Glazing

At the time of writing the methodology for EPC assessments of existing dwellings is RDSAP 9.94. The much anticipated RDSAP 10 update has not happened yet and a significant methodology overhaul from RDSAP to the Hone Energy Model (HEM) is proposed for 2025.

In an EPC assessment curently, using RDSAP, an assessor can summarize the area of glazing in a property by selecting from the following list:

• Much less than typical
• Less than typical
• Typical
• More than typical
• Much more than typical

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.

If either 'Much less than typical' or 'Much more than typical' are chosen, then the assessor has to provide measurements of every window in the property. Information for each window would include: Orientation (North, South etc), Frame material, Glazing Type (Single, Double etc), glazing gap (for double glazing). These options are generally chosen if there is a mixture of different glazing types in the property.

In the future when RDSAP 10 is released, the above list will not be available, and the assessor will have to measure every window regardless. The idea is that this will make EPC assessments more accurate.

In terms of types of 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.

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:

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. 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 you 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 documentation to your energy assessor in this situation.

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.

Information to follow here regarding how to determine whether double glazing is declared in an EPC assessment as pre-2002, 2002 or later, or Unknown (not that it makes any difference to my property).

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 asumed a 'Typical' amount of glazing and for the pre-2002 double glazing we've assumed a uPVC frame with 16mm glazing gap:

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 difference would 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.

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.