Heat loss and heat gain – Are your windows ready for winter?

Summer has been and gone and the autumn leaves are falling fast; winter is well and truly on its way. Many homeowners dread the cold snap at this time of the year. Typically, a household loses 10% of the heat it generates through its windows, but it doesn’t have to be that way.

The installation of energy efficient glazing with the right specification can be the difference between having living space that’s considered a heat loss area or one that’s comfortable to spend time in all year round.

So, how does heat loss work?

Radiation through glazing is in fact the biggest cause of heat loss through windows. It is said that two-thirds of all energy lost via a typical window is due to radiation. That’s because inner panes of glass absorb the heat generated internally and end up transmitting it to the cooler outside pane via convection and conduction and, eventually, outside.

All glazed units are given a U-value, which denotes its level of thermal transmittance expressed in units of Watts per square metre per degree of temperature difference. The lower the U-value of a glazed unit, the more thermally efficient it is.

In addition, some internal heat is lost through windows due to convection inside the glazing cavity. In some cavities, air within the cavity is warmed by the inner pane. This rises and is eventually replaced by cooler air, resulting in a convection current which continually transfers heat from the inner pane to the outer pane.

At Livingwood, all the Reynaers aluminium window and door systems we manufacture offer industry-leading thermal insulation. This negates the issue of heat loss, with glazed units that offer unrivalled U-values. Not only are cavities filled with low conductivity gas, we also offer a patented insulation technology in the chamber of the insulation strip to offer outstanding air tightness.

Thermal breaks – often using polyamide sections between glass panes – also help to stop heat being conducted through the glazed frame itself.

After the radiation of heat through glazing, air leakage is the second greatest contributor to heat loss in homes and workplaces through windows. This is particularly prevalent in older or badly fitted windows which feature minor gaps of material that used to offer a vital source of ventilation.

What is heat gain and the factors influencing it?

On the flip side, glazed systems are also known to absorb heat, resulting in a net gain of heat for interiors. Heat gain works in the following ways:

  1. Energy absorbed by panes is transferred inwards through radiation and convection.
  2. Solar gain transmitted directly through the glass panes.

There are three influential factors that can encourage heat gain in domestic and commercial properties:


Heat gain can occur more if a window does not have sufficient levels of shading, particularly in the warmer, summer months. It’s pretty straightforward to achieve necessary levels of shading for windows that are south-facing, but the sun’s lower angle can make it trickier to protect east and west-facing glass panes.


The orientation or direction of your windows will influence levels of solar gain throughout the year. Windows and doors facing directly south will be subject to peak absorption during the summer, while east and west-facing windows will experience peak absorption in mornings and afternoons respectively in the warmer months.

A window’s ‘G-value’

Aside from a U-value, a window also has a G-value, which measures the extent to which glazing prevents heat from entering a property via sunlight. The G-value is given in a fraction, demonstrating the amount of heat enters a window from the sun’s rays between zero and one. The lower a G-value, the less solar heat it transmits. The best way to minimise G-values is to incorporate a solar control or low-emissivity glazing specification into window and door systems to absorb and reflect UV rays without passing them and their heat to interiors.

At Livingwood, our combined technical insight into passive house construction and low-energy buildings enables us to specify the right energy efficient windows and doors for your bespoke project.

Our entire product range is designed to help your property operate on a level that’s better for you and the wider environment. For outstanding thermal performance and contemporary style, make sure you view our full range of aluminium windows today.