Frames

Timber

Well maintained timber window frames and sashes may still be in good condition. Poorly maintained windows may be affected by rot at sash and frame junctions and need replacement. Hardware such as stays and hinges are likely to be worn or poorly fixed into the timber.

If sashes have warped and jammed, or have generally deteriorated, it may be due to building settlement, rot, or paint build-up:

• if settlement is the cause of jamming, re-levelling the house may remedy the problem
• windows or doors may need to be removed and planed/sanded to give sufficient clearance to allow  them to open freely
• if the window sashes have rotted, unless minor, they will need to be removed and repaired with a new section of timber or replaced with a new sash
• if sashes are sticking because of paint-clogging, they may be freed by running a knife or blade between the sash and the frame to remove the paint. Windows where all surfaces are painted with acrylic paint are especially prone to jamming. If this is the case, repaint with enamel paint.

Aluminium and steel

Steel windows if not painted and well maintained may require significant work (including removal) to restore them. The glass will need to be removed and re-installed to prevent moisture entry into the glazing rebate.

While the aluminium itself should be reasonably sound if it has been regularly cleaned, early aluminium windows were manufactured with plastic glazing beads which are now likely to be quite brittle and glazing rubbers will have shrunk. Also the sealant used during manufacture is likely to have deteriorated and may be allowing some water through the joints. Hardware on early windows is also likely to be worn and nearing replacement.

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Thermal efficiency

Windows will be single glazed unless they have been replaced very recently. Single glazing has poor thermal performance and can result in internal condensation problems. Retrofitting insulating glass units (IGUs) is the best option for permanently improving the thermal efficiency of windows but can be costly.

Methods available to retrofit timber joinery with IGUs include:

• complete replacement of the window
• replacement of glazing only
• replacement of the sashes only
• installation of an adaptor system to allow joinery to accept IGUs
• remachining timber joinery to accept IGUs.

Figure 1: Double glazing retrofitted to a 1950s house.

Secondary glazing

Secondary glazing is a less expensive option than insulating glass units. With this, plastic film, magnetically attached plastic sheet, plain or low-E glass is installed inside the existing glass with a still air gap between them.

Research (largely carried out at BRANZ) found that secondary glazing gave R-values from 0.36 to 0.57 m2 K/W. This confirms that secondary glazing can be used as a functional alternative to retrofitted insulating glass units (IGUs) in existing domestic single-glazed window frames. (In fact, the performance exceeded the expected performance of IGUs retrofitted into the existing framing due to the secondary glazing effectively insulating the framing.

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Airtightness

Timber window frames and sashes are better insulators than similar-sized aluminium frames that are not thermally broken. However, timber windows tend to be relatively air leaky and therefore less efficient in terms of maintaining and retaining heat within the building.

Rough framed openings for windows were constructed to give sufficient gap between the window and the framing to allow the window to be installed plumb and square using folding wedges. This gap was then covered with architraves on the inside and facing boards on the outside. The gap can be a significant source of air leakage. To reduce the air leakage, remove the architraves insert a suitable diameter PEF rod and apply an expanding foam air seal as detailed in Acceptable Solution E2/AS1.

Install draught-stopping foam seals to seal the gaps between the opening sashes and the window frames where there is a noticeable gap between the sash and the frame.

Thermal efficiency can be further improved by installing heavy drapes. Velcro seals down each side of the window and the installation of pelmets will further reduce draughts.