirements for the glazing on your project, you must first calculate the TER and TFEE of the proposed building. The formula for this is stated in building regulations. The TER and the TFEE are the minimum energy performance requirements for the planned house build.

The TER and TFEE are based on the expected energy requirements of a ‘Notional Dwelling’ as stated in SAP 2012 Appendix R.

If the house is built exactly to the notional dwelling glazing specifications in SAP 2012 appendix R then it will meet the CO2 and fabric energy efficiency targets required. However, designers are free to vary this glazing specification provided they achieve the same or better overall level of CO2 emissions and fabric energy efficiency performance, and that the glazing achieves the minimum performance requirements stated in Criterion 2.

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glazed minimal extension

Criterion 2 of Part L1A and Glazing

Criterion 2 of L1A details out the minimum performance of individual building elements. This allows designers to create a flexible house design as long as they use building materials that reach AT LEAST the minimum performance requirements AND the whole project achieves TER.

The minimum requirements of individual building elements ensure that you cannot achieve the TFEE by solely depending on the very good performance of one area of the building. These minimum standards ensure that a good level of insulation is built into every part of the building envelope.

In order to achieve the TER and TFEE the overall specification of the building will need to be much higher than the minimum requirements.

The U values for the glazing should be calculated using standard calculation methods as stated in BR 443 Conventions for U-value calculations and are weighted U values (Uw values).

Table 2 from L1A lists the minimum performances of all building materials in a new build house, including the glazing and windows. All required and minimum Uw values are taken with the glazing installed vertically. Please note that the actual Uw value of glazing will change if it is installed at an angle. Building regulations are aware of this.

Windows, roof glazing, glazed doors, glass walls etcUw 2.00 W/m2K

The Glazing Performance in the 'Notional Dwelling'

SAP 2012 Appendix R gives full detail of the pre-existing ‘notional dwelling’ glazing specification. As stated above, this is not a requirement but is a prescribed route to achieving the required TER and TFEE rates. Designers are free to deviate from this specification in order to achieve their design goals as long as the minimum insulation values are achieved and the overall project achieves the TER and TFEE rates.

Overview of notional dwelling performance from Table 4 in Building Regulations Approved Document L1A:

Suggested Specification
Opening element (including windows and doors)A maximum proportion of 25% of the total floor area *
Windows, roof glazing, glazed doorsUw value 1.4 W/m2K, G Factor 0.63**
Curtain WallingUw value 1.5 W/m2K
Solid DoorsUw value 1.0 W/m2K
Semi-glazed DoorsUw value 1.2 W/m2K
VentilationNatural (trickle vents in the windows)
Airtightness5.0 m3/hm2

*part L1A also requires the building be designed to bring in an adequate amount of daylight which needs to be balanced.

**higher g-values could be used to increase the amount of solar gain and reduce the energy requirement of the building for space heating. However, designers need to specify the g factor of the glazing based on a balance of solar gain and heating energy demands.

It is important to note that the above is not a requirement, it is just a suggested path in order to achieve the required TER and TFEE rates for new build houses. There are various building design software or algorithms available for designers to play with their building envelope and ensure that the TFEE and TER are met.

Specifiers can ensure that the TER and TFEE are achieved whilst allowing design flexibility by improving the energy efficiency of the building fabric, using more efficient system measures or using low/zero carbon technologies as appropriate.

glazed minimal extension
glazed extension to modern home in London

Criterion 3 of Part L1A and Solar Control Glazing

As part L1A is all about balancing thermal insulation of the building envelope with the energy demands of running the house, careful consideration needs to be taken when it comes to

  • limiting overheating in a space in summer;
  • allowing heat gain to supplement heating in winter and;
  • balancing that against the energy requirement for lighting.

If a house is designed with poorly selected glazing then additional energy might be required to cool the space in summer increasing the energy demand of the building. If a house is designed without sufficient glazing additional energy will be required to light the space artificially with lighting and heat the space in winter.

A balance must be achieved in all designs.

The effects of overheating from solar gain in the summer can be reduced by careful specification of the glazing. The design must consider the orientation of glazing and its size. Solar protection via shading or solar control coatings on the glass is other suitable inclusions into the glazing design.

SAP 2012 Appendix R suggests a G value to the glazing of 0.63.

The building must also be designed in keeping with the ventilation requirements of Building Regulations Approved Document F.

SAP 2012 Appendix P includes instructions on how designers can check a design for solar gain and ensure their proposed design does not have a high risk of high internal temperatures. You need to do this even if the new build house is going to have air conditioning or other forms of mechanical cooling.

When including shading solutions within the architectural glazing design Criterion 3 (2.41) also requires that consideration is given to the amount of natural light entering the home. Building Regulations refers you to BS 8206-2 ‘Code Of Practice For Daylighting’ for further guidance on maintaining adequate levels of natural lighting in a home.


Higher g-values would also compley with the recipe as increasing solar gains reduces the space heat load. However, designers should be aware of the impact of g-value on the risk of overheating and optimise their choice accordingly.

Extract from Table 4 in Building Regulations Approved Document L1A.

Approved Document L1A and Conservatories or Porches

A conservatory or porch is defined as an addition to a building that is outside the normal external building envelope. If a conservatory or porch is included within the design of a new build house you need to follow the guidance from Approved Document L1B if;

  • The thermal separation between the conservatory/porch and the house adheres to the thermal performance requirements L1A as stated in Table 2 and 4.
  • The houses heating system is not extended into the conservatory or porch.

Useful Reading:

What is the difference between a Glass Extension and a Conservatory?

Building Regulations Part L1B for Glazing

If there is not a sufficient thermal barrier (using materials with performances in accordance with Table 2 and 4) between the conservatory/porch and the dwelling, then you need to design the additional area in line with the requirements of L1A.

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Front Door by Urban Front, Structural Glazing by IQ Glass

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Criterion 4 of L1A and Thermal Breaks

Once a new build house has been designed and specified, must also be built to specification and be of high quality. If the house is not built correct or uses poor quality workmanship/materials the energy performance of the building will be affected. It may not insulate as designed or may use more energy than expected.

This is where the DER and DFEE come into play. The contractor calculates the DER and DFEE to demonstrate that the building has been built correctly and to the specification from the designer.

In order to achieve this the building must be built with no thermal bridges (as far as is reasonable). These thermal bridges tend to be caused by gaps within various elements, for example, between the glazing and the insulating wall. To ensure that no gaps occur around the installation of architectural glazing expanding foam should be used between the frame/fixing of the glazing and the structure. In cases of structural glass installation, this small dimension of foam is covered on both sides by structural silicone for sealing.

The calculated Dwelling Fabric Energy Efficiency (DFEE) rate must not be greater than the Target Fabric Energy Efficiency (TFEE) rate.

Approved Document L1A, Section 1.4.

Criterion 4 of L1A and Air Permeability

As listed in Table 4, Approved Document L1A has requirements for the air permeability of a building.

This factor is for the air permeability of the entire building. In order to achieve the required levels, you should ensure that each element of architectural glazing in the house design has a high level of air permeability.

Opening windows and doors are tested for air permeability in accordance with EN 12207. Class 4 is the highest test result possible and indicates the system has been tested with pressures up to 600 Pa.

Useful Reading: Air Permeability in Windows and Doors

Structural glazing is not covered by this testing standard. Fixed structural glazing is instead designed with bespoke fixing details (by the architectural glazier) to ensure the frameless glazing is airtight and built in conjunction with the abutting building finishes.

The high specification sealing technique as detailed above (Criterion 4 of L1A and Thermal Breaks) will ensure that all glazing installations are airtight.

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Architectural Glazing by IQ Glass on RIBA Award Winning Old Fort Road

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Moonstone Contemporary Home with Specialist Glazing by IQ Glass

Criterion 5 of L1A and Maintenance of Architectural Glazing

Approved Document L1A looks at the building as a whole; ensuring the building envelope is insulating but that also running the building uses energy efficiently. Part of achieving the DFEE throughout the lifetime of the building is ensuring that the owner of the dwelling understands how to operate all the fixed building services correctly to maximise efficiency.

One way to ensure this is to provide the homeowner with an O+M (Operations and Maintenance) Manual for the building and all integrated technologies on building completion.

Your architectural glazier should provide the contractor/client with an O+M manual for all the glazing installed on the new build house. This document will include the operating instructions for all architectural glazing. In relation to L1A Criterion 5, this will also include instructions for the operation and maintenance of;

  • Electrical glazing systems such as heated glass/privacy glass.
  • Solar control glazing (cleaning and maintenance).
  • Automated louvre systems used as solar control methods.
  • Electrically operated windows and doors.
  • Auto venting rooflights.

If heated glass is being used as the heat source for the house you may have to improve the required fabric parameters, or you could specify an element of renewable energy (Approved Document L1A Appendix C, 5).

Appendix C of L1A and Glazing U values

Appendix C of Approved Document L1A explains the ways in which you need to communicate the evidence of compliance to Building Control.

The TER and TFEE need to be sent to Building Control at least one day before construction work begins.

Within the report, you should highlight any areas of the specification that deliver much higher performance than expected/required. This allows building control to give particular attention to these key features.

In the glazing specification, if the proposed glazing achieves a Uw value better than 1.2W/m2k building control will give it particular attention and may request further supporting evidence of the performance (if not already supplied)

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Treeside Modern New Build with Specialist Glazing by IQ

What Next?

If you are looking at the glazing specification for a new build house, make sure you speak to one of the experts at IQ.

They will be able to offer advice and guidance to ensure you hit all required building regulation requirements whilst achieving the architectural glazing design you design.

Visit the contact us page for all the ways in which you can get in touch with the team for specification support or request a quotation.

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Rebecca Clayton
Technical Sales
Rebecca is Head of Marketing at the IQ Group and has worked in glazing specification for many years. She has a broad range of technical knowledge about all our glazing products and offers technical advice and guidance to architects for specification. Her easy to digest technical advice is often quoted in magazines and publications. You might also recognise her as one of the IQ Glass CPD presenters.
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