If your windows are cold in winter, you are not alone.
Cold and draughty, single glazed windows are notorious for having poor thermal performance.
They are a significant source of heat loss, making rooms slow to heat up and hard to keep warm.
Poorly insulated windows are liable to cause significant temperature gradients within a room, especially if large.
Consequently, homes become less comfortable and, with ever increasing energy bills, more expensive to maintain.
These problems are especially noticeable in older period properties and listed buildings, where windows may have aged poorly and deteriorated over time.
This article discusses heat loss through energy inefficient, poorly insulated windows (and doors) and, what can be done to improve them.
Mechanisms of Heat Loss
Heat loss through a window occurs in two main ways; heat transfer and/or air leakage (draught).
Here we consider the first type, heat transfer, with draught being considered separately.
Types of Heat Transfer
There are three types of heat transfer through a window, namely:
- By convection, in effect drawing warm air towards the cold window surface.
- By conduction, where heat passes directly through the window fabric (glass, timber, steel etc).
- By the colder surface of the window absorbing infra-red radiation from the room.
The glass is the most conductive part of the window but heat is also lost through the frames fabric, albeit at a lower rate.
Quantifying the Rate of Heat Transfer
A U-value measures how effectively a building material—like a wall, roof, or window—insulates. It indicates the rate of heat transfer through a particular structure (or fabric).
U-Values do not measure heat lost through air-leakage (draught).
A lower U-value means less heat escapes, making the structure a better insulator and more energy efficient.
A typical 4mm thick sheet of glass has a U-value of 5.4 (W/m²K).
U-Value Window Comparison
The following typical U-values are presented to add context for comparison:
| U-Value (W/m²K) | Description |
|---|---|
| 1.3 – 1.5 | triple-glazed, argon filled, low-E coating |
| 1.7 – 2.0 | double-glazed, argon filled, low-E coating |
| 1.8 – 2.1 | double-glazed, air filled, low-E coating |
| 2.4 | window with secondary glazing |
| 2.7 | double-glazed, air filled |
| 4.8 | single glazed window |
| 5.4 | 4mm sheet glass |
(assumes wood or uPVC frame, 16mm or more air gap)
It can be observed that a window with secondary glazing will halve the rate of heat loss through a window, by way of heat transfer, as compared to a single glazed window.
Importantly, this does not consider the additional heat loss by way of draught, which in older buildings, is usually the greater of the two mechanisms of heat loss.
Hence the overall thermal improvement of secondary glazing will usually be much greater than just the stated U-Value improvement, due to the highly significant additional reduction in uncontrolled airflow.
Special Considerations – Cold Bridging
Cold bridging (more accurately called thermal bridging) occurs when a material with relatively high thermal conductivity creates a path that allows heat to bypass insulation and flow more easily from the warm side of a building to the cold side.
This is a problem particular to metal-framed (Crittall) windows, where the frames high thermal conductivity increases both U-Value (5.8 W/m²K) and, subsequent rate of heat loss (compared to timber and uPVC frames).
Where cold bridging is a possibility, for best results, it is advised both glass and frame is insulated in its entirety.
How WindowSkins Can Help With Cold Windows
Around 16% of household heat is lost through single glazed windows by way of heat transfer.
Current research suggest that by fitting WindowSkins (or any equivalent window insulation), a near halving in the rate of heat transfer can be achieved. This would equate to an 8% reduction in heat loss (solely by heat transfer).
Furthermore, around 22% of household heat is lost through uncontrolled air leakage (draught). Assuming 11% is attributable to the air gaps around the window frames (perhaps more for sash windows), it may be assumed that a further 11% of heat loss can be prevented.
Hence, by fitting WindowSkins (or any equivalent window insulation), it would be fair to say that the combined heat loss (through heat transfer and air leakage) can be reduced by around 19%.
Of course, this does depend on other factors such as the overall energy efficiency of the walls, floors, roof etc.
In practical terms, this would translate to either a significant increase in comfort (a faster heating, warmer home) or, a significantly lower annual energy bill (but most likely some of both).
For landlords, WindowSkins presents a cost effective way to improve a failed EPC (Energy Performance Certificate) rating.
Are you ready for a warmer, quieter, more comfortable home?
Arrange Your Free Quote Today
Bibliography / Further Reading
Taylor, J. ‘The Conservation and Thermal Improvement of Timber Windows’ (1996)
https://www.buildingconservation.com/articles/windows/thermal.htm
Wrightson, D. ‘Sash Windows – Painting & Draught-Proofing’ (2001)
https://www.buildingconservation.com/articles/sashwindows/sash_windows.htm
Bowden, J. & Cragg, A. ‘Sash Window Repair – Still in the Frame’ (2006)
https://www.buildingconservation.com/articles/stillintheframe/sash_window_repair.htm
Wood, C. ‘Thermal Performance of Historic Windows’ (Updated 2012)
https://www.buildingconservation.com/articles/thermal/thermal.htm
Taylor, J. ‘Secondary Glazing’ (2011)
https://www.buildingconservation.com/articles/secondary-glazing/secondary-glazing.htm
Makri, L. ‘The Thermal Upgrading of Historic Fenestration’ (2020)
https://www.buildingconservation.com/articles/fenestration/fenestration.html
Historic England. ‘Energy Efficiency and Historic Buildings – Draught Proofing Windows and Doors’ (Updated 2016)
https://historicengland.org.uk/images-books/publications/eehb-draught-proofing-windows-doors/heag084-draughtproofing/
Historic England. ‘Energy Efficiency and Historic Buildings – Secondary Glazing for Windows’ (Updated 2016)
https://historicengland.org.uk/images-books/publications/eehb-secondary-glazing-windows/heag085-secondary-glazing/
Historic England. ‘Energy Efficiency and Retrofit in Historic Buildings’ (Updated 2024)
https://historicengland.org.uk/advice/technical-advice/retrofit-and-energy-efficiency-in-historic-buildings/
DECC & BRE. ‘The Government’s Standard Assessment Procedure for Energy Rating of Dwellings (2012 edition)’ (Updated 2014)
https://bregroup.com/documents/d/bre-group/sap-2012_9-92
Jordan, S. ‘The Window Sash Bible: A Guide to Maintaining and Restoring Old Wood Windows’ (2024)
https://amzn.eu/d/02zBAbuw
