Why U-Value Should Be Your Top Priority for Windows

Right off the bat, a U-value denotes how effective a material is at insulating heat and preventing its escape.

Unless you’ve been immersed in products like windows for the years we have, you’d be forgiven for being unfamiliar with such terminology.

So, we want to pass on our knowledge to you in case U-value is a term that’s foreign to you, often mentioned in the promotional material for replacement windows.

Let’s decode it in more depth to turn you into as much of an expert on U-values as we are.

U-values and replacement windows are directly correlated as the U-value unit of a window is a measurement of its insulating qualities.

The lower it is, the greater the standard of insulation you can expect a window to deliver and usher in abundant warmth and energy efficiency within the home.

Contrarily, a high thermal transmittance (an alternative U-value meaning) means that a window performs poorly from an insulating standpoint and leaks out a significant volume of heat.

UK building regulations stipulate that brand-new window styles must attain a minimum thermal transmittance of 1.4 W/m²K, so anything below that legal benchmark is acceptable.

Search for replacement designs that have an exceptionally low thermal transmittance to maximise long-term energy savings and ignite significantly elevated home comfort.

To no great shock, single glazed windows rank last for thermal transmittance when compared to double glazing and triple glazing.

Their U-value can be as high as 6.0 W/m²K, signalling very substandard insulation, while on average, a double glazed window accomplishes in the region of 1.2 and 3.7 W/m²K. In respect of triple glazed windows, it’s not uncustomary for a triple glazed unit u-value figure below 1.0 W/m²K to be reached.

Use these learnings to consider the implications your choice of glazing installation will have on insulation and energy efficiency.

Moving away from windows for a second, there are separate building components that contribute to overall home insulation and its energy efficiency rating, like walls and roofs.

Here follows a list of the typical U-values of some of these components, given with the assumption they’re well insulated:

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

For fear of overwhelming you with too much complex language and technical details, let’s stick to the basics and reemphasise that low thermal transmittance indicates that a window offers impressive heat retention and insulation – is a lower U-value better? Always!

However, if you are intrigued as to how it’s calculated, you first must identify heat loss, which a heat flux meter can determine, and then measure the window area, multiplying the width and height together in metres. Finally, uncover the difference between the internal and external temperature.

Several free U-value calculators are available online if Maths isn’t your strong point.

Energy-efficient glazing is fundamental to window performance and home insulation standards.

It’s the name used to refer to window glazing purposely designed to restrict heat loss and boost thermal insulation. Low-E glass is its principal component, featuring an invisible coating that reflects heat.

When present in double or triple glazing windows, your winters at home will become notably cosier, with unwanted heat kept out in summer, also converting into more affordable energy bills.

Selection of energy-efficient glazing calls for scrutiny of U-values and concerns of privacy, security and sound insulation demands.

Delving into energy-efficient glazing some more, its development arose out of a want to uplift indoor temperature regulation in buildings so that reliance on heating and cooling systems could be shrunk, helping energy become a reduced source of dependency.

We’ve already gone into Low-E glass and that’s not its sole dominant feature.

Energy-efficient glazing incorporates insulating glass units, separated by a gas to alleviate heat transfer for thermal efficiency benefits, and is paired with materials offering low thermal conductivity.

Not a definitive list, but one that outlines some of the uppermost types of energy-efficient glazing:

Includes a microscopic thin coating that reflects heat and enables visible light to travel through. Great at restricting heat loss in winter and keeping interiors cool in summer.

Its two glass panels are separated by a spacer occupied by air or an inert gas. More insulating than single glazing, it can lower noise and heating and cooling costs.

Coloured UPVC windows involving triple glazing have three glass panes and two glass-filled spaces separating them. They exceed double glazing for thermal insulation and are especially recommended for homes in exceptionally cold climates.

Stems the quantity of solar heat that invades a building while at the same time, maximises the extent of daylight it receives. Often used to guarantee a manageable internal temperature in locations exposed to searing conditions.

Intelligent enough to permit certain wavelengths of light to get in e.g. visible light, and block others from doing the same e.g. infrared heat. Stops excessive solar gain to sustain occupant comfort.