Installing a uPVC or timber window

Installation means setting the window into the opening. Related work usually has its own items in the price list. Agree these with the company in advance so there are no misunderstandings:

• Removing and disposing of the old window
• Internal window board and external sill
• Making good and plastering the reveals
• Fitting blinds, shutters or insect screens
• Projecting installation on brackets in externally insulated houses

When you get a quote, ask whether the installation includes sealing tapes. Some companies fit them as standard, others only on request. A window without tapes goes in faster, but the joint between frame and wall stays unprotected.

An experienced fitter has a routine, but every opening is a little different. In older houses the opening is often out of square, or it is wider at the top than at the bottom.

Checking the opening and preparing the window

The fitter first measures the opening and compares it with the window. There should usually be a gap of roughly 10 to 20 mm around the frame so it can be filled properly with foam. If the gap is too narrow, the foam cannot get in. If it is too wide, it has to be built up first. On larger windows the sashes are taken off and the glazing units removed. This makes the frame easier to handle and protects the glass while it is set in place.

Before the frame goes in, the sealing tapes are stuck to it: a vapour-tight tape on the inside and a vapour-open tape on the outside. They go on first because once the frame is in the opening, it is hard to reach.

Setting, levelling and fixing

The frame stands in the opening on packers, usually plastic or hardwood. The packers carry the weight of the window into the masonry, so they stay in place after fitting. The frame is levelled horizontally and plumbed vertically with a spirit level, and the diagonals are checked. If the diagonals differ by more than the manufacturer allows, the sashes will not close properly later.

The fixings hold the window firmly in the opening. The usual options are:

• Frame fixings or concrete screws through the frame into the wall, common in solid brick and concrete
• Steel fixing straps or lugs attached to both the frame and the wall, suitable for hollow blocks, aerated concrete and where you do not want to drill through the frame
• Installation brackets for a projecting installation in the insulation layer

Fixings usually go roughly 15 cm from each corner and then at intervals of about 70 cm. The profile manufacturer sets the exact spacing. With screws through the frame, the fitter must not overtighten them. Otherwise the frame bows and the sash will rub.

Filling and sealing the gap

The gap is filled with expanding polyurethane foam. It is filled to about two thirds, because the foam keeps expanding. Too much foam can push the frame inwards and distort the window. You see this most on wide balcony or patio doors. Once the foam has cured, the excess is trimmed off and the tapes are stuck to the reveal. The inner tape keeps moist room air out of the foam. The outer tape protects the foam from rain and UV light but lets moisture escape outwards.

Hanging the sashes and adjusting

Finally the sashes are rehung, the glazing refitted and the hardware adjusted. A sash should close easily without rubbing, and the gasket should press evenly all the way round. The fitter also checks the night vent or micro-ventilation position and the mis-operation lock, which stops the sash being tilted and turned at the same time.

The procedure is similar for both materials. The differences are in the details, and these affect both the time needed and how carefully the fitters work.

uPVC windows

A uPVC frame is lighter and is not harmed by damp on site. It has steel reinforcement inside, and the fixings go into that. Dark colours and woodgrain foils on uPVC expand more in heat. On long frames manufacturers therefore recommend keeping exactly to the fixing spacing and not clamping the frame rigidly. Peel the protective film off the profiles as soon as possible after fitting, usually within a few weeks. In sunlight the adhesive bakes on and becomes very hard to remove.

Timber windows

A timber frame is heavier, and the surface is more easily scratched and marked by moisture, lime or mortar. Fitters therefore protect timber windows more thoroughly, covering them with film and a tape that will not lift the paint or lacquer. Timber moves with humidity, so it should not go into a damp new building where plastering or concreting has only just finished. If that cannot be avoided, the rooms need plenty of ventilation. Where screws go through the frame, the holes are closed with caps or plugs in a matching colour.

Where in the wall the window sits

For both materials, the position of the window within the wall thickness matters. In an uninsulated solid wall the window usually goes roughly in the middle third. In a cavity wall it is normally set in line with the cavity, so the cavity closer or insulation meets the frame. In a house with external insulation it pays to move the window towards the outer face of the masonry, or right into the insulation layer, so the insulation can lap over the frame. Otherwise you get a cold bridge, and the inside reveal can feel cold in winter.

Good preparation shortens both the installation and the clean-up. Before the agreed day, it helps to have the following sorted:

• A clear space of roughly 1.5 to 2 metres in front of each window, with plants, curtains, curtain poles and furniture moved
• The floor covered, and any furniture that cannot be moved protected
• Access to a power socket for tools
• An agreed place to put the new windows and to load the old ones
• In a block of flats, the building management or owners' association informed, and the neighbours warned about noise
• Parking space for the van

Permissions and the building management

Replacing windows like for like in a house often needs no separate planning consent. Many countries still require replacement windows to meet building regulations, especially for energy performance and sometimes for fire escape and safety glazing. In some places the work has to be notified to the authority or done by a registered installer who certifies it. Listed buildings and conservation areas usually have stricter rules on appearance and materials. Check with your local building authority before work starts. Enlarging an opening is a structural change, and so is removing the brickwork under a window for a balcony door or making a new opening. That needs a structural engineer and the building authority.

In a block of flats the windows form part of the facade. Frame colour, sash layout and any change to the opening usually need approval from the building management or owners' association. Ask before the work, not after it.

A single "per window" item does not mean every window takes the same effort. The amount of work depends mainly on:

• The size and weight of the window, since a large triple-glazed unit is heavy and needs more people
• The floor and access, since a ground-floor window is easier than one on the fourth floor without a lift
• The state of the reveals, since crumbling plaster or an uneven opening has to be made good
• The type of wall, since hollow blocks and aerated concrete need different fixings than concrete
• Unusual shapes, arched and raked windows, which are harder to level
• Balcony and patio doors with low thresholds, where the connection to the floor and the waterproofing need attention
• Scaffolding or a lift platform for external work at height

In modern blocks of flats installation is fairly predictable, because the openings are straight and the sizes repeat. In old brick houses, and where the old windows date from before any insulation work, removal often shows that the reveals are in worse condition than they looked.

A window is where several trades meet. If the order is wrong, something will have to be done twice.

• The company that supplies and fits the windows should take the measurements, so it is responsible for the sizes
• If you plan external insulation, decide the window position before ordering, because a projecting installation is hard to add later
• Call in a qualified electrician before installation if there will be electric shutters, external blinds or opening sensors
• The external sill goes in after the window and before the facade is finished, the internal board after the foam has cured
• The plasterer finishes the reveals only after the tapes are on and the foam has cured
• Painting comes last, once the reveals are dry

In practice it often happens that the windows are replaced and the reveals are finished six months later, with the foam exposed in between. If there is no other way, at least the outside should be protected with tape or temporarily covered.

Most window problems do not appear on the first day. They appear during the first or second winter.

Missing or badly applied tapes

Warm, moist room air gets into the foam, condenses in the colder part of the joint and gradually soaks the foam. The result is dark patches and mould in the corners of the reveals, often on one side of the window only. On the outside, unprotected foam yellows in the sun, crumbles and stops sealing.

A window held only by foam

Foam is not a structural fixing. A window held only by foam moves over time. The sashes start to rub and the hardware wears faster. Mechanical fixings are always needed, even if the window seems solid at first.

Too much foam and an overtightened frame

A bowed frame is hard to see by eye. You notice it because the sash does not seal in the middle, there is a draught, or the window is stiff to close. On balcony doors the threshold often distorts.

The wrong position in the wall

If the window sits too far out in an uninsulated wall, or too far in on an insulated one, the reveal is often cold in winter and gets condensation. Why does the area around the window get wet and not the whole wall? Because the wall is thinnest there, and heat escapes fastest.

Blocked drainage

There are drainage slots in the bottom of the frame. If foam or plaster blocks them, or the external sill covers them, water stays inside the profile and can cause problems in frost.

Two fitters usually install an ordinary window in a flat in one to three hours, including removing the old one. A whole flat is usually done in a day. A house takes one to several days, depending on the number of windows and the access.

PU foam can usually be trimmed after a few hours and, according to the manufacturer, normally cures fully within about a day. In cold weather it takes longer, and a winter foam made for low temperatures should be used. The reveals are finished only after the foam has cured and the tapes are on. The plaster in the reveal then needs time to dry before painting. How long depends on its thickness and type.

Fitting in winter is possible. The windows are replaced one at a time so the rooms do not get cold, and the fitters keep to the temperature limits the manufacturer gives for the foam and the tape adhesives.

Leave the installation itself to professionals, especially for larger windows, work at height and in blocks of flats. The units are heavy and fragile, and a faulty installation usually voids the warranty. What you can do yourself is prepare the room and, later on, check and fine-tune the window.

1. Move furniture and other items to about two metres away from the window.
2. Take down curtains, curtain poles and blinds, and clear plants off the window board.
3. Cover the floor with strong sheeting or cardboard and tape it down so it does not slide.
4. Cover any furniture and electronics that stay in the room to protect them from dust.
5. After installation, peel the protective film off the frames before the sun bakes it on.
6. Try every handle position: closed, open, tilted and the vent position.
7. With the window closed, put a strip of paper between the sash and the frame and try to pull it out at several points around the frame.
8. If the paper slides out easily anywhere, report it to the fitting company, or turn the locking cams with an Allen key as the manufacturer's instructions describe.
9. Once a year, clean the gaskets and drainage slots and oil the hardware with a lubricant made for window hardware.

If a sash rubs on the frame, do not force it into adjustment. The cause is often a bowed frame or poor packing, and that is a warranty matter for the installer.

Check the window at handover, while the fitters are still there. Go through the following:

• The frame is level and plumb, checked with a spirit level
• The sashes open and close easily in every position
• The gasket seals all the way round and the paper strip does not pull out easily
• The glass and frames are free from scratches and cracks
• The fixings or fixing straps are in place and visible before the reveal is covered
• The inner and outer tapes are continuous, without folds or gaps
• The foam is trimmed and not spilling onto the frame
• The drainage slots are clear
• The vent position and the mis-operation lock work
• You have received the warranty certificate, care instructions and hardware adjustment guide, plus any compliance certificate required locally
• The site is clean and the old windows have been taken away as agreed