While window frames and timber cills are an individual repair, if they aren’t properly maintained, they can cause damage to the common structure of the building. Close windows, as well as lintels and stone cills that are part of the structure of the building, are common repairs.

Having everyone’s windows the same style and painted in the same colours can greatly enhance the look and value of your home. It’s a badge of quality, so it is worthwhile maintaining what you’ve got.

And there is much that can be done to increase the energy efficiency of your existing windows.

Identical windows. Enhancing the building.

Repairing your windows

Preventive maintenance will save you a lot of money.

Regular repainting

Timber cills and the bottom rail of windows will decay first, and badly decayed cills can allow water in. Repaint timber and metal windows every five years, or less if the windows are in more exposed locations.


Get the mastic around your windows checked every painting cycle. If it needs to be replaced, the traditional burnt sand and boiled linseed oil mastic lasts longer and retains its flexibility better than ordinary, trowel applied builders’ mastic. Polysulphide mastics applied with a gun are not good at filling large gaps and are best not used on stone buildings.

Making sash window cleaning easier

‘Simplex’ fittings allow sash and case windows to be opened inwards for cleaning.

Simplex window system. Easily obtained fittings allow the lower sash to be swung inwards and easy cleaning of both upper and lower sashes. Image courtesy of Annie Flint.

Improving energy efficiency

Older tenements often have large areas of window, so treating them can make a significant impact to your comfort levels and fuel use.

The draught felt from windows may not be cold air coming in through the frame or between sashes but a convection down-draught, a movement of air caused by the difference in temperature between the window surface and the air in the room. You can check by resting a card against the window. If it blows over, then you need to tackle the draught.

More on energy efficiency

Replacing windows

It is recommended that you consider all the options before you think about installing new windows. This is because:

  • the cost of installing good ‘A’ rated new windows is high and the payback times are very long – the Energy Saving Trust estimate that installing A++ rated double glazed windows will save just £30 – £35 a year in the typical flat
  • cheap window installations placed in existing window frames will actually cut down on light received and passive solar heating gain
  • uPVC will decay faster than timber
  • the fixings and fittings on a uPVC window may be more difficult to repair or replace than fittings on a timber window
  • uPVC material can contribute to fires, more so than timber
  • retaining existing timber windows is better in conservation and sustainability terms

A cheap uPVC window installation. Note the deep uPVC frame added to the depth of the old timber frame. Up to 10 square feet of daylight can be lost in a bay window of an older Victorian tenement. Less sunlight means less gain from solar heating.

If you decide to replace your windows, make sure you get the proper consents as there are lots of regulations about window design, safe cleaning and cill heights. If you are a landlord, or if you are carrying out major repairs, you may be asked to comply with these standards – check with your Council’s Building Control team.

Check if you need planning consent:

  • if you are in a listed building or a conservation area, you will certainly need planning permission for new windows
  • many Councils also require all flat owners to apply for consent for window replacement – check with your local planning department

Rotten window frame. This can let water into the structure of the building, leading to rot in joist ends. Rotten cills can be replaced without the need to replace the full window.

Window replacement alternatives

So, what are the alternatives to window replacement?

  • use a sash window repair specialist
  • add draught proofing
  • blinds and curtains
  • shutters
  • secondary glazing
  • slimline double glazing in existing frames

Used alone or in combination, all these methods can have a considerable impact on energy efficiency.

Defective window mastic. This problem can let rain into your building’s structure. Note also the butterfly hinge which allows the bottom sash to hook onto the hinge and open inwards for cleaning.

Fitting new windows

If you are having new windows fitted to reduce noise, if standard double glazed units are used and both glass panes are the same thickness, the result can no better than single glazing.

Gaps around new windows are common. Here, care is needed to ensure the new window is tightly fitted and ideally special tapes are used to seal joints between windows and linings. Sometimes, installers use expanding foam but this can crack eventually.

The gap between window frame and external stone is normally sealed with a linseed oil mastic which should not be painted. Modern polysulphide mastics are often used but these rarely take up the variation in space between the case (window frame) and the stonework.

Ventilation is required into the room, so this can be provided by inbuilt vents in the top of the frame called ‘trickle vents’ to make sure you don’t get condensation in your home.

Energy efficiency alternatives

DIY draught-proofing

All good DIY shops will sell a range of draught proofing materials suitable for windows and the cost of using these is repaid very quickly in energy savings. You can seal a sash window with paint but this is not recommended as once this is done, you cannot open it or clean it, and repainting it is much more difficult.

Sash window specialists

Sash window repair specialists can repair and draught proof timber windows more cheaply than installing adequate-quality new windows. Window frames, cills and sashes can be repaired or partially replaced by splicing new timbers into the side of the case, and draught-proof seals or brushes can be chased into the frames of each sash.

Existing sashes can be replaced with new double glazed sash windows, although the weight of glass will require the lead counterweights to be renewed.

Rotten cill and broken mastic. The cill can be replaced and the window sash repaired. The mastic should be entirely renewed.

Slim double glazing

Slim double glazing panes can be installed in existing windows to maintain the original appearance and this is often a suitable approach in conservation areas and listed buildings. However, this method should not be used if you have historic glass (crown glass).

A number of firms produce these ‘slim’ double glazing panes. These are very thin double glazed units which can be puttied into place in existing sashes. The units are double walled panes of glass, normally filled with an inert gas or a vacuum. They aren’t as energy efficient as modern double glazing units, but do have the advantage of making use of existing window frames. Some types are more efficient that others, and the edge seal can make a difference.

The cost of installation will depend not just on the overall size of the window but the number of individual panes of glass in each window.

Overhauled sash window. This window has had slim double glazing added.

Secondary glazing

Secondary glazing can also be used to reduce energy loss and has the added advantage to being more effective in reducing airborne noise than double glazing. Different opening arrangements can be made to suit your window. For large sash and case windows, it may be best to install the secondary pane in an aluminium frame which can be hinged open for cleaning, or a two pane sash which slides into an aluminium sub-frame fixed to the existing window frame.

A cheaper solution is to use glazed glass or acrylic panels with metal frames which are held into place with magnetic strips attached to the original frame. If your windows are very large, then the magnetic tape alone may not be strong enough to hold the glazing and the panels may need to be screwed to the frame. Paintwork also needs to be in good condition to support the magnetic tape. The drawbacks to this solution are that the glass may prevent ventilation and a storage place needs to be found for the panels if you don’t want to use them in summer.

An even cheaper solution is to use plastic glazing film – but you get what you pay for. The film needs to be applied using double sided tape. The tape can refuse to stick to poor or greasy paintwork and removing it at the end of the season can remove paint at the same time. The plastic film (single-use) is tightened up by heating with a hair dryer but is easily torn and not easily cleaned. It also makes your windows hard to open. If you are an owner, we recommend you invest in more permanent solutions.

Comparing the effectiveness of secondary and slim double glazing
Glazing and frames Reduction in heat loss U-value W/m2K
Secondary glazing system 63% 1.7
Secondary glazing and curtains 66% 1.3
Secondary glazing and insulated shutters 77% 1.0
Secondary glazing and shutters 75% 1.1
Double glazed pane fitted in existing sash 79% 1.3

(figures from Historic Environment Scotland)

Blinds, curtains, and shutters

A range of thermal and draught excluding blinds can be purchased or made. For greatest impact, get blinds which create air pockets in their layers. These thermal blinds can have a honeycomb structure or use a reflective layer of material or mylar in their construction.

Draught excluding blinds fit into vertical runners but if you make your own blinds, you can construct a hinged timber piece to fit on the wall and fold over the edge of the blind to keep out draughts. If making or buying curtains, go for full length, look to use a heavy fabric, and add thermal linings. Reflective mylar sheets, felted materials with reflective strips or even old blankets, are ideal.

Dealing with radiators below windows. Warm air is prevented from going behind curtains which are tucked in behind a timber upstand attached to the cill. Image courtesy of Annie Flint.

Make sure the curtains don’t cover the radiators. If you can’t move the radiators, create a shelf fully around the window area, above the radiator and fit the curtain length to the shelf. Consider using curtains and blinds together for extra impact and flexibility.

Comparing the effectiveness of curtains, blind and shutter solutions
Curtains, blind, and shutter options Reduction in heat loss U-value W/m2K
Fitting and shutting heavy curtains 14% 3.2
Modern roller blind 22% 3.0
Modern roller blind with low emissivity plastic film fixed to the window facing side of the blind 45% 2.2
Victorian blind 28% 3.2
A ‘thermal’ duette honeycomb blind 36% 2.4
Closing shutters 51% 2.2
Victorian blind and shutters 58% 1.8
Modified shutters, with insulation inserted into panels 60% 1.6
Victorian blind, shutters, and curtains 62% 1.6

(figures from Historic Environment Scotland)

Many older tenement flats have shutters already fitted. Some of these may just be decorative. These won’t have hinges fitted. Shutters designed to open may have been painted up but can be refurbished with a bit of DIY. Draught proofing and additional insulation on the reverse of the panels by way of an aerogel insulation board can also be added to increase effectiveness. New shutters can also be made. These can be fitted into the old shutter recess, if there is one, or fixed to the window frame or case.

Thermal image – shutters in use. Showing benefit of using traditional shutters. Image courtesy of Changeworks.

Comparing the effectiveness of different shutter solutions
Shutters Reduction in heat loss U-value W/m2K
Closing shutters 51% 2.2
Modified shutters, with insulation inserted into panels 60% 1.6
Victorian blind and shutters 58% 1.8
Victorian blind, shutters and curtains 62% 1.6
Secondary glazing and insulated shutters 77% 1.0
Secondary glazing and shutters 75% 1.1

(figures from Historic Environment Scotland)

Effects on your EPC Rating

Permanent solutions, such as installing secondary glazing or slim glazing, will have a positive effect on your EPC rating. Removable solutions, such as thermal curtains and blinds, will not impact on your EPC rating but will save you energy and money.

Who pays?

Window frames, the moving parts of the window, and the mastic that seals the frames in the wall are generally an individual responsibility. Everything around this is generally a common repair (check your titles to see what applies to you).

Even though windows are an individual repair, you have a legal duty to maintain them. The main thing to do is prevent water trickling into the walls and causing rot.

It is worth joining with other owners to get all windows painted and the mastic repaired at the same time.

Around your windows

The stone and brick in the walls around your windows are generally a common responsibility. The elements that require your attention are:

  • lintels – supporting the walls above the window opening
  • cills – need to shed water to help keep the walls below dry
  • canopies, architraves, and hoods are not just attractive – they help protect the windows from the weather
  • mullions – the vertical stones between windows

Close windows – a common repair

Make sure you can open the close windows to allow ventilation to the stair. This can help prevent condensation occurring on the skylight. Repaint every 5 years, or more often in exposed locations.

If you have stained glass windows or painted Victorian glass, then this should be conserved. It is possible to remove painted glass panels and sandwich them between toughened glass to give them more protection, however, the weights to the sash would need to be adjusted.

Other types of windows

Professional help recommended?

Although the work may appear straightforward, ensure your builder or tradespeople have the skills for the job. If in any doubt, get professional help to specify and organise the repair.

Further information:

406 305 Under One Roof

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