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The key to successful property renovation is deceptively simple — buying the right property in the first place. Prospective renovators, therefore, tend to steer clear of houses with potentially expensive, ‘headline-grabbing’ defects, such as structural movement and decrepit roofs.

But there are some parts of the building where the ‘scare threshold’ is generally a lot lower and buyers are less sensitive to defects. Windows are one such area, despite the fact that the cost of installing new replacements throughout can easily run into five figures — not much less than for a major re-roofing job.

Perhaps this relaxed attitude is down to the fact that local DIY stores stock a wide range of off-the-shelf windows that a competent person can fit. Or maybe we should blame the double-glazing industry for seductively portraying window replacement as a desirable and easy home improvement.

Either way, when it comes to window talk there’s no shortage of sales-driven advice, which doesn’t always stand up to closer scrutiny. For example, fitting double glazing is widely believed to be a great way to reduce energy bills by cutting heat loss from your property. But the fact is, windows tend to only leak around 10 per cent of a typical home’s total heat loss, compared to 35 per cent through the walls.

According to the Energy Saving Trust, based on the resulting savings in reduced bills, it can take well over 50 years for the cost of installing new windows to be paid back. One thing the salesmen don’t stress either, is that much of this improvement is simply down to draught-proofing, thanks to snugly fitted new frames.

Another common misconception is that PVCu windows last forever and are maintenance free. In fact, there are a number of problems that can develop over time, such as defective handles, damaged seals and the glass misting up. Plus, anything made of PVCu on the exterior of the house will eventually succumb to the effects of UV light, becoming brittle and fading or yellowing. So it’s not unusual for PVCu windows to need replacement within 25 to 30 years.

When it comes to upgrading old windows, complete replacement isn’t the only option. In some cases, refurbishing can make more sense. The optimum solution will depend on factors such as the condition of the existing units, the type and location of the property, and, of course, the size of your budget. In older buildings the quality of original windows is generally far superior to modern equivalents, so it’s usually a better option to salvage and restore them. By contrast, softwood windows dating from the 1960s to 1980s can be especially prone to rot, and are likely to require complete replacement.

Of course, the bottom line with refurbishment projects is whether an improvement will add significantly more value than the cost of carrying it out, and you might find that offering prospective buyers a more affordable sale price could be a better inducement than the lure of shiny new glazing. On the other hand, replacing horrible old anodised aluminium units afflicted with unsightly black mould growth can do a lot to transform the market appeal of a property.

Period Properties

Ripping out original windows in historic buildings can be a sure-fire way of diminishing both their character and market value, as well as possibly contravening planning laws. A good alternative can be to overhaul and retain them, perhaps incorporating double-glazed units in the form of secondary glazing that opens to the inside (which will provide superior sound-deadening qualities than conventional double glazing alone).

Alternatively, new double glazing can sometimes be fitted to existing window frames; super-thin double-glazed units have been developed. A couple of caveats, however: this option is better suited to casements rather than sliding sashes, where the extra weight can cause problems with counterbalancing. You may also need to use a router to enlarge the glazing rebates to accommodate thicker glass. And bear in mind that conservation officers generally resist the loss of original glass.

There is, however, one scenario where replacement can actually be welcome in historic properties, and that’s where inappropriate replacements have already been installed. That said, listed building consent is still required for listed homes, even if you’re hoping to do the decent thing and rectify past wrongs.

As old buildings tend to move in tune with seasonal changes to ground conditions, replacement windows need to tolerate this without distorting; something that timber frames are generally better suited to than aluminium or PVCu.


Planning consent is not normally required to replace windows, unless buildings are listed or located in Conservation Areas (where it applies to the ‘principal elevation’, usually the one facing the road). However, replacement windows installed after April 2002 should have Building Regulations approval (the exception is where you’re just replacing the glass, not the whole window).

Building Regulations
The Building Regulations define new windows and doors as ‘controlled fittings’ (Part L1B) which must meet maximum permitted heat loss standards based on either a ‘C’ Window Energy Rating (WER) or a minimum whole window U value of 1.6W/m²K. Replacement windows also need to comply with requirements for ventilation, e.g. with trickle vents in window frames and opening lights.

At least one window per floor should be large enough to allow escape from fire, too (not normally a problem as the stipulated minimum opening area need only be 0.33m², with width and height no less than 450mm).

In most cases, installation work will be carried out by a FENSA-registered installer (visit, who can ‘self-certify’ the installation and provide the necessary compliance certificate. So an application to Building Control only needs to be made when windows are replaced on a DIY basis or by an installer not registered as a ‘competent person’.

Lead Paints
When it comes to overhauling old windows, bear in mind that lead-based paints were widely used as late as the 1960s, so layers of old lead paint will be lurking under later coats of modern gloss. But there’s only a potential risk is lead compounds are ingested or inhaled, usually from sanding. Simple precautions should therefore be taken against ingesting dust when rubbing down.

rotting timber frame on a window

Problems with Timber Frames

Identification & Implications
Rot is a particular problem and soft, decayed wood, flaking paint and sticking windows are all signs. Rot tends to be caused by water penetration and pooling leading to deterioration of the timber; this in turn tends to be the result of a lack of maintenance or where corner joints have worked loose.

Lower frames, joints and cills are especially vulnerable. Flaking paint may conceal early signs of decay, particularly to bottom rails and lower sashes.

To test for rot, check to see if the wood is soft and spongy, and can be dug away with a screwdriver. Loose joints are another common problem.

Remedial Work
It’s usually possible to repair timber windows.

  • Small areas of rot can be cut out and filled with putty or filler.
  • With larger areas, it should be possible to cut away the damaged wood, treat the remaining sound wood with preservative fluid, and graft new sections of replacement timber into place. (Preservative tablets can also be inserted into holes drilled in the frame; these release a preservative into the timber.)
  • A primer and undercoat followed by two topcoats of exterior paint should ideally be applied as a finish.
  • Cracked or loose glazing putty must be renewed too.

Cills are a notorious weak point, and can often be repaired by cutting the rotten face back to sound wood and then planting in a new piece of seasoned wood, using glue and non-ferrous screws. It’s important to ensure that there’s a clear drip groove on the underside of the outer cill, in order to disperse rainwater safely away from the wall. The drip grooves on old cills can sometimes become blocked with paint; water can soak through the wall below as a result.

Where the entire cill has to be replaced, taking out the old and replacing it with a new softwood cill will cost in the region of £110* (based on a 100 x 75 x 1,200mm cill).

Loose joints are another common problem. The simplest form of repair is to strengthen the corner of a window with an angle bracket made from non-corroding brass or stainless steel. Once any decay has been cut out and filled, the bracket can be secured in place and concealed.

Alternatively, loose joints can simply be reglued. This normally requires taking the windows out and clamping the glued frames in a workbench. Where larger areas of timber are defective, they can be cut out and new matching sections to frames, rails, stiles and glazing bars scarfed in.

Repairing Rotten Windows

Period window specialist Ventrolla provides a nationwide repair and renovation service for timber sliding sash and casement windows and doors. One issue that it can tackle is rot to timber windows (which is often the result of water penetration or pooling on or around the frame and/or cill).

1&2. Here, decaying timber on the cill and frame has been routed out to expose sound timbers.

rotten timber window frame
repairing a window cill

3. A new cill has been fitted and new timber spliced into the frame to make good.

repairing the window frame and cill

4. A specially formulated epoxy resin, VR90, has been used to fill any cavities and the new timber moulded to match the existing profile of the timber.

filling gaps in a rotten timber window with epoxy resin

5. It’s then primed ready for final decoration.

repaired window primed and ready for decorating

Ventrolla offers a free no-obligation survey and quotation.

Misting up Window Panes

One of the most common defects encountered with double glazing is their tendency to ‘mist up’, sometimes within a surprisingly short period of time after installation. Condensation forms inside the sealed unit cavity between the inner and outer panes. Even a tiny pinhole in the seal around the edge of the sealed unit can allow water vapour to penetrate the inner space between the glass panes.

Units normally include some form of desiccant material to absorb small amounts of moisture, but this eventually reaches saturation point. So when the temperature drops, the vapour condenses into droplets on the cold surface of the glass. In warm weather the moisture may temporarily disappear.

Misting up is typically caused by the erosion of the edge seals, which become permeable. This can happen where water has accumulated over time under the lower edge; often the result of poor design or careless installation of the sealed units within the frame, with inadequate spacing blocks and drainage.

Once a sealed unit has started to mist, there’s not a lot that can be done other than replacement (although short-term fixes sometimes work). Where one or two windows have misted, there’s a good chance the others will develop similar issues. This is why it’s important to ensure new installations are covered by a valid insurance-backed warranty.

Remedial Work
The solution is to take out and replace either the glazed sealed unit itself or, more typically, to take out the whole window.

  • Before cutting out the old windows, it’s important to check whether there’s a suitable lintel supporting the masonry above. If not, temporary support will be required.
  • If the old glass can’t be easily removed without damaging it or cutting, it’s a good idea to cover both sides with clingfilm to reduce the risk of small shards flying off.

A couple of things to bear in mind include checking with the contractor in advance whether the price includes making good to internal plaster and decorations and externally to render, etc. This is something that commonly gets ‘overlooked’. When it comes to timber windows, the opening should also be lined with a strip of damp-proofing material, too.

Replacement windows should have integral security locks, but bear in mind that an emergency escape may be necessary in the event of fire, so consider fitting ‘thumb turn’ rack bolts, or cylinder locks that can be opened without a key from inside.


  • Taking out and renewing a softwood casement window (measuring 1,350 x 1,200mm) will cost in the region of £600.
  • Installing a PVCu casement 1,500 x 1,050mm in size will cost around £500.
  • Replacing a Victorian-style sliding sash (of 1,500 x 1,100mm) will cost £1,100.
  • Taking out and renewing the glass panes in a casement window (measuring around 600x800mm) will cost around £120.

Damp and Draughts

An obvious sign is rain and draughts entering through gaps in and around the windows. Condensation forming on windows or reveals internally and dripping down – sometimes mould is evident, too (above) – are also clues. Externally, obvious gaps between frames and surrounding reveals/jambs may be visible.

Condensation can be relieved with improved room ventilation and extractor fans to expel moist air. It’s common for the sides of the walls around window frames (the reveals) to suffer from damp and mould due to cold bridging, as the masonry surrounds act as a pathway between the cold outdoor and warm indoor air.

fitting draught proofing strips

Draughtproofing windows can be enormously cost-effective

Remedial Work

  • Gaps between the frames and surrounding masonry wall reveals need to be sealed with a suitable silicone mastic, or larger gaps pointed up in a weak mortar.
  • To prevent cold bridging when refitting windows in cavity walls, special insulated plastic ‘cavity closers’ should be installed.
  • Draughtproofing windows can be enormously cost-effective, too, while fitting good-quality internal secondary glazing is a great way to stop draughts, minimise heat loss and increase sound insulation.

Find more information on draughtproofing in this article on insulating old homes.

Corroded Metal Windows

Signs include rusted frames, which may have warped or become distorted. Cracked glass, leaded lights with loose cames and small glazing panes that have buckled are also symptoms.

corroded metal windows

The most serious issue that can afflict iron or steel windows is rust. As the metal corrodes it expands, causing distortion which can eventually lead to the glass cracking. But even windows that appear to be beyond repair can often be successfully overhauled.

Remedial Work

  • Where rust only affects the surface, it can simply be cleaned off with a wire brush (but take precautions as red lead was a commonly used primer).
  • Corroded sections of old wrought iron windows can be cut out and new replacement sections welded in.
  • Repairs to cast iron are more difficult as it’s almost impossible to weld; it may instead require ‘cold-metal stitching’.
  • Once all rust is removed, apply a good-quality zinc primer before painting.
  • Old metal windows may have also acquired multiple coats of paint which, once stripped, can greatly improve their functionality.
  • Where old leaded lights have become loose and draughty, fitting secondary glazing internally can sometimes provide an effective solution. In more severe cases, the entire leaded light may need to be removed and professionally rebuilt wholly or partially using new lead.

Defective Lintels & Structural Movement

Any opening in a wall is a weak point and older solid walled buildings can suffer problems where hidden wooden lintels have become rotten. Clues to this can include cracking over window openings externally to the brickwork (see image below) to brick arches, stonework or render. Cracking to internal plasterwork can develop where internal timber lintels are defective.

cracking brick over lintels

Movement can also occur in more modern cavity wall properties in the form of stepped cracking to the external masonry above a window opening; this can be caused by a lack of support to the outer leaf during replacement window installation works (more on this later).

Implications & Remedial Work

Timber lintels

Timber was the traditional material for lintels spanning openings above doors and windows. So in older solid walled buildings, even where the outer face of the wall has a brick arch or stone lintel, there is often a secondary beam sitting behind it made of wood, often plastered over.

Even where an old timber lintel has started to rot, it may not automatically need replacing. The important thing is to expose it and allow it to dry out. This often requires the removal of cement renders or impervious modern paints that are trapping damp.

More Recent Buildings

When it comes to more modern buildings, it’s just as important that the contractors carry out checks before removing the old windows. For example, many properties built between the 1940s and 1970s have no lintels over window openings because the original frames were designed to support the walls above. But replacement windows aren’t designed to support such loadings.

If there is no lintel, suitable temporary support must be provided and a new lintel inserted, with guidance from a structural engineer. (Hence the requirement for Building Regulations consent when old windows are replaced with new windows.)

Where existing windows have already been replaced in cavity walls without lintels, stepped hairline cracking to the wall above the opening is very common. The opening will need to be stabilised, and where necessary, a new lintel inserted to support the weight of the outer cavity.

A similar but more serious problem can occur with bay windows, particularly to 1930s houses. The original windows often had discreet integral columns supporting heavy loadings from roofs. So it’s again important that replacement windows are designed to provide sufficient structural support.

Sticking and Stiff Sash Windows

Identification & Implications
Sashes are heavy and prone to jamming, or they may slide down uncontrollably.

Sash windows are fairly straightforward to overhaul. Common problems include broken sash cords and jammed pulleys which are obvious and easily fixed, but other defects are less obvious.

  • Where the original 2mm-thin glass has been replaced with modern thicker glass (which must be a minimum of 3mm), the counterbalancing iron or lead weights in the side boxes will need to be adjusted.
  • The timber beading to the frames that holds the sashes in alignment can also break or work loose, and it is common for old windows to have been ‘painted closed’.

Remedial Work
To overhaul sashes you really need to remove them from their frames, which can be done from the interior by carefully prising off the nailed timber beading.

  • Sash cords are fairly easy to replace.
  • Pulleys can be lubricated.
  • Beading can be prised off, dismantled and rubbed down.
  • Distorted wood should be planed off, followed by lubricating the sides with candle wax, and oiling the pulleys.
  • Any paint should be stripped from the sash channels in the frame and protected with linseed oil. Removing paint is normally best done by burning off (taking care not to leave burn marks), but chemical strippers may need to be used near glass to avoid cracking.
  • Refixing beading with brass screws rather than nails is a better solution.

There are companies, such as Ventrolla, who can also undertake the remedial work for you.

Read more on the care and repair of sash windows here

All images from Shutterstock, apart from those supplied by Ventrolla showing repair of rotten windows.

09/01/2014: Ian Rock, who is a surveyor, photographed at his home near Bicester in Oxfordshire. Picture: John Lawrence 07850 429934About the Author

Chartered surveyor Ian Rock is director of the survey price comparison website and author of the Haynes Period Property Manual.

Image: John Lawrence

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