Understanding how windows are made is an important part of understanding what makes a good window. Window frames are an important part of the window – not just structurally, but because different materials and finishes impact on the energy efficiency, security and overall appearance of the window.
How are windows made?
Window frames are constructed by taking long lengths of ‘profile’, cutting them to size, and fixing the ends together to create a rectangle (or other shape!). Different materials are fixed together in different ways – timber windows can be ‘joined’ together with interlocking sections, whereas uPVC and aluminium frames are welded or fused together under high heat.
Some uPVC and aluminium windows are ‘mechanically joined’ to replicate the same process as a wooden frame, giving a more classic look. Better quality uPVC windows are now made using a ‘smooth welding’ process, resulting in a seamless and more polished finish.
Timber window frames & joinery
Timber windows are made by fitting together solid lengths of wood that have been pre-cut and styled by machine or hand. Good quality timber windows are made using engineered wood that has been layered and compressed to increase the structural stability of the frame. Timber is naturally insulating, and the frames don’t require any other materials or parts.
uPVC window frames & chambers
You might think that because timber frames are made from solid pieces of wood that all frames are created in this way, but it’s not the case. Timber is a naturally insulating material, but uPVC windows work by trapping air in pockets within the frame, reducing thermal conductivity and stopping heat from escaping.
These are called ‘chambers’ – the more air chambers a window frame has, the more energy efficient the window can be. This is because each chamber provides an additional barrier for the heat and the cold to contend with. The outerframe and the sashes can each have a different number of chambers, but numbers can range from a 3-chambered frame to a highly energy efficient 9-chambered frame.
Aluminium window frames & thermal barriers
You might have heard that aluminium window frames are ‘cold’ and not as energy efficient as uPVC. Well, the problem with older aluminium windows was that the profile could only be ‘extruded’ in one piece, meaning window frames used to be made entirely from metal – the worst material for letting the heat escape!
Over the years, advances in manufacturing led to windows being made in two parts, allowing for a polyamide thermal barrier to disrupt the transfer of heat. Like uPVC windows, aluminium frames also contain energy efficient chambers.
Window frame styles
If you have a keen eye for detail, you’ll want to choose the right frame styling and detailing for your new windows. Windows can have clear, sharp lines or a softer, curvier outline depending on whether you choose bevelled or ovolo frames.
Another popular design choice is windows with ‘flush’ sashes, meaning that windows sit completely flat, with minimal frame depth. You might want to choose flush windows if you want to replicate the flat façade of traditional timber frames, however these windows do not offer the weatherproof qualities as regular frames. At the other end of the spectrum, ‘stormproof’ windows are designed so that the sashes overlap the frames, to protect against the elements.
Like fashion, window styles have changed over time, and there are certain features that can tell you what era a window belongs to. Georgian windows are overlaid with astragal bars that divide the sashes into six over six panes, Edwardian sash windows sported sash horns, and 1930s windows use arched frames to give a property its style.
Good window companies understand the differences between different historical window styles and can advise you about which period features you should consider, which is especially important if you live in a period property.
Previously, you could only expect to achieve fine detailing with timber frames, but it’s now possible to replicate this style using modern uPVC or composite materials. Some uPVC frames are manufactured with a ‘woodgrain effect’ finish, which emulate the texture of timber.
Window seals, or gaskets, sit between the frame and the glazing to stop air leakage and water ingress, resulting in a fully airtight and weatherproof window unit. Gaskets keep your home warm and dry, saving your frames from becoming damp and rotting. It can be easy to overlook the importance of good quality seals, but they really do help the all-round performance of a window.
Gaskets are pre-fitted to windows before installation, and with modern windows you won’t need to add any additional draught excluding seals.
Three types of seal are needed to make a window fully weathertight:
- Window opener seals help support the window all around its edges
- Outer weatherproof seals prevent water from getting in from the outside
- Inner weatherproof seals keep the window airtight and keep draughts out of your home
Rubber vs. Q Lon window seals
Until very recently, window seals were thick, black and rubbery, and stood out starkly against white frames. Seals have slimmed down since then, and are now more discreet. They also offer better insulation and contribute to a much higher energy performance.
Seals are available in either rubber or a material called Q-Lon. The difference is that rubber tends to flatten once it is subject to repeated pressure, altering its shape and ultimately hindering its performance. Eventually, rubber seals can collapse, and this is one of the main reasons why older windows fail after a number of years.
Q-Lon seals, however, have a compression recovery of 99%, meaning they will spring back to their original shape every time. They are also available in a range of colours, which really improve the overall appearance of the window set.