Technical Glazing Terms

What is a U Value or Rating?

A ‘U value’ is the term frequently used to describe the thermal performance of glazing and window installations. The term ‘u-value’ (or U factor) isn’t a very specific term and can be used to describe various elements of thermal performance. All elements of U factors are generally calculated using BS EN 673:1998, the standard test method for determining thermal transmittance in glass and glazing.

U Values in Glazing

A calculated U value indicates the heat loss through a material or installation and is measured in units of W/m2K.

When the ‘U value’ decreases means that less energy is able to travel through the material; so the lower the U value the better the thermal performance.

When it comes to architectural glazing specification, there are various elements of ‘U values’ that are relevant:

  • Ug Value = indicates the thermal performance of the glass panel.
  • Uf Value = indicates the thermal performance of the framework.
  • PSIG = indicates the thermal performance of the glazing spacer.

To calculate the overall thermal performance of an architectural glazing installation you combine these thermal elements together, along with the sizes of the opening, opening configurations and framing design, to give you an overall U value for the installation referred to as the Uw value.

dover-court-road-side-infill-extension-800 x 1000 product image

“The U Value is….” It is important to make the distinction between the Uw value and Ug value when specifying glass as some glaziers with low performing systems will quote the Ug value as a generalised ‘U Value’ in the hopes of making their system seem more insulating than it is. Always ensure you know what ‘U Value’ your glazier is quoting.

Building Regulations U Values for Glass

It is the Uw value that is important when attempting to achieve good thermal performance in glass installations.

It is the Uw value that matters when looking to adhere to Building Regulation requirements for thermal performance in glazing.  When renovating or extending a home the glazing elements must have a Uw value of 1.6 W/m2K or better. For a new build house you have to look at the energy requirements of the building as a whole but the glazing cannot have a Uw value of worse than 2.00 W/m2K.

You can read more about Building Reg’s thermal performance requirements for glazing in our following technical articles:

Building Regulations Part L1A for Glazing (new build homes)

 Building Regulations Part L1B for Glazing (extensions/renovations)

All Uw value requirements for glazing are taken with the glazing in a vertical position. This is because the thermal insulation of glazing changes depending on what angle it is installed in. For this reason Building Regulation requirements for glazing and Uw values are always stated for glass in a vertical position. You can find out more about this thermal phenomena in our technical article here: Thermal Performance of Horizontal Glass

home-farm-close-glass-extension-800 x 1000 product image

Improving Uw Values of Glazing

To ensure a minimum level of insulation all glazing units should be double glazed as a minimum with a low e coating and an argon gas filling. The low e coating reflects radiant heat energy from inside back in, minimising the amount of energy that escapes through the glazing. An argon gas filling is heavier than air so reduces the speed of the convection currents in the gas cavity that help heat escape.

All insulated glass from IQ is specified to this standard as a minimum. This glass specification will produce a glass unit with Ug Value of 1.0-1.1 W/m2K.

There are various ways of improving the Uw value of an installation. Obviously, improving the individual U factors in the calculation will improve the overall thermal performance.

Using higher specification glazing and specialist coatings on the glazing can improve the Ug value of the glass (the centre pane performance).

To ensure a good Uf value (which will ensure a good overall Uw value) the fixings or framing that you use should be fully thermally broken. You can read about why this is important in our technical article: Thermal Breaks in Metal Windows and Doors

Having true glazing bars within a framed window or door will also affect the Uw value of the glazing causing a reduction. You can find out more about how glazing bars affect the Uw value of window systems here at our technical article: True vs Applied Glazing Bars in Windows and Doors.


Triple Glazing vs Double Glazing

Triple glazed units have a better Ug value and are more highly insulating than double glazing units. To improve the thermal performance of a glass installation you could use triple glazing, however there are various considerations that you need to go through before you specify triple glazing on a project.

Triple glazing units have an additional glass pane in them and another gas cavity. This makes triple glazed units a lot deeper than double glazing.

This is not a problem when designing structural glass installations as the fixings are all designed around the glass specification. Triple glazing can easily be used in structural glazing whilst maintaining the frameless glass design that is wanted.

Many framed glazing systems are not designed to hold a deeper glazing unit and therefore triple glazing cannot be used in them.

For example, the minimal windows® luxury sliding door system has a very slim 21mm sightline, making it a favourite of architects all over the UK. However, the system has a maximum glass thickness of 32mm meaning that triple glazing is not possible.

The minimal windows 4+ is an upgraded version of the slim architectural sliding door that has been designed to accepted deeper glazing units (up to 54mm deep) and was specifically designed to allow for triple glazing units.

Triple glazing is also heavier so careful note must be taken of the weight of the glass units, especially when it is being specified in opening elements.

park-road-triple-glazing-800 x 1000 product image

U Value Standardisations

It is important to note that all ‘standard’ or ‘typical’ Uw, Ug or Uf factors given for architectural glazing installations are standardisations.

The size of the glazing installation, the configuration of the glazing and the glass to frame ratio will all effect the overall thermal performance of a glazing installation. The Uw value for your glazing installation can be easily calculated, however we do need to know the particulars of the installation to provide an accurate calculation.

For example, a 2-pane minimal window® installation, measuring 5m tall by 3m wide with a double-glazed unit would achieve a Uw value of 1.3 W/m2K. If we changed exactly the same opening size to a 3-pane configuration (introducing more frame into the same sized opening) and kept the glass specification the same, the expected Uw value would then be 1.39 W/m2K.

As you can see from above, even a small change to the architectural glazing installation can cause a change in the Uw value. We can easily do this calculation for you based on your exact opening size and sliding door configuration to generate the Uw value for your project.

number-44-minimal-windows-house-800 x 1000 product image

Further Reading

Building Regulations Part L1A for Glazing (new build homes)

Building Regulations Part L1B for Glazing (extensions/renovations)

True vs Applied Glazing Bars in Windows and Doors

Thermal Performance of Horizontal Glass

Thermal Breaks in Metal Windows and Doors

IQ Glass Product 800x1000 glazed extension

What Next?

If you are looking at the specification of the architectural glazing on your project the team at IQ are happy to help. Just visit our ‘Contact Us’ page to find out all the ways in which you can get in touch with us. Our highly technical team are able to answer questions about the glazing specification on your project, as well as offer solutions to improve the Uw value of your glazing if required.

You may also want to organise to visit our architectural glazing showroom in Amersham for a one to one technical meeting or arrange for one of our very popular architectural glazing CPD’s.