Improving the thermal performance of traditional windows

Based on research conducted by Dr Paul Baker of Glasgow Caledonian University (GCU) for Historic Scotland resulted in a report “Improving the thermal performance of traditional windows”. The report summarises the results of research on the thermal performance of traditional windows and methods of reducing heat loss carried out by the Centre for Research on Indoor Climate & Health, Glasgow Caledonian University (GCU) on behalf of Historic Scotland. Whilst most of the work was laboratory based using a sash and case window, some in situ measurements were carried out in a tenement in Edinburgh. Historic Scotland carried out a series of thermographic surveys to complement the thermal performance tests.

The report has concluded that:-

Laboratory measurements of the U-value of a traditional sash and casement window showed that there was no significant difference before and after draughtproofing of the window. The whole window U-value is 4.4 W/m2K. 72% of the heat loss through the window will be via the single glazing.

The airtightness of the window was improved considerably by draught proofing, reducing the air leakage by 86%. The window is tighter than the recommended 4000mm2 trickle vent for domestic new build.

All the options tested in the GCU Environmental Chamber reduce the heat loss through the glazing. Shutters are the most effective option of the traditional methods, reducing heat flow by 51%. By insulating the shutters heat loss can be reduced by 60%. Further improvement would be possible with a purpose designed set of shutters. Improved blind designs also have the potential to reduce heat loss.

High performance secondary glazing and replacement double glazed panes offer improved thermal performance throughout the day. Careful installation of the secondary glazing also results in improved air-tightness.

All the options offer improved thermal comfort due to higher surfacetemperatures compared with single glazing alone.

The in situ U-value measurements confirm in practice the performance oftraditional shutters and show the potential benefits of low emissivity glazing in a secondary glazing system.

Effect of the options on U-value
Effect of the options on reduction in heat loss through the glazing


U‐value (or thermal transmittance co‐efficient) is a measure of how much heat will pass through one square metre of a structure when the temperature on either side of the structure differs by 1 degree Celsius. The lower the U‐value, the better is the thermal performance of a structure. The U‐value is expressed in W/m2K.

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