At the time of writing the methodology for EPC assessments of existing dwellings is RDSAP 9.94. The much anticipated RDSAP 10 update is due in Q1 2025 and a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026.
Glazing plays a part in the energy performance of a building, so during an EPC assessment the type and quantity of glazing is recorded.
This information then contributes to the calculation of a building's EPC rating.
The whole dwelling is considered, 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 legislation, meaning that newer windows are generally 'better' than older ones.
The cost of replacement windows however is fairly high compared to the potential increase in EPC rating, but this might still be an option for you if other avenues are limited.
During an EPC assessment for an existing dwelling, curently an assessor can simply summarize the area of glazing in a property by selecting from the following list:
For the middle three options, this currently would be the only entry required to indicate the quantity of glazing. For these options, an algorithm in RDSAP calculates an assumed area of glazing, and this suffices in most cases where the glazing throughout a property is of a consistent type, or is of a resonably typical quantity.
If either 'Much less than typical' or 'Much more than typical' are chosen, then the assessor has to record information about every individual window in the property. This would suit cases where either there is quite a mix of window types in a property, or the quantity of glazing is unusual for the type of building, or if an assessor particularly wants to record the quantiy of glazing accurately. The information gathered for each window includes:
At the time of writing, the methodology for conducting EPCs for existing dwellings is RDSAP version 9.94. In the near future RDSAP 10 will be released (circa Q1 2025), and assessors will no longer be able to choose the quantity of glazing from a drop down list. Instead they will have to measure every window regardless. The intention of this change is to make EPC assessments more accurate.
Note that a door may be treated as a windows within an EPC assessment if it is considered to be highly glazed. If measured, and over 60% glazed then it is classed as a window.
With glazing, for an improved EPC rating you would look to install glazing that has better thermal performance than your current glazing.
This would typically be double glazed instead of single glazed, or triple glazed instead of double glazed.
As you might expect, glazing companies thoroughly recommend upgrading your glazing 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.
We'll take a look at an example property in a moment to see how much difference it can make.
First though, let's look at the different types of glazing that are available within the assessment, and the thermal performance (U-Values) that are used by RDSAP for each one:
Glazing Type |
Default U-Value (W/m2K) |
Default g value (W/m2K) |
Single | 4.80 | 0.85 |
Secondary | 2.40 | 0.76 |
Double (pre 2002) - 6mm gap (any frame) Double (pre 2002) - 12mm gap (uPVC) Double (pre 2002) - 16mm gap (uPVC) |
3.10 2.80 2.60 |
0.76 |
Double (unknown install date) | 2.60 | 0.76 |
Double (2002 onwards) | 2.00 | 0.72 |
Triple | 1.80 | 0.68 |
The information in this table is from Table S14 from SAP 2012 and is for windows in walls. Additional figures are available for roof windows. The references to 2002 are valid in England & Wales (where I live). The date designations are slightly different in Scotland and Northern Ireland.
This table includes the U and g values for each type of glazing, which are used by RDSAP in the calculation of the EPC rating. These values are a measure of thermal performance as follows:
U Value - This measures the rate of heat transfer (ie loss) through a window or door, indicating how well it insulates. A lower U-value means better insulation. U-values are expressed in W/m2K.
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
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 a U value of 4.80, compared to the others which are between 1.8 and 3.1.
Changing away from single glazing to any of the other type of glazing will therefore give a decent performance inprovement.
Comparing that with changing from standard 2002+ double glazing to triple glazing, we can see that will only improve the U value from 2.00 to 1.80 which is not so much of an improvement.
Having said this however, these figures are the performance values assumed in RDSAP. If you have triple glazing for which you have U and G values supplied by the manufacturer, and these are better than the default values in RDSAP your assessor can overidde 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 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 to 2.40. That's not far at all from the performance values for double glazing.
In the near future, when RDSAP 10 is released, an energy assessor will be able to record double glazing fitted from 2023 onwards in addition to the types in the table above. Glazing installed from 2023 will comply with building regulations introduced in 2022 which enforce even tighter U values for glazing in the UK.
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 him evidence the age of the windows.
Convention 3.12 defines how to identify the double glazing age for an EPC assessment (Unknown/Pre 2002/2002 Onwards) as follows:
For my property, which we cover in our Case Study 2 page, here are the EPC ratings that result, when we select different types of glazing. This is a 1950s mid-terraced property with a condensing gas combi boiler, but without cavity wall insulation.
I have assumed a 'Typical' amount of glazing and for the pre-2002 double glazing we've assumed a uPVC frame with 16mm glazing gap:
Description | EPC Rating |
Single Glazing | 66D |
Secondary Glazing | 68D |
Double Glazing (Pre 2002, UPVC Frame, 16mm Gap) | 68D |
Double Glazing (2002 onwards) | 68D |
Double Glazing (Unknown Install Date) | 68D |
Triple Glazing | 68D |
The figures make insteresting reading, but they won't necessarily relate to your property, which will have many differences compared to mine. 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 2 SAP points that moving from Single glazing to Double glazing makes, and also with my property, how all the other forms of glazing produce the same EPC rating of 68D.
I therefore won't be changing my double glazing, which is already 2002+, to triple glazing as there is no obvious gain for my EPC rating, and doing so would incur significant financial costs.
Just to check, I repeated the assessment with cavity wall insulation added to the property, and the results indicated similarly with 70C for single glazing and 72C for all other glazing types.
Click here to return to our 'Improve Your EPC' main page, and see if there is another way you could improve your EPC rating.
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