It is estimated that heat loss through windows and doors can account for up to 25% overall loss from a building, so that addressing this can result in significant improvements to a building’s energy efficiency.

Repairs, draught-proofing and other low risk options

The repair and refurbishment of windows can improve the thermal performance of historic windows. Where draughts are causing an issue, draught-proofing the windows and doors can have considerable improvements.

The use of shutters and heavy curtains for windows can make a significant improvement in reducing heat loss in windows. Historic England, Historic Environment Scotland and the SPAB have researched the improvements to heat loss achieved by these light-touch options. Historic England’s research found that heavy curtains reduced heat loss in sash windows by 39% and well-fitting shutters reduced heat loss by 64%. Other benefits to shutters are achieved during warmer months as they can help with ventilation and avoiding over-heating.

Repairing, refurbishing and draught-proofing historic windows and doors can preserve the special interest of a building. From a sustainability perspective, it also retains embodied carbon.

Secondary Glazing

Secondary glazing is often a good energy efficiency measure that can improve building performance, whilst having little impact to a building’s fabric. Research has found that in some instances secondary glazing performed better than double-glazing. Details pertaining to secondary glazing are available in Historic England’s Guidance Traditional Windows: Their Care, Repair and Upgrading. Detailed design will need to consider appropriate materials, position and fixings into the window reveals, and how the design will respond to the existing window. Dependant on the form pf the windows, and its surround, secondary glazing may not always be possible.

Double-glazing

The approach to window replacement and repair will be bespoke to each building and as such it is difficult to prescribe set approaches. In all cases it is important that a baseline understanding of the significance of existing windows in undertaken to inform approach.  Generally double-glazing should only be considered where the existing windows (if of heritage significance) are beyond repair. There is however a wider consideration, for example a double-glazed window replacement should not be applied to an elevation where all other fenestration is of significance and single glazed. In this circumstance there would likely be an intrusive aesthetic effect to architectural interest.   

Should windows be not of significance then there may be opportunity for an appropriate double-glazed replacement. However, the replacement of windows that are not beyond repair, results in the loss of embodied carbon. It should also be noted that double-glazed units have a limited lifespan, generally warranties vary from 10-20 years. For example, the units will require replacing again when the seal fails as this will impact the vacuum between the glazing panels.  Installing secondary glazing, if well maintained, may have a longer lifespan, particularly as there is no reliance on the vacuum between panels.

Where the installation of double-glazing is acceptable, any replacement windows should not be uPVC windows. They tend not compatible with the building fabric and can result in an increase of moisture issues and build up. Historic England’s Guidance Traditional Windows: Their Care, Repair an Upgrading succinctly outlines the issue with uPVC windows in the historic environment.

Further information on windows and doors

Historic England’s Traditional Windows: their care, repair and upgrading

SPAB Briefing Windows and Doors

CAT, Windows

Draught-proofing

Historic England’s Draught-proofing windows and doors

STBA, Window draughtproofing

STBA, Door draughtproofing

Refurbishment

STBA, Window refurbishment

STBA, Door refurbishment

STBA, Window Shutters Refurbishment

Secondary glazing

Historic England’s Secondary glazing for windows

STBA, Secondary glazing

Replacements

STBA, Energy efficient glazing

STBA, High performance doors