Windows & Glazed Doors


Windows and Glazed Doors

A key element in any new build, extension or renovation project, the right choice of fenestration can dramatically improve the appearance, energy efficiency and security of your home.

Picture: IQ Glass.

For repairing, fitting or replacing existing doors and windows in a house, planning permission isn’t normally needed. Double glazing can be installed under Permitted Development, provided the building isn’t listed.

New door or window openings of a similar appearance to those used in the construction of the house won’t require planning consent, as long as any upstairs windows in side elevations are glazed with obscure glass and fitted in a nonopening frame (unless the opener is more than 1.7m above the floor of the room in which the window is installed).

Building Regulations apply to thermal performance and other areas such as safety, air supply, means of escape and ventilation.

Doors and windows need to comply with the requirements of Building Regulations Part L in relation to the amount of heat that can pass through the glass and framework.

Window installation options

There are two options when it comes to installation.

You could use someone registered with a competent person scheme such as Certass  or FENSA who will be approved to carry out the work to comply with Building Regulations. Once the installation is completed a certificate will be issued as proof that the work was done by a registered installer.

Alternatively, you could use an unregistered installer or do it yourself, in which case approval can be sought from the relevant body – either your local authority or an approved inspector. They will check the replacement windows or doors for compliance.

Energy efficiency

Double glazing is now standard in new homes, reducing heat loss and offering sound insulation. It works by hermetically sealing a layer of air between two sheets of glass so that the air acts as an insulant. This reduces heat loss to the outside and allows windows to be larger while still complying with Building Regulations.

Replacing the air with an insulating gas, such as argon, will further increase thermal performance. Coat the glass that faces inwards with a reflective coating and this thermal performance is improved even more.

The key to reducing heat loss lies in the width of the air space between the panes of glass - the wider the gap the better the insulation.

Factory triple glazing is becoming increasingly popular for low-energy homes. Energy savings from triple glazing are relatively small, but comfort levels inside the house are increased, with fewer cold spots. The additional weight of triple glazed windows will also reduce sound vibrations.

Energy-efficient windows and doors are available in a range of frame materials and styles. They vary in their energy efficiency, depending on how well they stop heat from passing through the window, how much sunlight travels through the glass, and how little air can leak in or out around the window.

The BFRC Scheme is the UK’s national system for rating the energy efficiency of windows. BFRC assesses the performance of windows and doors and gives them a rating between A++ and E.

Genuine BFRC-rated products are labelled with a small official sticker on the inside of the frame and often a large rainbow label on the surface of the window or door: clear, visible proof that it will perform as promised.

British Fenestration Rating Council chart showing the energy performance of the various bands of windows.



Glazing and U-values

U-values measure the rate of heat loss by the entire window / door through the frame, sash and glass.

The lower the U-value figure, the better the window or door will keep heat inside a building on a cold day.

There are two U-values are used for glazing:

  • Ug-value - the thermal transmittance of the glazing itself (or the centre-pane U-value); and
  • Uw-value - the thermal transmittance of the whole window – glazing and frame together.

Building Regulations and Passivhaus have requirements for the whole window Uw-values of windows and doors.

Building Regulations

For new builds the backstop is 2 W/m2K, but 1.4 W/m2K is recommended for the Target Emission Rate.

For retrofit, it is 1.6 W/m2K, which is a window with Energy Rating C.

For Passivhaus, the Uw-value should be less than 0.8 W/m2K when modelled with glazing Ug of 0.7 W/m2K.

Choosing windows

Windows are like the eyes of a building, and a simple alteration to size or shape can change the entire character of a property, as well as the quality of light entering its interior. Windows may have come a long way since they were first introduced into our homes in the early 16th century but, with so much choice, selecting their size and design can be a minefield.

A large fixed window adds drama to this bedroom. image:

It’s important to make your choices early on in the design stage as the size and position of window openings are part of the house plans. Any late variations may need to gain planning approval and could be costly. 

Crittall-style doors and windows are currently a favourite design choice.

Popular window types

The image slider below shows ten window styles.

Fitting bespoke windows is going to be more expensive than purchasing off-the-shelf units, but many standard window sizes come in a variety of styles, which are achieved by altering the glazing bar configuration.

If your home is in a Conservation Area, is listed, or of local historic value then you may be required to match your existing window dimensions and specification exactly to conform to the conservation officer’s requirements.

The dilemma if you are building or renovating a period-style property is how best to maintain authenticity while still meeting Building Regulations to create a low-maintenance, energy-efficient home.

Glazed Doors

Glazed doors provide a seamless transition between outside and inside living image:

Below is our quick overview of the differences between the glazed door styles that are available.

Folding/sliding doors

Also known as bifolds, folding/sliding doors are made up of several glazed panels, and can both slide and concertina in on themselves, effectively opening up the entire wall of a house and allowing unrestricted access. They can open left or right, inside or outside, and can even fold around corners. It’s also possible to install bifolds with a single door which can be used on its own for easy access.

There will need to be space for stacking inward-opening doors, and the design of bifolds means more framework is on view when the doors are closed.

Combined with flush thresholds, folding/sliding doors extend a room seamlessly onto the garden and allow unhindered access.


Bifold doors can be opened up completely on warm days to provide access to the garden.  image:

Bifold doors are not limited to new builds or extensions but can be used to replace older patio doors or create new walls of glazing in traditional or contemporary styles.

It’s important to plan ahead if fitting bifolds, in order to make decisions about drainage and floor levels and to ensure that Building Regulations are met.


Some companies offer a fitting service, which will ensure that the doors are correctly installed by experienced fitters.

Sliding glass doors

Sliding glass doors tend to be made up of two or more large glass panels, opening and closing in line on a track, with one pane sliding behind the other, offering flexibility regarding how far the doors may be opened. They have large areas of glass, but the view will be broken even when the doors are fully open because the frames don’t recess completely.


Sliding doors offer slim sightlines. image:


Available in traditional and contemporary styles, the size, design and frame material of sliding doors should all be carefully considered to ensure the doors suit both your home and lifestyle.

If you’re building an extension or creating a new opening, this will usually need to be completed before measuring up for sliding doors to ensure an accurate fit and this can sometimes delay progress if the doors need to be made to order. Ideally, the supplier or your builder will be responsible for both measuring up and installation to avoid costly mistakes.

Choosing an experienced fitter should ensure the doors operate smoothly on the track and all Building Regulations are met.

Lift and slide doors

Lift and slide doors, unlike simple sliding doors, are moved inside the frame itself on wheels. When the handle is turned the casement is lifted slightly allowing it to be moved aside. As the casements are often heavier than conventional sliding doors, the wheels simplify opening and closing and glide easily. The casement lowers the profile into the frame as well. This makes lift-and-slide systems significantly denser and provides a high thermal insulation value as well.

Lift-and-slide door systems can provide a high thermal insulation value.


Slide and turn doors

Also known as slide and swing or slide and stack doors, slide and turn doors provide an alternative to sliding or bifold doors.

Individual, non-connected panels slide to one end before turning and stacking.
Slide and turn doors have the narrow sightlines of sliding doors with the facility to be completely opened out like bifold doors.


French doors

French doors are more traditional in design and are often seen in older homes. They are usually made up of two outward-opening or inward-opening doors and may be fitted with fixed panes beside them to increase the glazed area.
French door frames usually create a step, rather than the level threshold that patio doors offer.


PATIO DOORS - 5 THINGS TO CONSIDER by Matt Higgs, director Kloeber

1. Design

What colour best suits your internal colour scheme and the external design of your house? Give careful consideration too to the type of frame and choice of materials – wood, composite and aluminium all have different characteristics. Remember, the thickness of the frame will affect the view (sightlines). Examine how the doors open and close, and whether you can open up the whole door or just part of it. Sliding doors can have large panes in relatively slim framework whereas bifolds will generally have more panels.

The combination of glazed doors, windows and roof lights gives this space a real wow factor. image:

2. Space

A consistent floor level will help to merge inside and outside spaces, making your terrace or patio like an extension of the room beyond the doors. Bifolds are really good for opening up your house into the garden. Bifolds offer up to 90 per cent of the aperture whereas sliding doors will always have panes that can’t all be open at once, unless they retract into pockets in the wall. Always consider the amount of panels as early as you can as this affects costs and how you use the space.

3. Performance

Glass will always let out more heat than a solid wall, so it’s a good idea to consider triple glazing or high performing double-glazed units. Different frame materials also have different thermal properties (U-values) with timber usually being a good choice. Consider how the doors fold or slide and how well they seal between panels. Thermal and weathe rtesting reports should be available from reputable companies selling high performance sliding or folding doors.

4. Price

If you’re looking for a large feature door for your home, don’t sell yourself short. A cheap door is likely to let you down and not perform well. You tend to get what you pay for, so buy the best that you can afford. Do your homework and ensure you know exactly what you’re buying and how it performs against other options.

5. Weather

How much your house is exposed to weather can be a real factor in determining your choice. If you’re in a very exposed area then go for a low-maintenance material with good Pascal (air permeability) ratings. Always ask for weather-testing reports on what you’re buying. Quality doors that achieve Secured By Design will have this.


A fully glazed wall adds drama to the rear of this new home. image:

Choosing Materials for Windows and Doors


Timber doors and windows have a traditional look and feel. When treated correctly, exterior wooden doors and windows will last for more than 100 years and are beautiful to look at with a natural texture.

When treated correctly, exterior wooden doors and windows will last for more than 100 years

Wood can be adapted in size and repainted in different colours, but it is vulnerable to the elements, needs to be treated and maintained, and can be prone to twisting and warping. Cheap wooden doors and windows can come apart at the joints and may rot if not sealed correctly.

Softwood is cheaper than hardwood and will soon deteriorate if not regularly painted or sealed.

Hardwood is more durable, and offers good thermal insulation, but requires regular maintenance.


uPVC doors and windows are often cheaper than timber versions, have excellent weather resistance, are easy to maintain and require no painting or staining.

uPVC windows are well insulated, more stable than timber, and usually come with multipoint locking.

It will be difficult or impossible to alter the size or colour of plastic doors and windows once they are fitted, however, or to change locks and door furniture with ease. uPVC doors can look flimsy or cheap and may reduce the value of period homes.



Steel windows and doors have been installed in buildings since the 1930s and are currently enjoying a return to fashion.

Steel frames combine slender sections with the modern benefits of thermal performance, security, durability, and ease of maintenance. 

Today’s steel window is double glazed with thermally efficient units, and polyester powder coating means it will not need to be painted for at least 20 years.

For more information visit the Steel Window Association website.


Popular for contemporary homes, aluminium windows and glazed doors are available in virtually any colour. Aluminium frames are finer than uPVC and are lightweight, strong and maintenance free. Aluminium frames are often more expensive than other window and door materials.


Composite doors and windows are made, as the name suggests, from a mixture of materials. They tend to be more expensive than other options. In a composite window, timber is often clad externally with a weatherproof material such as aluminium to create a robust, low-maintenance material on the outside with the attractiveness of timber inside.

Roof Lights versus Lanterns

Roof lights and lanterns are an excellent way of bringing additional natural light into dark interiors.

Lanterns sit proud of the roof, and can have some fancy ornamentation. They were popular in the Victorian era and can often be found on period properties.

Roof lights, on the other hand, tend to have slim clean lines and contemporary frames which are well suited to modern designs. Fitted with the appropriate glazing, they can even be walked on, making them ideal for flat roof terraces.

Lanterns usually have standard dimensions, with the light broken up and reflected by the intersections in the frame. 

Roof lanterns are ideal for rooms that are already well lit but require a wow factor.

Flat glass roof lights are generally slightly cheaper per square metre, but the price difference isn’t a deal breaker. However, their superior insulating properties mean that they can potentially bring in more warm air than escapes, even in winter, reducing heating bills.

There is little to choose between flat glass roof lights and roof lanterns when it comes to ventilation as both types can be opened.

Individual flat glass roof lights can be up to four square metres with the added flexibility to join individual units together to create limitless expanses. There is also more flexibility with their shape and distribution.

Lanterns, however, can be built bigger overall as they have smaller panes in the frames. Typically, they are square or rectangular, although different shapes are available, if your budget will stretch to that.

Because of the slope, a lantern will be easier to keep clean, and self-cleaning glazing is avalaible on suitable pitches.

When choosing between roof lights and lanterns, maximising daylight through the roof will be the top priority, followed by ventilation and insulation.