Thermal Performance Of Traditional Windows

Technical Report

This is a short summary of the findings relating to secondary glazing and double glazing only showing that secondary glazing was the most effective overall option, as it reduced heat loss through the window by 63%.

The full (31 page) report can be found here: Thermal Performance Of Traditional Windows

From Page 2:

“Windows are the most targeted building element for replacement to reduce heat loss in

The window that was tested, provided by Historic Scotland, was a typical timber single paned sash and casement window.

The thermal performance of the window was tested at the National Physical Laboratory using a guarded hot box system, in order to get an industry-standard measurement of the window’s properties. The thermal transmittance of the window (known as the U value) was measured as being 4.5 W/m²K.

Draught proofing is a common practice to prevent wind from blowing in through traditional windows. The test window was draught proofed, and although the U value of the window was not improved, the airtightness of the window was improved considerably, reducing the air leakage by 86%. The window is tighter than the recommended 4,000 mm² trickle vent for domestic new build.

A series of heat loss reduction measures were tested. These measures are all standard steps that people can take and are widely available, including the use of curtains, shutters, blinds, and secondary glazing. All the options were tested on the window in the Environmental Chamber at Glasgow Caledonian University, and all were shown to reduce the heat loss through the glazing to varying degrees.

Secondary glazing was the most effective overall option, as it reduced heat loss through the window by 63%.
From Page 9:

Table 3: NPL guarded hot box results for the sash and case window before and after draught proofing.

Standardised thermal transmittance (U value)   W/m²K  
before draught proofing 4.5
after draught proofing 4.2

The difference in the U-values is not statistically significantly different since the overall measurement uncertainty is ± 5.5%. The average value is 4.4 W/m²K.

Whilst the glazed area is 55% of the total window area, approximately 72% of the heat is lost through the glazing assuming an indicative centre of pane glazing U-value of 5.7 W/m²K as given in CIBSE Guide A [8].

From Page 13:

Table 5: The effect of the various options on reduction in heat loss through single glazing, the estimated U-values and measured  average surface temperatures

Type of System Reduction in heat loss U value W/m²K Temperature of interior (warm) room-facing surface °C
Secondary glazing system 63% 1.7 19°
 Double Glazing 55%  1.9 18°

From Page 14:

Installing the secondary glazing clearly gives an improvement which is comparable to the best of the options examined prior to its installation, however the secondary glazing has the advantage that its benefits can be realised both day and night.

Replacing the single glazing with the Slimlite double glazed panes also produces a significant improvement.

From Page 17:

Over the range of pressure differences, the draught proofed window shows an 86% reduction in air leakage compared with the as-received condition.

The carefully sealed secondary glazing system provides a further reduction in air

leakage (97% compared to the as-received condition)

From Page 18:

5 Conclusions

Measurements of the U-value of a traditional sash and casement window showed that there was no significant difference before and after draught proofing of the window

High performance secondary glazing and replacement double glazed panes offer improved thermal performance.

Careful installation of the secondary glazing also results in improved air-tightness.

From Page 19:

The in situ U-value measurements show the potential benefits of low emissivity glazing in a secondary glazing system.”

The full (31 page) report can be found here

APG are well established family business and work to the highest standard, we fellow international guild lines for reference please see The Conservation and Thermal Improvement of Timber Windows For further information please Contact us today on 02871 357 444. -or-  E-mail us.