Choosing windows

Windows are one of the most important elements of any self-build or renovation - we bring you the lowdown on how to choose the right type for your home
by Build It
28th November 2012

Whether you’re planning a brand-new self-build, undertaking a conversion or renovating a property, the choice of windows is too important to be an afterthought.

A glance through Build It’s self-build homes shows how crucial windows are to the look and character of a home – from contemporary feature windows that make a bold statement to discreet styles that blend sympathetically with a period house. And even if you’ve opted for open-plan living with a designer glass wall, you’re almost certain to have some part of the house in need of conventional glazing too. So how do you make sense of it all?

Planning permission

With new self-builds, windows will naturally be part of the overall design when you apply for planning approval, and gaining it may involve specifying a particular style of window, for instance, if you are building in a conservation area. But generally, if you are simply replacing windows in an existing building, or fitting double-glazing, you won’t need planning permission. If you are also changing the outline of the house, for instance by creating a new bay or dormer window, it’s a different matter and may require permission depending on permitted development rights, so it’s worth contacting your local council to find out.

Building Regs

In a new self-build, overall energy efficiency of the property is the main consideration – so highly insulated walls and roof, for instance, may compensate for windows with a less than ideal U-value or WER rating.

Jargon buster

Low-E or low-emissivity glass has a fine, almost invisible coating of metal oxide on one surface, which lets in heat and light from the sun but stops heat from leaving the room, reducing heat loss considerably

Mullion A vertical bar of wood, stone or metal dividing a window into two or more parts

Secured by Design (SBD) A national police initiative aimed at ‘designing out’ crime. SBD windows include crime prevention features such as espagnolette bolts that lock the window into the frame at both top and bottom, or 100 per cent laminated glass

Trickle vent Common form of ventilation that provides a steady flow of fresh air. Usually installed in a window frame – cheap (£25-£30) and maintenance free

U-value Measure of heat loss – the lower a window’s U-value, the better

Like so many other aspects of a new self-build, the windows must be approved by Building Control. Replacement windows in England and Wales (Scotland has its own, comparable standards) have only come within the scope of Building Regs since April 2002, but since last year they have been subject to more stringent standards than ever for thermal performance – partly due to the government’s drive to increase energy efficiency.

Building Regs also apply to areas including safety, means of escape and ventilation. In effect this means the new windows must be the same or better than the old ones. So if the previous window also provided fire escape access by opening fully, you can’t replace it with, say, a fixed window. And if there are trickle vents in the old windows the new ones must have them too. If you have deep windows reaching less than 800mm from the floor, they must have toughened or laminated safety glass.

If you’re replacing windows, by far the easiest route to compliance is to use a contractor registered with the Fenestration Self-Assessment Scheme (FENSA), a body set up by the Glass & Glazing Federation to approve installers. Otherwise, you’ll need to get full local authority Building Control approval yourself. You must be able to produce a certificate when you sell your house to prove that either Building Control has approved your replacement windows or they were installed by a FENSA supplier.

Window styles

Leaving out special cases such as roof windows and French windows (both outside the scope of this overview), the two main – and familiar – frame styles in Britain are sash (where two or windows slide up and down to open; sometimes called vertical sliders in the trade) and casement (side-hinged).

Period styles such as Georgian, Victorian or Edwardian houses usually call for sash windows, and a great many companies offer a range of traditional sash styles, while 1920s and 1930s properties will more often require casement windows, with Art Deco and Modern Movement homes using metal frames (see below).

Traditional sashes are operated by a weight-and-pulley system, but modern versions often have smoother smoother running spring and spiral balances. You can also find sashes where the top and bottom windows separately tilt inwards so they can be cleaned from indoors.

Frame materials

There are three main material options for frames – wood, PVC-U (both available for sash, casement and tilt styles) and metal (mostly casement). Each material has its pros and cons and its devotees and detractors.

Wood This material is the traditionalist’s choice, particularly for sash windows. It combines aesthetic appeal with good insulation. What’s more, as a renewable resource is the most environmentally friendly option. The main drawback is that wood is more trouble to maintain, needing regular repainting and potentially treatment for rot and decay. Nevertheless, they’re highly popular with both self-builders and renovators.

PVC-U These windows offer reasonable insulation and are low maintenance – suppliers claim PVC-U frames have lasted 45 years with no more than cleaning in mild detergent from time to time. They are are also touted as a cheaper solution than wood or metal, but it’s not a good idea to go for the cheapest option; more expensive frames are reinforced with steel or aluminium or better structural integrity. You can get frames in wood-grain and coloured finishes, though some people find them unattractive and you won’t be able to use them in a listed building. Frames can also have problems with expansion in sunlight.

Metal Frames made of aluminium or steel are the most common, but you can also get them in other more expensive metals, such as cast iron or architectural bronze. All have the advantage of being very strong, long-lasting and with about one-third the expansion of PVC-U. Metal frames should need little maintenance, and they have a low profile (particularly compared with PVC-U). They are often a good choice when replacing period windows or imitating period styles, from grand Georgian homes to modernist boxes, and also work well in a sleek contemporary context. The main disadvantage is that metal is a relatively poor insulator, although designs should usually include a ‘thermal break’ to reduce the flow of heat through the material.

Composite materials Composite frames have grown in popularity in recent years. They usually consist of a timber internal frame, sheathed with a durable metal outer, such as powder-coated aluminium. These frames offer the characterful warmth of timber internally, but the durability and low-maintenance virtues of metal on the external face. They are often specified in exposed locations.

Types of glass

There are a huge number of different glazing options, from tinted to toughened. Bear in mind that if you are considering replacing your windows for a more energy efficient version, you may be able to simply replace the glass and, with sash windows, have them draught-sealed and refurbished – thus avoiding the hoops of Building Regs approval.

  • Low-E glass, such as Pilkington K glass, lets sunlight in and stops heat getting out.
  • Tinted glass – green, blue, bronze or grey – limits the entry of sunlight but does nothing to keep heat in the room.
  • Noise reduction can be supplied by double-glazing (or even expensive triple-glazing), but you can also get glass than incorporates a layer of special laminate for acoustic performance.
  • Self-cleaning glass, such as Pilkington Activ, has a microporous coating which reacts with sunlight to break down dirt, then encourages rain to wash it away.
  • Safety glass can be toughened or laminated, or both. Toughened glass has been heat-treated to fragment into tiny pieces when it breaks, rather than into dangerous shards. Laminated glass cracks when it is hit and only breaks with difficulty. Both can help windows to meet Building Regs on security, but laminated glass is potentially a liability in places where you may need to break the glass in case of fire.

Energy Ratings

These were introduced three years ago as a way of standardising the energy performance of a window, which has previously been expressed as a U-value (see Jargon Buster box).

The WER score is based on the total flow of energy through a window and takes into account its U-value, G-value (how much energy it absorbs from the sun) and ventilation. The rating applies only to complete windows – the frame and glass together, not separately. This means you will only find it on window installers’ products or sometimes on the manufacturers’ windows if they are factory-glazed.

Just like electrical goods, window energy ratings are shown as letters in a scale from A to G, with A the best and G the worst. The minimum level for replacement windows under the Building Regs is 1.6W/ m2K, which is in band C. Windows in new dwellings have a slightly easier-to-hit target of 2.0W/m2K, but must be specified in conjunction with walls, floors, ceilings, etc to ensure the property hits the overall maximum tolerance permissible under Building Regulations.

Case Study: The self-build

When Andrew Dundas built his new home near Ilkley, its position on the border of a conservation area meant it was subject to strict planning restrictions. The design was inspired by the work of architect Charles Voysey and the Arts & Crafts movement, and he needed all the details to suit the period. Andrew had other criteria too: his plot was on the edge of a steep slope which offered impressive views across the valley. “We wanted to let as much light into the building as possible and use natural materials to complement the style of the house, while taking advantage of the most upto- date window technology,” he says. After a long search, Andrew opted for mullioned casement windows from Andersen Windows – 30 in total. While these had the slim wood profile he wanted, they also had a special Perma-Shield coating, which meant they would be as low-maintenance as a thicker PVC-U alternative. Energy efficiency was also a deciding factor, as Andersen’s soft coat Low E argon filled double-glazed units offer U values of 1.4, allowing for larger glazed areas. “We were so impressed with the thermal efficiency, we felt that, from an energy point of view, we could almost have built the whole house from glass.”

Case Study: Replacing windows

When Milly Smith moved into a five-bedroom house in Fulham, south London, five years ago, some of the windows were in desperate need of repair. “They had draughts coming in and some of the frames were rotten,” she says. She had two lots of windows replaced by The Sash Window Workshop, which reproduced the existing window style (see pictures) but with toughened glass and draught-sealing. The job was finished within a day. “The new toughened glass now gives better security and the draught-sealing keeps the house much warmer – I feel it has been a great investment,” says Milly.

Image: Oak casement windows by Kloeber

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