Building Regulations and Planning Permissions

Building Regulations Part L1 | Architectural Glazing to New Build Dwellings

Thermal Performance Requirements for Windows and Glazing  

What Uw Value Do I Need for My House Build or Extension in the UK?  

How do I know what thermal performance I need for my windows?  

The thermal performance (or Uw value) you need to achieve for windows or glazing on your house build in the UK are dictated by ‘Building Regulations’. Building Regulations are a set of governmental rules and guidelines to ensure all building works in the UK are safe, accessible and are built in line with the government’s environmental policies.  

There are regional differences with Wales, Scotland and Northern Ireland having their own variations of Building Regulations in line with local policies. However, many of the key targets and requirements are the same or very similar. The following article details the specific requirements from Building Regulations for England. You can find out a little more about other regulations below:   

• Guide to Scottish Building Regulations 

• Overheating in New Dwellings in Scotland 

• What Are The Differences Between Welsh Building Regulations And English Building Regulations? 

• Overheating in New Dwellings in Wales 

The thermal performance requirements for house builds or renovations come from Approved Document L of Building Regulations which provides guidance on the ‘Conservation of Fuel and Power’. Approved Document L is split into two volumes:  

• Approved Document L, Conservation of fuel and power, Volume 1: Dwellings 

• Approved Document L, Conservation of fuel and power, Volume 2: Buildings other than dwellings 

In this article we will be specifically looking at the thermal performance requirements of windows for dwellings (houses) which comes from volume 1. For information about the thermal performance requirements of windows and glazing to non-residential buildings you can refer to this article on our dedicated commercial glazing website:  

• IQ Projects: Building Regulations Approved Document L, Volume 2 and Glazing 

What key terms do I need to know to understand Building Regulations?  

Approved document L 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 house build:  

• Dwelling = a self-contained unit designed to accommodate a single household. This could be a single house or an apartment within a larger building.  

• Standard Assessment Procedure (SAP) = the approved procedure for assessing the performance of a new build dwelling. 

• A Notional Dwelling = an example building specification that achieves all building requirements of Approved Document L. This can be used as a building template for a simple build.  

These terms are used to describe the targets that the new build dwelling must be designed to achieve:  

• Target Primary Energy Rate = the maximum primary energy use for the dwelling in a year. This is expressed as kWhPE/(m2,year). It is determined using the Standard Assessment Procedure. This is influenced by the fabric and fuel. 

• Target Emission Rate = the maximum CO2 emission rate for the dwelling. This is expressed as kgCO2/(m2.year). It is determined using the Standard Assessment Procedure. This is influenced by the fabric and fuel. 

• Target Fabric Energy Efficiency Rate = the minimum dwelling fabric energy efficiency (kWh/(m2,year)). It is determined using the Standard Assessment Procedure. This is influenced by the fabric of the building. 

These terms are used to describe the actual performance values of the house as built:  

• Dwelling Emissions Rate = the dwelling CO2 emission rate (kgCO2/(m2.year)). It is determined using the Standard Assessment Procedure. 

• Dwelling Fabric Energy Efficiency Rate = kWh/(m2.year). It is determined using the Standard Assessment Procedure. 

• Dwelling Primary Energy Rate = kWhPE/(m2,year). It is determined using the Standard Assessment Procedure. 

Thermal performance in glazing is denoted by a Uw value. This is the overall weighted U value of a whole window construction.  

Read about Uw values in more depth here.  

What is the Uw value requirement for glazing to a new build house?  

For a new build house (or new dwelling as termed in Building Regulations) you have to ensure that your overall house build adheres to the target performances set out in Approved Document L as a whole structure.  

Architects or the specifier of the house build must undertake calculations to determine what the target primary energy rate, target emission rate and target fabric energy efficiency rate of the new build house are. This is done using SAP.  

The minimum thermal performance requirements for windows and glazing on a house build are listed in Table 4.1. These are called the ‘limiting standards’.  

Extract of Table 4.1 

However, in order to meet the target fabric energy efficiency rate of the house build, the energy efficiency of some elements will need to be significantly better than the limiting standards in Table 4.1. 

The SAP calculation will tell you what the Uw value of your glazing will need to be to achieve the target energy efficiency of the overall build.  

What is the Uw value requirement for glazing to a house extension or renovation?  

For work to existing dwellings (houses), the Uw value of the new windows or doors need to meet the limiting standards of Table 4.2 and also be no worse than what is already existing. This includes glazing to house extension as well as replacement glazing.  

Extract of Table 4.2 

“Why do some people say you cannot have more than 25% glass in an extension?”  

This is a myth! What building regulations actually says is:  

• For enlarging window openings: “If the area of windows, roof windows, rooflights and doors exceeds 25% of the total floor area of the dwelling, compensating measures should be taken to improve the energy efficiency of the dwelling.” (Section 10.5)  

• For extensions: use the SAP to show that the “dwelling primary energy rate, the dwelling emission rate and the dwelling fabric energy efficiency rate for the dwelling and proposed extension do not exceed those for the dwelling plus a notional extension.” (Section 10.10).  

Essentially, you can absolutely use more than 25% glass as long as you ensure the overall performance of the extension meets Building Regulations. This will typically involve improving the thermal performance of the windows or other aspects of the building fabric. You have to have a calculation that shows this.  

What if I have to maintain the character of a building with my replacement windows?  

When the replacement glazing is to a traditional building there may be limitations on the changes you can make to the glazing. If you need to maintain the character of the existing building, installing double glazing or new thermally broken frames may be detrimental to the existing design of the house. 

If this is the case, you can either: 

• Use glazing with a maximum Ug value (centre pane performance) of 1.2 W/m2K.  

• Use low emissivity secondary glazing. 

• Any replacement window, door or rooflight must also be draught-proof and use insulated cavity closers on installation where possible. 

If in the replacement of a window or door you are also enlarging the window opening, then you need to check the percentage of glazing against the total floor area of the glazing. If the area of windows, roof glazing or glass doors exceeds 25% of the total floor area of a house you need to achieve a higher thermal performance than that detailed in Table 4.2. 

What is the Uw value requirement for glazing to a conservatory or a porch?  

For a building element to be classed as a conservatory or a porch it must have thermal separation from the main house or dwelling. If it is thermally separated like this, the conservatory or porch is exempt from the energy efficiency requirements of Building Regulations.  

The glazing or windows should meet the minimum standards in Table 4.2 above.  

You can read our more in-depth article “What is the Difference Between a Glass Extension and a Conservatory?” here.  

Are there any exemptions from the thermal performance requirements of Building Regulations?  

There are certain exemptions from Approved Document L Volume 1 if adherence to these rules would detract or harm the visual character of a building. 

This includes: 

• Listed houses (Section 1 of the Planning (Listed Buildings and Conservation Areas) Act 1990) 

• Houses in a conservation area (Section 69 of the Planning (Listed Buildings and Conservation Areas) Act 1990) 

• Houses included in the schedule of monuments (Section 1 of the Ancient Monuments and Archaeological Areas Act 1979). 

• Historical or traditional houses where there is a need to match the external appearance or character of the existing building. 

Extensions or renovations to these types of buildings are treated on a case-by-case basis by the local authority. The building control body should take advice and guidance from the conservation officer as to what is deemed appropriate for that building. 

The conservation officer will determine what works can be undertaken to the sensitive building to maintain the character of the building. Any works should adhere to Document L wherever reasonable and possible within the guidance given by the conservation team. 

What air tightness is needed for my glazing?  

Air permeability is the phrase used by Building Regulations to measure the air tightness of a building. New build houses have to meet a certain requirement for air tightness.  

The limiting air tightness for new build houses (the worst performance allowed) is 8.0m3/(h·m2) @ 50Pa and 1.57m3/(h·m2) @ 4Pa. This equates to Class 2 in the European air permeability test EN 12207.  

You can learn more about air permeability in windows and glass doors here.  

Similar to thermal performance, the design air permeability (which is the target air permeability for the project) is set at the design stage. This is then assessed once the building is complete through an on-site test.  

Why do some people say you need a Uw value of 1.2 for a new build house?  

The requirement for a 1.2 W/m2K Uw value for a window to a new build house is based on the ‘Notional Dwelling’ framework. This is a ‘cookie cutter’ house design which is known to be suitable under SAP calculations. It can be used as a more simplified way to achieve compliance with Building Regulations.  

However, your SAP calculation will tell you what the Uw value requirement for your windows and glazing will be. These must be better performing than the limiting standards in Table 4.1 or 4.2.  

Designing out thermal bridges in glazing design 

Once a new build house has been designed and specified, it must also be built to specification and be of high quality. If the house is not built correctly 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. 

The position of the glazing within the build-up of the insulated wall is important in ensuring the continuity of insulation. 

“Windows and doors should be installed in such a way that the thermal integrity of the insulated plane is maintained” (Approved Document L Volume 1, 2021 edition, paragraph 4.15). 

This includes: 

• Ensuring minimal tolerance around the window or glass door. 

• Installing the glazing so that it overlaps the inner face of the external leaf to be continuous with the insulation of the external wall. 

• Installing the window or glass door in openings where continuous cavity closers have been used. 

Paragraph 4.15 also details the importance of detailed drawings for the build. IQ create bespoke technical design drawings to show where our glazing sits within the wall opening to ensure this aspect of the requirement is met.  

These drawings are also important in ensuring the limitation of thermal bridges within the design. Paragraph 4.17 details the importance of detailed drawings of junctions to reduce thermal bridging. 

Considerations of Document L with Other Areas of Building Regulations 

Approved Document L Volume 1 only considers the thermal insulation and energy efficiency of a building. It is designed to move all homes towards a carbon natural future. However, it cannot act alone. 

Architects and specifiers must also consider all other aspects of Building Regulations when designing bespoke homes.  

The below other Approved Documents from Building Regulations have cross over with the thermal insulation of the building and energy efficiency and must also be considered. 

• Approved Document F for Ventilation 

• Approved Document O for Overheating 

• Approved Document Q for Security 

Further Reading 

• What building regulations do I need to consider for architectural glazing? 

• Air Permeability in Windows and Doors 

• What is the Difference Between a Glass Extension and a Conservatory? 

• Glass Designs for Overheating: Approved Document O 

• Building Regulations Part Q for Glazing 

• What is a U Value or Rating? 

• What is the difference between U Values and a BFRC certificate? 

• What Are the Differences Between WER Ratings and Uw Values? 

• How to Improve the Thermal Performance of a Window 

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 desire. 

Click here to get in touch with the technical team for specification support or request a quotation.