Heat loss through windows happens in three ways: conduction through the glass and frame, draughts through gaps, and radiation through the glass itself. An old single-glazed window lets through roughly three times as much heat as a modern sealed unit. Worn seals add more on top. Most of it can be reduced, and not always by replacing anything.
We make timber windows, so you might expect us to say the answer is always new ones. It isn’t. Sometimes it’s a strip of draught seal or a pair of lined curtains. Sometimes the frame is sound and only the glass has had its day. This guide explains how well your windows insulate your home, where the warmth goes, puts rough numbers on it and sets out the options in order of cost. Then you can decide what’s worth doing in your own house.
Whole-window U-value 1.4 W/m²K
Triple glazing available
Warm edge spacers as standard
Factory finished
Around 20 working days
In this guide
What You’ll Discover in This Article
- Why windows are the weak spot in a wall
- The three routes warmth takes, and where it escapes
- How to put a rough figure on it
- Cheap fixes to try first
- What to specify when new frames are the right answer
01 · The weak spot
Why Windows Are the Weakest Point in Your Home’s Thermal Insulation
A modern cavity wall with insulation might reach a U-value of 0.3 W/m²K. An old single-glazed sash sits nearer 4.8 W/m²K. Each square metre of that window lets warmth through about sixteen times faster than the wall around it.
How Much Heat Is Lost Through Windows
It depends on the glass, the frame and the gaps. In a draughty Victorian house, the windows can be the biggest single route out. In a well-built new home, they’re still the part of the wall most prone to heat loss, simply because glass is thin. They’re also the main source of heat gain on a sunny day.
What a U-Value Measures
The U-value tells you how quickly heat passes through a material: the watts lost through one square metre for each degree of difference between inside and out. Lower is better. Most window and door makers quote it, and it’s the clearest guide to thermal performance. Compare whole-window figures, not the centre of the glass alone, because the frame and spacer bars matter too.
02 · How heat moves
Three Types of Heat Transfer Through a Window
Windows lose heat in three ways. Knowing which one dominates tells you what to fix, and which fix will save energy rather than just money.
Heat Conduction Through Glass and Frame
Conduction moves warmth through solid material from the warm side to the cold side. A single pane of glass offers almost no resistance. A sealed unit traps air or gas in the gap between the glass, which slows it dramatically. Two panes of glass with a low-emissivity coating are now the baseline for good window glazing.
Air Leakage and Draughts
Warm air escapes through gaps, and cold air is drawn in to replace it. On old sash windows, the gaps are at the meeting rails, the parting beads and the staff beads. On a casement, they’re usually where the opening light meets the window frame. A loose window can lose as much through air leakage as through the glass.
Radiation and Solar Gain
Warm surfaces in a room radiate towards the cold glass, and some of that energy passes straight out. A low-E coating reflects heat back into the room. Glass also lets the sun in, which is measured by the solar heat gain coefficient. On a south-facing wall in winter, that free warmth helps.
03 · Weak points
Where Heat Escapes Through Windows: The Weak Points
When a room feels cold near the window, the cause is usually one of these.
Single Glazing
The obvious one. Single glass has a centre-pane U-value around 5.8 W/m²K, and nothing else you do to the window will match the gain from upgrading it.
Failed Sealed Units
A unit that has misted between the glass panes has lost its seal and much of its insulation. The frame may be fine. Often only the unit needs replacing.
Worn Seals and Gaps
Seals harden and flatten over the years. Our guide to window seals on wooden windows shows how to spot one that’s stopped working.
The Joint Around the Window
Gaps between frame and wall are often missed. If the frame wasn’t packed and sealed when it went in, air moves straight past it. A poor installation makes high-quality joinery look cheap, and perform badly too.
Cold Bridging
Metal frames without a thermal break conduct warmth straight through, and a metal frame loses heat faster than a wooden one. You’ll often see condensation on the frame itself.
04 · The numbers
Calculating Heat Loss Through a Window
The sum is simpler than it sounds: U-value, times area in square metres, times the temperature difference.
A Worked Example
Take a sash 1.0 m wide and 1.6 m high, so 1.6 m². With 20°C inside and 0°C outside, the difference is 20 degrees.
| Window | U-value (W/m²K) | Heat loss at 20 degrees | Over a heating season |
|---|---|---|---|
| Old single glazing | 4.8 | 154 W | about 369 kWh |
| Double glazed timber | 1.4 | 45 W | about 108 kWh |
| Triple glazing in timber | 0.8 | 26 W | about 61 kWh |
The seasonal figure assumes around 2,000 degree days, typical for much of England. Under the October 2026 price cap, gas costs about 7.97p per kWh. With a 90% efficient boiler, upgrading that one window saves roughly £23 a year from conduction alone. Ten windows like it would save around £230, before you count draughts or the effect on your overall energy use. It’s a modest energy saving per window, but it adds up across a house and helps reduce your energy bills every winter.
What the Numbers Leave Out
They ignore air leakage, which can be as large again on a loose sash. They also ignore comfort. A cold pane makes a room feel colder than the thermostat says, so people turn the heating up. Warmer glass removes that effect, and most owners notice it before they notice the bill.
05 · Cheap fixes
Cheap Ways to Reduce Heat Loss Before You Replace Anything
If the frames are sound, start here. Cutting heat loss this way keeps the heat inside your home for a fraction of the cost of new joinery. These options are cheap, reversible and usually acceptable on listed buildings.
Draught-Proofing
Brush or compression seals set into the frames leave less room for heat to escape and cut air leakage at low cost. It’s the cheapest improvement for an old sash, and a sash specialist can fit them in a day or two.
Thermal Curtains and Blinds
Tests at the University of Salford’s Energy House found that well-fitted blinds help reduce heat loss by up to about a third. Window coverings work by trapping a layer of still air against the glass, and heavy thermal curtains do it best, improving heat retention after dark. Drawn at dusk, the right window treatments significantly reduce heat leaving the room overnight.
Thermal Shutters
Historic Environment Scotland measured a 51% cut in the warmth escaping through a sash when closed timber shutters were used. Fitted shutters do the same job in a house that never had them.
Secondary Glazing
A slim glazed frame fixed to the inside of an existing window helped reduce the heat lost by 63% in the same research. Historic England’s guidance on secondary glazing explains why it’s often the preferred option for listed buildings, where the original windows have to stay.
06 · New windows
When Energy Efficient Windows Make Sense
It makes sense to replace your windows when frames are rotten, too slim to take a sealed unit, or so distorted that draught-proofing can’t close the gaps. The same principles apply to roof windows.
What to Specify
Ask for the whole-window U-value, or the energy performance rating, to compare different types of window fairly. Look for low-E glass, a gas-filled cavity and warm edge spacers, which reduce heat transfer at the edge of the unit. Our timber windows built for energy efficiency reach 1.4 W/m²K as standard, and triple glazing goes lower. If you want the detail on the glass itself, see how double glazing works.
Why Wooden Frames Help
Timber frames insulate naturally. Wood conducts heat over a thousand times more slowly than aluminium, so the frame stays close to room temperature and doesn’t become a cold bridge.
What Building Regulations Require
In England, Approved Document L sets a maximum U-value of 1.4 W/m²K for replacement windows, or an equivalent energy rating. A FENSA-registered installer can self-certify the work.
Ventilation and Condensation
A well-sealed window still needs fresh air in the room. Trickle vents let a small, controlled amount through, so less heat is wasted than through an open window and you reduce energy use overall. That keeps condensation off the glass and helps minimise heat loss in your home without sealing the room completely.
Frequently Asked Questions
Which windows are the coldest?
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Single-glazed windows with worn seals. Warmth leaves through the glass and the gaps at the same time.
Does double glazing make a home warmer?
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Yes. Modern windows with good double glazing in a timber frame reach around 1.4 W/m²K, against roughly 4.8 W/m²K for single glazing, and the inner pane feels far less cold.
What’s the cheapest way to cut heat loss from windows?
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Draught-proofing and lined curtains. Both cost little and work on any window.
Is triple glazing worth it?
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On exposed, north-facing elevations, often yes. Elsewhere, the extra saving over good double glazing is smaller and takes longer to pay back.
Do wooden frames lose more heat than metal ones?
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No. Timber is a natural insulator and conducts heat far more slowly than metal.
Summary
Conclusion: Fix the Cheapest Losses First
Seal the gaps first to stop heat loss where it’s easiest, then use curtains, blinds or shutters at night, and consider secondary glazing for period homes. When frames reach the end of their life, choose new ones by their whole-window U-value and pick a careful fitter.
Ask us for a quote with your sizes and a few photographs, and we’ll tell you honestly whether new windows are the right answer.
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