Thermal Performance of Glazing Explained

In construction, glazing systems such as windows and doors are given U value which dictates their thermal performance.

This is tested by measuring the heat flow rate through the element as well as monitoring the temperature on either side under controlled conditions.

Overall U values for glazing systems are generally calculated using BS EN 673:1998, the standard test method for determining thermal transmittance in glass and glazing.

It is written in the unit of W/m2K, this essentially means the amount of energy (Watts) that travels through the surface of the material at normal temperatures (Kelvin).

Therefore the lower the value the better, as less heat travels through the system.

 industrial style aluminium casement window with glazing bars

U values explained

There are three U values that relate to the thermal performance of windows, doors and roof glazing, these are the Ug, Uf and Uw value (also known as the overall U value).

The Ug value refers to the centre pane performance of the glass unit. This tends to be the lowest and therefore the best out of the three U values.

Glass is not a conductive material and factors such as specialised coating and double or triple glazing can bring the Ug value down further.

Uf value is the thermal performance of the frame, this tends to be higher than the Ug value.

For the Uw value, also known as the overall U value, the Ug and Uf value are combined taking into account size and configurations.

Some companies give the Ug value as the U value which can lead the customer to believe the systems have a higher thermal performance than they do.

This is why is it important to always make sure your glazier is quoting the correct ‘U values’.

modern aluminium framed glazing from sieger
modern aluminium framed glazing from sieger

Building regulations

Building Regulations Part L documents state the standards windows and door have to meet in England and Wales, these can be found online through the online planning portal.

The current version of the Building Regulations L1A sets the energy efficiency standards for new build homes.

This document states that windows, roof lights and glazed doors must have an overall U value of 2.0 W/m2K or below.

This allows architects more freedom with design, however, they must ensure the overall energy efficiency of the house satisfies the TER (Target CO2 Emission Rate) and TFEE (Target Fabric Energy Efficiency) rates.

Building Regulations Part L1B covers the values required for existing buildings.

In accordance with this document, the overall U value for windows and rooflights cannot be higher than 1.6W/m2K and doors with over 60% of the internal face glazed cannot be above 1.8W/m2K.

Our team at Sieger can advise you on which systems are best for your type of project.

All Sieger window and door systems come with a low e coating and argon gas filling as standard and by utilising triple glazing, we can ensure Sieger windows, doors and rooflights adhere to regulations.

Thermal Bridging & Breaks

An issue that used to occur with aluminium frames was thermal bridging due to aluminum being a conductive material.

With modern glazing advances this is no longer the case as thermal breaks can be used to stop heat travelling through the aluminium.

Aluminum has come a long way in terms of aesthetics, security and thermal performance.

All Sieger frames are thermally broken to ensure maximum thermal performance.

Each frame has a polyamide barrier within the frame, which is made from non-conductive materials such as nylon.

As aluminum is a conductive material, the cold air outside brings the temperature of the frame down significantly.

When this reacts with cold air from inside the property is can cause a condensation build up on the inside of the glazing and within the glass panes.

This is referred to as thermal bridging and it is one of the most significant heat loss areas in buildings comes from thermal bridging.

Having a thermal break with a non-conductive material prevents thermal bridging from occurring.

When true glazing bars are used, such as in the Sieger Legacy Systems this can affect the Uw value as a significant amount of heat is lost through air leakage where the glass and frame meet.

Sieger uses warm edge spacers between panes in a double or triple glazed unit to reduce the energy loss at this point.

modern aluminium framed glazing from sieger
modern aluminium framed glazing from sieger

Single, Double or Triple Glazing?

Single glazed has the lowest value, the higher glazed it is the lower the U value will be.

Sieger systems come with double glazing as standard, however triple glazing can be specified in most systems.

Triple glazed units have a better Ug value due to the additional glass pane and second gas filled cavity.

By using triple glazing the thermal performance can be improved. However not every system is suited to triple glazing.

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

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.


Coatings that can improve the Uw value

Both low emissivity coatings and solar control coatings can improve the U value of a glazing unit.

Low emissivity coatings, commonly known as a low e coating, reflects radiant heat energy from indoors back into the interior space.

This minimizes the amount of heat that can escape out of the home.

Solar control coatings prevent heat from travelling through glass in either direction.

This coating prevents infrared rays from the sun entering the property, and those that do penetrate through are then kept withing the property.

Even minor changes to the architectural glazing installation can cause a change in the Uw value.

Our Sieger team can easily do this calculation for you based on the product, exact size and configuration to generate the Uw value for your glazing system.

For more information contact our technical team who will be able to answer any questions and discuss your project in more detail.

To talk to the team, call 01494 722 882 or email [email protected]

oversized slimline aluminium windows

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