Glazing

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 10. A significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026. In addition, government consultations indicate future changes to the format of EPCs and minimum rating levels required for private rental properties. Approaches to improving EPC ratings will therefore change in the future.

Glazing is a component of 'building fabric', which is a term that encompasses all the external structural elements that enclose a building, and separate the inside from the outside.

During an EPC assessment, all windows are measured and information is recorded about their type, orientation etc. This information then contributes towards calculation of the building's EPC rating.

Glazing across the whole dwelling is recorded, including any extensions, but glazing in a conservatory is assessed seperately within the methodology.

The thermal performance of windows has improved over recent years due largely of course to tighter Building Regulations, meaning that newer windows are generally 'better' than older ones.

The cost of replacement windows however is fairly high in relation to the potential increase in EPC rating, but it's nonetheless an option to consider, and you may see Recommendations on EPCs to upgrade glazing.

Under the current methodology for existing dwellings (RDSAP 10), an assessor records information about each window in a property individually.

In the past (prior to 15th June 2025) this was optional, and most of the time, a generalisation was recorded by choosing an option for glazing area from: Much less than typical / Less than typical / Typical / More than typical / Much more than typical.

The information gathered for each window now includes:

• Frame Width
• Frame Height
• Area (width x height)
• Type (Window or Roof Window)
• Glazing Type (Single, Double etc)
• Frame Type (Wood/Metal/uPVC)
• Building Part (Main Building / Extension 1-4 etc.)
• Glazing gap (for double glazing)
• Frame Factor
• Location (Man Wall/Alt Wall 1/ Alt Wall 2 etc.)
• Orientation (North, South etc)
• U value (if known)
• g value (if known)
• Draught proofed (Yes/No)
• Permanent Shutters (None/Internal/External etc.)

Some of these items are optional and depend on the selection of 'Glazing Type'.

Doors that have a glazing content of less than 60% are recorded as doors in an EPC assessment.

However, doors that have a glazing content of 60% or more are treated as windows, and recorded in the same way as described above.

The 60% threshold relates to the proportion of glazing in the door relative to the area of the whole door frame.

There are two numerical measures used to represent the performance of glazing: U and g values.

These are used by the EPC methodology to consider the level of heat-loss from the property through the glazing, and also any the level of potential thermal gain from the sun.

Both of these affect how much fuel is used by a heating system in maintaining a fixed temperature in the property, and of course this affects the final EPC rating which is based on financial cost.

U and g values can be described as follows:

U Value - This represents the rate of heat transfer (ie loss) through a window or door. A lower U-value means better insulation and lower heat loss. U-values are expressed in W/m2K. This is Watts per square meter per degree (Centigrade/Kelvin) temperature difference between inside and outside of the window.

g-value - This measures how much solar heat passes through a window or door. G-values are expressed as a number between 0 and 1, with 1 indicating maximum solar gain and 0 indicating no solar gain.

RDSAP, which is the methodology used in creating EPCs for existing dwellings, will ordinarily use U and g values from a lookup table, based on the type of glazing that is recorded in the assessment.

However, if the glazing has been supplied with sufficient documentary evidence from the manufacturer or supplier, then U and g values can be entered manually, to overide those that would be used by default.

Having touched on U and g values, we'll now look at the options available in an EPC assessment to describe the 'Glazing Type' of each individual window. The options are:

• Single Glazing
• Secondary Glazing
• Double Glazing (Pre 2002)
• Double Glazing (2002 to 2021)
• Double Glazing (2022 onwards)
• Double Glazing (Unknown install date)
• Triple Glazing (Pre 2002)
• Triple Glazing (2002 to 2021)
• Triple Glazing (2022 onwards)
• Triple Glazing (Unknown install date)
• Single Glazing (known data)
• Double Glazing (known data)
• Triple Glazing (known data)

Notice that Double Glazing and Triple Glazing are broken down into three datebands. The default U values are better for the more recent datebands because of increased requirements of building regulations introduces over the years. These therefore give better thermal performance and contribute towards a better EPC rating as a result.

By the way, the dates I've used in this list are for England & Wales, as I'm based in England, but they do differ slightly for Scotland and Northern Ireland, and are listed in the next table.

In the list above, the options with 'known data' are the options that would be chosen if sufficient documentary evidence was available, providing the exact whole window u and g values of your glazing. We'll cover that scenario a bit more later.

So, in terms of the U and g values that are used by default in an EPC assessment of an existing dwelling, these are availeble to view in table 24 of the RDSAP 10 Specification document on the BRE website:

Notice the row that you woud expect to be marked up as glazing age '2002 to 2021' is marked up as '2002+'.

Notice also, for double or triple glazed windows with an unknown install date the methodology uses the u and g values as per the 'before 2002' glazing age. 

So, this table indcates the U and g values for each type of glazing that are used by RDSAP 10 by default towards calculation of the EPC rating, and are a measure of the thermal performance of the glazing:

So when we look at the table, it's easy to see that single glazing is by far the worst performer in terms of heat loss from a property, because it has the highest u values.

For example the methodolgy uses a u value of 4.80 for single glazed windows in a wooden frame. The other glazing types are better and range between 1.4 and 3.1. The latest windows have a u value of 1.4

Changing away from single glazing to newly fitted double or triple glazed windows will therefore give the greatest performance improvement.

If you compare that with changing from standard 2002-2021 double glazing to 2023+ double/triple glazing, we can see that will only improve the U value from 2.00 to 1.40 which is not so much of an improvement.

Having said this however, all these figures are the default values assumed by the RDSAP methodology. If you have glazing for which you have documentary evidence of the U and g values, supplied by the manufacturer, and these are better than the default values in RDSAP, your assessor can overide these values with the ones from the manufacturer, and they will be used for the EPC rating calculation instead. You would need to supply sufficient documentary evidence for this however.

I think the U values in the table above are quite interesting, particularly if you have single glazing in a listed building where you can't install double glazing because the local Conservation officer has specifically advised you that you can't.

Simply fitting secondary glazing on the inside of the existing windows will improve the U value used by RDSAP from 4.80 (in a wooden frame) to 2.20/2.90 depending on the type of glass in the secondary glazing. Within the assessment, that performance is not far off that of older double glazing.

As a generalisation, in order to improve your EPC rating you would be looking to install glazing that has better thermal performance (U - value) than your current glazing.

This would typically be double glazed instead of single glazed, or brand new glazing instead of older double/triple glazing.

As you might expect, glazing companies thoroughly recommend upgrading your glazing in order to improve your EPC rating.

However, replacing windows can cost a considerable amount of money, and you should enquire with a local energy assessor about how much your rating would improve before proceeding with any upgrade.

If you already have older double glazing you may decide it's not worth the financial outlay.

We'll take a look at an example property in a moment to see how much difference the glazing makes.

FENSA (the Fenestration Self-Assessment Scheme) is a UK-based organisation that oversees the installation of windows, doors, and other glazing products in compliance with building regulations.

FENSA was established in 2002 to ensure that installations meet the required standards for energy efficiency, safety, and performance, particularly in relation to the Building Regulations set out by government.

They cover installations of replacement windows, doors, roof windows, and roof lights fitted to existing homes. Glazing in new build properties is not covered by FENSA.

Once an installation is complete, FENSA registered installers issue a certificate that confirms the work meets regulatory requirements.

A search facility on the FENSA website can indicate whether a certificate was issued for your property since FENSA's formation in 2002.

This facility is available via the following link, and replacement certificates are currently available for a price of £25:

https://forms.fensa.org.uk/fensa-certificate

If your windows were fitted from 2002 onwards and you have a copy of this certificate, consider presenting a copy of it to your energy assessor as this will help them evidence the age of the windows.

During an EPC assessment energy assessors follow a number of published rules, known as 'Conventions', and these are publically available to view.

The ones for RDSAP 10 (for EPCs on existing dwellings) are available here:

https://bregroup.com/documents/d/bre-group/rdsap-conventions-v12-1-21-august-2025-final

Convention 3.12a decribes how to choose the age of double glazing. The instructions are:

• Choose unknown date if there is no evidence of the date.
• Multiple glazed units [ed: ie double/triple] can be dated via the following methods:

a) The manufacturing date on the spacer bar, or possibly on the frame.

b) There is documentary evidence confirming the date of installation of the window e.g. FENSA / CERTASS / Building Control certificate or manufacturers guarantee.

c) Property build date if after the following trigger dates:

Post 2002 / post 2022 (E&W)

Post 2003 / post 2023 (Scotland)

Post 2006 / post 2022 (NI)

In the absence of any of the above evidence.

d) Presence of thermal spacer bar indicates glazing age post 2002 (E&W), 2003 (Scotland) or 2006 (NI)

• If none of the above applies choose ‘unknown’.

As indicated in point a, if you can locate a manufacturer's datestamp on the window somewhere, then this can be used to age the window.

We've mentioned FENSA certificates in the section above (point b in the convention), and particularly in the case where you do not have a datestamp on your windows, or any other form of documenation, this could help an assessor choose a date range rather than 'Unknown' age.

It also makes sense that if the construction date of the property is post 2002 (England & Wales) etc. then an assessor should be able to confidently choose the 2002-2021 double glazing age in England & Wales (point c).

Point d refers to spacer bars and we haven't talked about them in detail so far. Let's do that now.

Spacer Bars are part of the structure around the edges of double glazed sealed units. They hold the two panes of glass away from each other with a fixed 'gap' between them. Sometimes, a datestamp can be found on the spacer bar, and this can be used to choose the glazing age.

Aluminium was a popular material for these spacer bars to be manufactured from, until October 2010, when following the introduction of tighter thermal regulations, manufacturers changed to 'warm edge' spacer bars made from plastic.

This reduced thermal bridging at the edges of double glazed units and this type of spacer bar did not exist prior to 2007/2008. Unfortunately, the majority of thermal spacer bars from that time don't have datestamps on them but they can be visually identified as such, and therefore qualify for glazing age 2002-2021 (E&W).

The perforations in the spacer bars allow moisture in the glazing gap to traverse into the bar and be absorbed by dessicant located there. There tends to be more holes in aluminimum spacer bars than in the plastic spacer bars.

The picture above shows aluminium spacer bars on the left and the more modern plastic 'warm edge' thermal type of spacer bar on the right, used since 2010.

Examples

Here we'll look at some examples of datestamps and spacer bars.

This is an alimuminium spacer bar in a double glazed sealed unit. There are no datestamps anywhere around the spacer bars, or printed on the frame either so this gets assigned a double glazing age of 'Unknown', unless there is other documentary evidence available.

This is also an aluminium spacer bar but with some information stamped into it this time, but there is no datestamp. This would also qualify as 'Unknown Date'.

This is a thermal spacer bar in a window from an extension built in 2022. There's no datestamp on the spacer bar, but visually you can tell it's a thermal spacer bar and this qualifies it as glazing age 2002-2021 (E&W). Further evidence would be required to qualify it as 2022 onwards.

These are markings on a uPVC door frame I had installed on the 27th October 2020, only about two weeks after the frame was manufactured. This door is much less than 60% glazed and is therefore classed as a door within an EPC assessment. However, had it been classed as a window due to having more than 60% glazing, then it would have clearly qualified for glazing age 2002-2021 (E&W).

This is a uPVC window frame, and a printed datestamp is visible of 30/09/2022. This currentyy qualifies for Double Glazing aged 2022+ (E&W) under the current EPC methodology RDSAP 10. Previously, under RDSAP 9.94 this would have onliy qualified for double glazing dateband 2002+ (E&W) because the 2022+ dateband was not available then.

While you wait for your energy assessor to come round, or even before you initially call them, review the age of your double glazing. You could then present any helpful evidence to the assessor when they come round, and most will be thankful for the information.

You might know the age of your glazing already because you lived at the property when the windows were installed. Otherwise, check the FENSA website, and the local authority building control portal and search for applications related to glazing.

Record any results you find, and consider purchasing a certificate from FENSA if they have one (and you don't have one already), or possibly a duplicate building control completion notice if you don't already have a copy.

Look at your double glazing, and see whether you can locate any manufacture dates on the spacer bars or uPVC frames.

Examine your spacer bars and identify whether they are plastic, aluminium, or possibly an even older type with a black tarry like substance.

In this section, we take our Case Study 2property and model the EPC ratings that result when we select different types of glazing.

This property is a 1950s mid-terraced property with a condensing gas combi boiler, cavity wall insulation and 200mm loft insulation.

The property has 10 windows and 3 doors. All the doors are recorded as doors because they contain less than 60% glazing.

Roughly half of the windows are West facing, and half are East facing:

The figures make insteresting reading, but they won't necessarily relate to your property, which will have many differences compared to this one. Always engage with a local energy assessor first, to see what improvement could be made with your property, before proceeding to spend any money.

What surprises me here is the relatively small difference of 3 SAP points that moving from Single glazing to modern Double or Triple Double glazing makes.

In fact there is no integer value gain in EPC rating for this property if I were to replace the 2002-2021 double glazing that is currently there to anything newer, despite the significant financial cost this would incur.

Even with a u-value of 1.00 and g value of 0.55, which are glazing values from a Passivhaus I'm currently helping the owner with, the EPC rating would still remain at 74C. There may well have been a fractional increase in rating behind the scenes, but not enough to roll the integer value up by a whole number.

Notice also, the improvement that is made by moving from Single Glazing to Secondary Glazing. Secondary glazing with normal emissivity glass scores the same on this property as double glazing with either pre 2002 or unknown installation date. Also, secondary glazing with low emissivity glass on this property scores the same as double glazing with a 2002-20022 installation date.

If you had a property where double glazing was not an appropriate measure to install, then Secondary glazing would be a very good option to investigate.

It's important that you contact a local energy assessor and ask them to model what glazing changes could make to the EPC rating of your particular property, before you carry out any works. This ensures you'll already know the outcome from carrying out any works you have in mind.

There are three conditions under which glazing recommendations can appear on your EPC certificate.

These will appear if the measure increases the SAP rating by at least 1 (one) SAP point:

Double Glazed Windows (replace single glazed windows with double glazed)

• This recommendation is considered when the existing dwelling has Single Glazed windows present.
• This recommendation appears if the existing dwelling has less than 80% of windows with multiple (ie double/triple) glazing.
• The example EPC rating provided after this recommendation has been applied is with all single glazed windows replaced by double glazing with
  U = 1.4 W/m2K (vertical windows) or U = 1.6 (roof windows) and g = 0.63.

Glazing Replacement

• This recommendation is considered when the existing dwelling has double glazing with PVC frames and 12mm gap, installed before 2002 (E&W) or 2003 (Scotland) or 2006 (NI)
• This recommendation appears if at least 80% of windows are of that type
• The example EPC rating provided after this recommendation has been applied is with those particular double glazed units replaced with new units giving whole window values of U=1.4 and g=0.63

Secondary Glazing

• This recommendation is considered when Single Glazing is present but the assessor has de-selected the 'Double Glazing Appropriate' option
• This recommendation appears if less than 80% of windows in the property are of multiple glazing
• The example EPC rating provided after this recommendation has been applied is with secondary glazing to single glazed windows with

U = 2.9 W/m2K (vertical windows) or

U = 3.2 W/m2K (roof windows)
and g = 0.76.
Otherwise, windows with single glazing changed to double glazing with:
U = 1.4 W/m2K (vertical windows) or
U = 1.6 W/m2K (roof windows)
and g = 0.63.

RDSAP Convention 2.16 discusses how secondary glazing should be dealt with, particularly in combination with double glazing. This might be useful to some people:

"If secondary glazing with single glazing, record as secondary glazing. (Normal or Low-E if it can be determined).

If secondary glazing with Age 1 double glazing, record as Age 2 double glazing.

If secondary glazing with Age 2 double glazing, record as Age 3 double glazing.

Age 1 means pre 2002 in E&W (2003 in Scotland, 2006 in NI)

Age 2 means 2002-2021 in E&W (2003-2022 in Scotland, 2006-2021 in NI)

Age 3 means 2022 onwards in E&W (2023 in Scotland, 2022 in NI)

If secondary glazing has been removed in summer, enter as above only if assessor can confirm that the panels exist and can be re-fitted. Evidence to be recorded on site notes"

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