Search for information about heat loss through windows and you will quickly find percentages claiming that a particular amount of a home’s heat always disappears through the glazing.
The problem is that homes are not all built the same way.
The amount of heat lost through windows depends on the size and condition of the glazing, the frames, installation and how the rest of the property performs.
There is no single percentage that accurately describes every UK home. Window heat loss needs to be considered as part of the complete thermal envelope.
Why do windows lose heat?
Windows separate a heated room from the colder outside environment.
Heat therefore moves through the glazing and frame, while gaps or defective seals can also allow unwanted air movement.
The rate of heat loss depends on how the window is constructed and its overall area.
What affects the performance of a window?
Glazing
The construction of the glazed unit influences how readily heat moves through it.
Frames
Frame materials, condition and construction also form part of the overall thermal performance.
Seals
Damaged or deteriorated seals can allow unwanted air movement around openings.
Installation
How the window meets the surrounding wall can affect draughts and localised thermal weak points.
Is heat loss through the glass the same as a draught?
No.
Heat can transfer through a closed window even when it is completely airtight.
A draught is different because it involves air moving through a gap or opening.
Both can affect comfort, but they are different mechanisms and should not automatically be treated as the same problem.
Why do large glazed areas matter?
The amount of window area within a room can significantly change its heat-loss characteristics.
A small bedroom with one modest window is very different from an extension with wide glazed doors and several large windows.
This is why the importance of glazing has to be considered in relation to the actual property rather than reduced to one nationwide percentage.
What about the rest of the building?
Windows form just one element of the thermal envelope of a house.
| Part of the home | What can influence heat loss |
|---|---|
| Windows | Glazing, frames, seals, installation and total glazed area |
| Loft or roof | Insulation depth, condition, coverage and continuity |
| Walls | Construction type, insulation and local thermal bridges |
| Floors | Floor construction, insulation and exposed surfaces |
| Air leakage | Uncontrolled gaps around different parts of the building |
Why can a poorly insulated loft matter more than the windows?
If a property’s windows are already performing reasonably well but the loft has large gaps or inadequate insulation, the loft may represent a more obvious opportunity for improvement.
Likewise, very good windows do not compensate for poor insulation elsewhere.
Suitable traditional loft insulation can reduce heat loss through the upper ceiling, but it does not change the performance of the glazing.
Each part of the envelope has its own job.
Where does thermal bridging fit in?
Heat loss can also become concentrated around junctions between different building elements.
These areas are known as thermal bridges.
A window opening, lintel, wall junction or another construction detail can therefore influence local surface temperatures even when the glazing itself performs well.
Why should you be cautious with generic heat-loss percentages?
Two houses can have completely different proportions of glazing, different roof insulation, different wall construction and different floor types.
Assigning the same heat-loss percentage to both can create a misleading picture.
If you are deciding where to invest money, understanding the actual weak points of the property is far more useful than relying on a generic graphic.
Our first article explains whether replacing windows will necessarily make the whole home warmer.
The visible weakness is not always the biggest weakness
Windows attract attention because you can see them and feel immediately when there is a draught nearby.
Insulation hidden above ceilings, beneath floors or inside walls is less obvious.
That does not make it less important.
A whole-home assessment helps separate what is easy to see from what is actually influencing the heating demand of the property.
Frequently asked questions
What percentage of heat is lost through windows?
There is no single percentage that applies accurately to every home. The contribution depends on the amount and type of glazing and the performance of the rest of the property.
Do bigger windows lose more heat?
The total glazed area matters, so a room with extensive glazing can have different heat-loss characteristics from one with a small window.
Is a draught the same as heat passing through the glass?
No. A draught involves air movement, while heat can also transfer through glazing and frames even when the window is airtight.
Can insulation reduce heat loss more than new windows?
It can in some homes, depending on which part of the building is currently the greater weakness.
Why should the whole house be considered?
Because windows, roofs, walls, floors and air leakage all contribute to the overall heating demand of the property.
In the next article: we compare two common investments and ask whether replacing windows or improving insulation should come first.
Read: Should You Replace Windows Or Improve Insulation First?
