Why U-Value Matters for Your Next Window Purchase

Diving straight into U-values, we can reliably inform you that it signals how successful a material is at insulating heat and stopping it from escaping.

We’re aware of this kind of window terminology because we have been involved in windows for what seems a lifetime.

Acknowledging that you’re not as immersed in windows as we are, let’s go into the meaning and significance of a U-value, a term you may have come across when shopping for new window designs.

By the end of this article, our goal is to have turned you into an expert on U-values, just like us.

Replacement windows and U-values have a close association as a U-value denotes the insulating prowess of a window, expressed in a unit.

A low unit value tells you that a window delivers impressive insulation and succeeds in offering a home top-level energy efficiency.

On the other hand, high thermal transmittance (a separate definition of U-value) symbolises that a window fails to offer quality insulation and is prone to enabling marked heat loss.

According to UK building regulations, new replacement windows in the UK must attain a thermal transmittance of at least 1.4 W/m²K.

If you can, buy windows with the lowest thermal transmittance to make the biggest savings on energy and benefit from a huge volume of elevated warmth at home.

Trailing last for thermal transmittance would be single glazed windows, exceeded by double and triple glazed windows.

A single glazed window can have a U-value as high as 6.0 W/m²K, which indicates a poor insulating design, while the average unit rate of a double glazed window is usually somewhere between 1.2 and 3.7 W/m²K. It’s not uncommon for some triple glazed windows to accomplish a figure of 1.0 W/m²K and below.

This information will help you make a more educated decision on replacement windows and identify how effective they are for long-term thermal efficiency.

Apart from windows, you must be mindful that other building components will influence home insulation and the energy efficiency rating it achieves, such as the walls of the house and its roof.

If well-insulated, the following components generally accomplish these U-values:

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

We could get into all sorts of complex language, but we won’t do so for fear of confusing you, so to put it simply, a low thermal transmittance represents that a window has excellent thermal properties – the lower the U-value, the better – that’s basically the most important thing to know.

But if you’re curious to know how to calculate it, your first task is to uncover heat loss, which you can track using a heat flux meter. After that, measure the height and width of the window and multiply them together in metres. Lastly, detect what the difference is between the internal and external temperature.

Need some help figuring it out? There are U-value calculators that you can use for free online to do the Maths for you.

A window is heavily dependent on energy-efficient glazing if it’s to excel at window performance and thoroughly insulate homes.

Energy-efficient glazing is deemed such as it is specially crafted to minimise heat loss and amplify thermal insulation. Low E-glass is a critical component of it and contains an invisible coating intended to reflect heat.

Inclusion of energy-efficient glazing in double or triple glazed units will warm up your winters in the home and keep excess heat at bay in summer, initiating a cheaper energy bill too.

Pay regard to U-values when selecting energy-efficient glazing and also be conscious of privacy, security and sound insulation requirements.

The development of energy-efficient glazing came about in an endeavour to improve the temperature regulation in buildings and make them less reliant on heating and cooling systems, minimising energy usage.

Low E-glass has already been covered, and there are additional elements to speak about.

Energy-efficient glass embraces an insulating glazed unit, with the panes separated by a gas to allay heat transfer for the benefit of thermal efficiency, and is combined with materials that provide low thermal conductivity.

Although not a comprehensive list, this will bring your attention to a few of the topmost forms of energy-efficient glazing:

Possesses a tiny thin coating designed to reflect heat and allow visible light to pass through. It limits how much heat loss occurs in winter and cools indoor temperatures in summer.

A two-paned solution, the sheets of glass are divided by a spacer filled with air or an inert gas. Transcending single glazing as an insulator, it is capable of reducing noise and energy costs.

Triple glazed windows are a step up from double glazed equivalents, featuring three glass panes and two spacers, rather than just one. Often installed in the coldest counties, they outdo double glazing for thermal insulation.

Stops too much solar heat from getting into a building, while also maximising light transference. Generally employed at locations exposed to high external temperatures to help them maintain a manageable internal temperature.

Will let in certain wavelengths of light e.g. visible light and intercept other types, such as infrared heat. Also manages to block excessive solar gain from manifesting inside buildings to reward occupants with comfortable living.