Thermal performance of timber windows - Westbury Windows and Joinery

Discover the thermal performance of our timber windows

Timber windows can be a great asset to your home. They look timeless and elegant, and the natural light and ventilation they let in can dramatically improve your quality of life. However, what about the thermal performance of a timber window?

If you find yourself stuffing the gaps in your windows with wads of cotton wool to keep out the draft in the winter, or shutting your blinds to keep out the heat in the summer, then it might be time to invest in thermally efficient windows. With the right windows, you can significantly reduce chilly spots, air leaks and draughts from your home. What’s more, you and your family can feel comfortable at any time of the year. As a result, the costs of heating your property significantly reduces as your home will no longer be losing warmth out of poorly performing windows.

Finding the right kind of window product for your home can be overwhelming with all the technical information available. Understanding how windows work when it comes to insulation can make the buying decision easy. 

Always check the u-Value of the whole window

When it comes to windows, thermal efficiency is not always straightforward. The industry determines a window’s performance based on its energy balance, instead of just insulation. This balance looks at solar gains versus heat losses, both of which are essential for maintaining a comfortable temperature inside your home at different times of the year. 

Solar gains depend on the share of solar irradiation passing through the window. During the cold season, you will want your windows to maximise on this effect. During the summer, lower solar gains and ventilated cooling are more appropriate. Heat losses depend on the outside temperature, and how easily heat escapes through the window. Solar irradiation also depends on the window’s orientation in relation to the sun. 

When looking for new windows for your home, always enquire about the U-Values, which are a measurement of a material’s insulation efficiency. The U-Value tells you how much heat a window is likely to let through. The lower the U-Value, then the better the energy efficiency a product has.

Do keep in mind when checking the U-Value that the number can differ depending on which part of the window has been tested. Many companies advertise a low U-Value number but measure from the centre of the glass in the pane, which is usually the section of the window with the highest insulation. Always check the U-Value for the whole window, as all products experience more heat loss around the edges of the pane.   

Let’s talk about timber vs uPVC windows

For various reasons, timber window frames had a bad reputation in the 20th Century. Throughout history, timber frames had always been popular and widely used. However, after the Second World War, Europe had to switch from commonly used Canadian Douglas Fir to cheaper, alternative timbers from Europe. 

Before long, builders and homeowners alike assumed that a timber window would soon become warped, rotten, cold and draughty, and they considered uPVC a better alternative. As far as we are concerned, this could not be further from the truth. Timber makes beautiful window frames, and it can be a durable, high-performing material. Providing the windows are constructed from the right kind of timber, they can last for 50 years or more. 

Perfectly performing timber windows

Thanks to extensive research and design innovation, homeowners no longer have to choose between form and function. At Westbury Windows and Joinery, we have investigated the performances of different timber materials, joinery methods and fitting techniques to result in a high performing window product. 

We design our timber window frames, sashes and doors with deep inside-to-outside profiles, therefore improving the thermal performance through the wood components. This technique was originally mastered by Scandinavians many years ago, to help keep out the cold weather. They used a standard frame depth of 115mm, which is what we use at Westbury today. Our sashes are 68mm deep, meaning they can incorporate a thicker and wider choice of energy-efficient and sound reducing glass. 

In addition to our window’s unique design, we use an engineered timber with Accoya® on the outside layer resulting in exceptionally low thermal conductivity. Accoya is a revolutionary material made from fast-growing, sustainably sourced Radiata Pine. Once felled, the pine undergoes a process that compresses and bonds laminates (thin slices) of the pinewood, rotating the grain by 90˚ at each layer. This cross-lamination gives Accoya up to 75% greater dimensional stability than solid hardwoods. The Accoya timber undergoes a process called acetylation, which re-plumps the cells of the cut lumber, making the timber harder and termite resistant. 

Such is the material performance of Accoya that it has a guaranteed lifetime of 50 years. We then spray-apply our timber windows with three coats of Teknos; a water-based, micro-porous paint explicitly formulated for external use on timber. The paint creates a layer that protects the external timber, making it resistant to bacteria, mould, and UV attack. 

What kind of u-values can you expect at Westbury Windows and Joinery?

Ideally, you want to choose window products that comply with building regulations and prevent heat loss from your property. Back in 2018, Part L building regulations for standard replacement doors and windows specified that a product should have a U-Value no higher than 1.6 for windows, and 1.8 for doors. Look for products of superior quality that exceed these regulations. Windows that have U-Values of just 0.1 or 0.2 under the regulation threshold do not make any difference to your carbon footprint, or energy bills for that matter.

Westbury Windows and Joinery expertly construct windows with a whole product U-Value of 1.4 W/m2K based on double-glazed glass and 1.1W/m2K, based on triple-glazed 4mm thick glass with a centre pane value of 0.6W/m2K.

In addition to this, our timber doors have an excellent whole product U-Value of 1.4W/m2K and under. We make them with 68mm deep door leaves, meaning that they can incorporate a much thicker and more extensive choice of energy-efficient glazing.  

Over the last 30 years, Westbury has built a reputation for design, quality and innovation, producing exceptional bespoke windows and doors to homes across the UK. Contact us today to discuss your project in more detail, or book an appointment to visit one of our showrooms.