The Importance of U-Value in Energy-Efficient Windows

Without keeping you in suspense, we’ll immediately divulge that a U-value characterises the effectiveness of a material at insulating heat and staving off its escape.

You could be excused for unfamiliarity with such terminology, whereas we wouldn’t be as we’ve been involved in the replacement window industry for decades now.

That makes us well-placed to educate you on U-values, something you will undoubtedly come across whilst browsing the replacement window ranges of companies like Orion.

In uncomplex fashion, let’s break down their meaning and go more into what they represent.

The two things are interlinked as the U-value of a replacement window is a measurement of its insulating properties.

Replacement windows with low U-values offer the finest level of insulation and optimum energy efficiency, making your home a significantly warmer place to live.

On the flip side, high thermal transmittance (an alternative term for a U-value) indicates a window with poor insulating abilities and especially prone to releasing heat.

Building regulations in the UK stipulate that all new replacement windows have to attain a thermal transmittance of at least 1.4 W/m²K and nothing less.

If you have the budget for them, purchase windows with the lowest possible thermal transmittance to maximise your energy savings and enjoy supreme thermal comfort.

Single glazed windows trail in last place for thermal transmittance, unsurprisingly in comparison to double glazing and triple glazing.

The worst offending single glazing can have a unit rate as bad as 6.0 W/m²K, which stands up badly against double glazing, which generally achieves between 1.2 and 3.7 W/m²K. Ranked first are triple glazed windows, capable of accomplishing a figure below 1.0 W/m²K.

Consider these measurements and the impact they will have on insulation and thermal efficiency when you come to select new window designs.

Windows are just one of a series of building components that influence home insulation and the energy efficiency rating of a dwelling – walls and roofs contribute to it too.

Below, we outline the average U-values of certain components, dependent on them being well-insulated:

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

We’ll seek to explain this as simply as we can, but before we do, just know that low thermal transmittance signals that a window is incredibly effective at retaining heat and providing insulation – a low U-value is always a good sign.

Calculating it entails identification of heat loss volume, something that a heat flux meter will uncover, and then you need to measure the height and width of the window area, multiplying the two in metres. The final task is to discern what the difference is between the internal and external temperatures.

Take the easy option and use a U-value calculator online to work out the final figure.

Energy-efficient glazing is imperative if outstanding window performance and home insulation is to be realised.

It refers to window glazing that’s forged to minimise heat loss and bolster thermal insulation. Low-E glass is fundamental to its ability to reflect heat, an invisible coating featured on the glass doing that.

Employed in double or triple glazed windows, winter warmth at home is assured and summer heat won’t cause your interiors to get too hot, plus there’s the added bonus of cheaper energy costs.

Heed U-values when in the midst of energy-efficient glazing selection and take into account security, privacy and sound insulation.

Covered in more depth, energy-efficient glazing arose from a yearning to improve temperature regulation within buildings so that heating and cooling systems could be left off more often and fewer units of energy consumed.

Now that we’ve explained Low-E glass, it’s important to also clarify a few other central features.

Energy-efficient glazing also alludes to insulating glass units, each being separated by a gas to temper heat transfer for the benefit of thermal efficiency, and it’s combined with materials that provide low thermal conductivity.

Whilst not a complete list, these are some of the foremost versions of energy-efficient glazing to make you aware of:

Low-E glass has a thin coating that reflects heat and gives visible light the capacity to infiltrate. It excels at controlling heat loss during cold spells and cooling interiors in summer.

A spacer bar filled with air or an inert gas keeps the two glass panels separated. A stronger insulator than single glazing, it can reduce noise levels and the cost of heating and cooling rooms.

Three glass panes occupy a triple glazed window unit and have two spacers keep them apart, similarly filled with air or gas. There’s a big uptake of triple glazing in the world’s coldest countries.

Fulfils the role of ensuring that a building doesn’t receive too much solar heat but still gains a sufficient amount of natural light. Often specified for buildings exposed to consistently high temperatures.

Used in the scenario where you want to attract certain wavelengths of light, such as visible light, and avert others, e.g. infrared heat. It will stop profuse solar gain for sustained home comfort.