If your windows are cold in winter, you are not alone. Cold and draughty, single glazed windows (and doors) 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 a major source of heat loss, allowing for uncontrolled air leakage, rattles and excess noise penetration. Consequently, homes become less comfortable and, with ever increasing energy bills, more expensive to maintain.
These afflictions are especially noticeable in older period properties and listed buildings, where windows may have aged poorly and deteriorated over time. It is a particular problem for timber sash windows, even though they may ostensibly, be in good working order.
This article discusses heat loss through energy inefficient, poorly insulated windows 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).
Heat loss through a traditional timber window are predominantly through air leakage (draught) but, for larger windows, the proportion of heat lost by heat transfer tends to be greater.
This text focuses on heat transfer with air leakage (draught) being considered separately on the Draughty Windows page.
Types of Heat Transfer
There are three types of heat transfer through a window, namely:
- Convection – the flow of heat energy from a region of high temperature to a region of low temperature
- Conduction – where heat passes directly through the window fabric (glass, timber, steel etc).
- Radiation – where the colder surface of the window absorbs infrared wavelengths 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
WindowSkins works by creating an airtight seal against the window frame, thereby trapping an insulating layer of air and blocking draught, much like double glazing.
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), a near halving in the rate of heat transfer can be achieved. This would equate to an 8% reduction in household heat loss by way of heat transfer.
Furthermore, around 22% of household heat1 is lost through uncontrolled air leakage (draught). Assuming half of this leakage is attributable to the gaps around the window frames (perhaps more for sash windows), a further 11% of household heat loss can be prevented.
Hence, by fitting WindowSkins, it would be fair to say that the combined household heat loss (via 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.
- assumes a traditional detached period home ↩︎
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 a balance of both).
Additional benefits such as reduced noise, air pollutants and condensation are a welcome bonus – all whilst preserving the windows original character.
For landlords, WindowSkins presents a cost effective way to improve a failed EPC (Energy Performance Certificate) rating.
And finally, it may also reduce the need for expensive overhauls or outright window replacement.
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. ‘Traditional Windows – Their Care, Repair and Upgrading’ (Updated 2017)
https://historicengland.org.uk/images-books/publications/traditional-windows-care-repair-upgrading/heag039-traditional-windows-revfeb17/
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
