Choice of double glazing materials
- PVCu is a good budget option
- Frame thickness determines structural strength
- Aluminium is best choice for commercial locations
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.
Rear view of large PVCu installation with both two pane
and four pane patio doors fitted.
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
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
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
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
properties. It has the twin benefits of being suitable
for the recreation of virtually any traditional design or
while incorporating the contemporary advantages of double
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.
This shares many of the features of PVCu, although aluminium
is more expensive and does not provide such efficient insulation.
White Aluminium Door and window installation. Note the patterned
glass in bottom of door.
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 advent 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
Aluminium windows can be fitted as 'direct fix' – ie
directly against the brickwork or, alternatively (and more
often) into a hardwood subframe.
Replacement sliding sash windows are also available but
are usually more expensive than the more common casement
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 required
although this necessitates the use of a specialist joinery
company with associated cost implications.