What You Need to Know About U-Value in Windows

Before going into greater depth about a U-value, let us tell you straight off that it represents how effective a material happens to be at insulating heat and arresting its escape from a house.

Window terminology like this speaks our language, but we appreciate that you might not have the same level of understanding.

That will hopefully have changed by the time you reach the end of this article and we’ll have imparted much of the knowledge of the U-value that we possess.

So without further ado, let’s delve into U-values more deeply.

Replacement windows and U-values are closely aligned because a window’s U-value signifies how well it insulates.

The lower it is, the higher the standard of energy efficiency offered, which is what you need to be guaranteed a cosy home.

To use an alternative U-value meaning, a high thermal transmittance signals that a window is a poor insulator and susceptible to allowing heat to leak out.

It is specified in UK building regulations that all replacement windows supplied in the UK have to attain a thermal transmittance of at least 1.4 W/m²K and nothing below that figure.

Therefore, try and prioritise windows with a low thermal transmittance in order to make a very noticeable saving on energy and hugely amplify home thermal efficiency.

Single glazed windows offer second-rate thermal transmittance if you compare them to double and triple glazed windows.

In some instances, their U-value can be as much as 6.0 W/m²K, which ranks unfavourably against a double glazed window and its average unit rate of 1.2 to 3.7 W/m²K. A triple glazed unit u-value of 1.0 W/m²K and less is not uncommon.

Make a mental note of this when you come to select new windows and use it to identify which will reward you with superior insulation and energy efficiency.

Windows are not the only building component that influence home insulation, as walls and roofs are a factor in this too.

So long as they are well insulated, next to each of the following components is their typical U-value:

• External Walls: 0.18 W/m²K
• Party Walls: 0.0 W/m²K
• Floor: 0.13 W/m²K
• Roof: 0.11 W/m²K

To keep things basic, a low thermal transmittance tells you that the window you’re observing is exceptionally good for heat retention and insulation – so, always look for designs with a low U-value.

If you want to calculate it, heat loss must be discerned first, which you can detect using a heat flux meter, and then you need to record the measurements of the window area i.e. its height and width, multiplying the two figures together. As a last job, uncover what the temperature difference is between the inside and outside.

Allow a U-value calculator to do the working out for you – there are several of these online that you can use for free.

A window can only perform to the highest level if it has energy-efficient glazing, vital also for the prevention of condensation on windows.

It gets its name because of its ability to constrain heat loss and intensify thermal insulation in the home. Low-E glass is a leading component, containing an invisible coating that reflects heat.

Double and triple glazing windows that feature energy-efficient glazing transform living spaces into cosy havens and stop them becoming profusely warm in summer, culminating in cheaper energy.

Observe U-values when assessing any energy-efficient glazing and check it will obey any security, privacy or sound insulation requirements you have.

There’s more to share about energy-efficient glazing. It was originally devised with the aim of increasing the standard of temperature regulation inside buildings and to minimise dependence on heating and cooling systems, shrinking energy usage.

Low-E glass is something we’ve already spoken about, so it’s time to mention some additional features.

Energy-efficient glazing co-exists with insulating glass units that are each independently separated by a gas to diminish heat transfer for the purpose of thermal efficiency and paired with materials providing low thermal conductivity.

Energy-efficient glazing comes in various forms and a few of the different types are detailed below:

Contains the thinnest of coatings, designed to reflect heat and allow visible light to filter in from outside. Does an excellent job of stemming heat loss in winter and cooling interiors in summer.

The two glass panes are kept separated by a spacer filled with an inert gas or air. A more effective insulator than single glazing, it can restrict noise and heating and cooling charges.

Any triple glazed coloured UPVC windows consist of three glass panels and have two spacers as separators. Their thermal insulation exceeds a double glazed window and triple glazing is regularly specified for homes located in cold regions.

Designed to control how much solar heat a building receives and maximise the amount of daylight it gets. Chosen most frequently for buildings that are regularly exposed to hot weather conditions.

Will enable certain wavelengths of light to pass through e.g. visible light, and intercept other light types from following suit e.g. infrared heat, so that excess solar gain doesn’t come to fruition.