Double Glazing Frame Materials, Picking The Right Frame

This article explores the different type of double glazing frame materials available today.


This shares many of the features of PVCu, although aluminium is more expensive and does not provide such efficient insulation.

When double glazed windows first became popular in the late 1960s aluminium was the usual choice of material due to its strength and durability. It is more resistant to warping, twisting or sticking when subjected to the elements. It is also virtually intruder-proof and neither absorbs water nor rots or rusts.

Its popularity declined with the arrival of the cheaper PVCu. However, aluminium remains an excellent choice for commercial locations and any circumstances in which strength is an important factor. It is advisable, if choosing aluminium, to specify frames with a thermal break as this improves the insulation properties.

Aluminium windows can be fitted as ‘direct fix’ – i.e. directly against the brickwork or, alternatively (and more often) into a hardwood sub frame.


PVCu, uPVC, PVC-U, and PVCU all essentially refer to the same substance. The most popular choice, this is an excellent material which has the advantage of needing little or no maintenance. The least expensive of all the available materials, it is most popular in white but is also available in mahogany and oak wood-grain styles.

Problems with discoloration are negligible and most suppliers will provide warranties against this. The design of the windows varies from company to company but points to look for include: Internal or externally-glazed windows – an option offered by most PVCu systems. Internally-beaded windows, where the glass is held in from the inside, are generally deemed more secure and burglar-resistant. However, there are also perfectly satisfactory externally-beaded PVCu systems on the market. Many feature either internal wedge gaskets or a double-sided tape that firmly fixes the external bead.

Thickness of PVCu wall – most PVCu systems for window and door construction are ‘multi-walled’ with internal reinforcement provided by either aluminium or galvanised steel box section. Wall thickness can vary from system to system, most being around 3mm or 3.5mm. In general the thicker the walling, the stronger the section. Ask your supplier to show you a sample section and establish whether the frames are fully reinforced. Be aware, too, that the greater the number of internal walls, the greater the strength of the building.

Depth or thickness of frame – the depth of frame extrusion can vary from as low as 50mm to more than 70mm, although most are in the 60-65mm range. This, too, has an effect on the structural strength of the window or door.

Note that PVCu is unacceptable to planners for use on listed buildings, nor is it popular with planners in conservation areas.

Sash windows

Replacement sliding sash windows are also available but are usually more expensive than the more common casement style windows.

They are made in both PVCu and timber. The main difference between PVCu and the more traditional timber box sash window is the method of holding the sashes in position. Instead of weights, pulleys and a cord, a pair of sophisticated spring and spiral balancers provides the sash retention and can carry weights of up to 40kg.

The traditional glazing bar arrangement may also be replicated on PVCu by concealing the glazing bars within the double glazed unit or by surface-mounting the bars onto the external faces of the unit.

One of the biggest advantages of timber sliding sash windows is the ability to replicate any period design feature. Although this necessitates the use of a specialist joinery company with associated cost implications.

One of the biggest advantages of timber sliding sash windows is the ability to
replicate any period design feature.


A more expensive material, hardwood is the choice of those seeking a traditional design with an authentic look and is a particular favourite for use in listed buildings or period properties. It has the twin benefits of being suitable for the recreation of virtually any traditional design or feature, while incorporating the contemporary advantages of double glazing.

Hardwood is available in a variety of stains such as mahogany and light oak, as well as various painted finishes and, while it does require periodic maintenance, this is not an onerous task thanks to modern paints and stains. As with PVCu, the frame thickness will affect the structural strength. It is also important to ascertain which jointing method is used – most suppliers use a traditional mortice and tenon joint but other systems do exist.

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