A-rated double glazing has a U-value of around 1.4 W/m²K; triple glazing gets to around 0.8. That is a real gain, but windows are usually only 10 to 20% of a home's heat loss, so triple glazing cuts your total heat loss by a few percent, not a quarter. For most London homes with uninsulated walls or a thin loft, double glazing plus wall and roof insulation is the better use of the money. Triple glazing earns its place in homes that are already well insulated, and where a heat pump or whole house retrofit is planned.
Key takeaways
- A-rated double glazing: U-value around 1.4 W/m²K. Triple glazing: around 0.8 W/m²K.
- Windows are typically 10 to 20% of a home's heat loss. Walls and the roof lose more, so insulate those first.
- Indicative price: uPVC double glazing £500 to £1,200 per window; triple glazing costs more per window, so get both priced on the same quote.
- Triple glazing is not automatically quieter. Acoustic laminated glass in a double glazed unit often does more for traffic noise.
- Triple glazed units are heavier and need frames and hinges rated for the weight; many period sash windows cannot take them.
- Windows and doors are 20% VAT. Insulation, solar and heat pumps are 0% VAT until 31 March 2027.
How do double and triple glazing compare?
Both are sealed units: panes of glass separated by a spacer bar, with argon gas in the gap and a low emissivity coating on the inner face. Triple glazing adds a third pane and a second gap.
| A-rated double glazing | Triple glazing | |
|---|---|---|
| Typical U-value (whole window) | around 1.4 W/m²K | around 0.8 W/m²K |
| Heat lost through the glazing | Baseline | Roughly 40% less than A-rated double |
| Indicative cost per uPVC window | £500 to £1,200 | Higher; a premium on the double glazed price |
| Weight of the glazed unit | Standard | Around half as heavy again for the same size |
| Noise | Good; better with acoustic glass | Similar unless acoustic glass is specified |
| Inside surface of the glass in winter | Cool near the glass | Closer to room temperature, fewer cold draughts |
| VAT | 20% | 20% |
The U-values are typical figures for a complete window including the frame. Individual products vary, so compare the whole window U-value on the quote, not the centre pane figure, which always looks better.
What does the U-value difference mean in practice?
A U-value is the rate heat passes through one square metre for each degree of temperature difference between inside and out. Lower is better. Going from 1.4 to 0.8 cuts heat loss through the glazing by a bit over 40%. The catch is where windows sit in the whole house picture: usually 10 to 20% of heat loss, with solid walls, an uninsulated loft and a suspended floor taking the bigger shares; see where homes lose heat. Cutting window losses by 40% therefore trims total heat loss by roughly 4 to 8%.
If your windows are old single glazing or early double glazing from the 1980s and 1990s, the jump to modern A-rated double glazing is where most of the gain is. The extra step to triple is smaller, and it costs the most per unit of heat saved.
How much more does triple glazing cost?
Indicative London prices for A-rated double glazing are £500 to £1,200 per window in uPVC and £900 to £2,000 in aluminium. A 3 bed house with 8 to 10 windows typically lands at £5,000 to £12,000. Triple glazing adds a premium on top because of the extra pane, the heavier unit and the stronger hardware. The size of the premium depends on the supplier and the frame, so ask for double and triple priced side by side on the same quote. Our double glazing cost guide breaks the double glazed figures down by material and house size.
On energy savings alone, neither pays back quickly. Windows are replaced because they are failing, draughty, misted or noisy, and the heat saving is a bonus. Both are charged at 20% VAT: windows are not on HMRC's list of energy-saving materials, unlike insulation, solar and heat pumps, which are 0% until 31 March 2027. See our 0% VAT guide.
Is triple glazing better for noise?
Not by default, and this surprises people. Sound insulation in a window comes from three things: the mass of the glass, panes of different thicknesses so they do not resonate at the same frequency, and a laminated acoustic interlayer that damps vibration. A standard triple glazed unit with three identical 4 mm panes does little more for noise than a double glazed unit.
For a home on a main road, under a flight path or near a railway, the better specification is usually a double glazed unit with one thicker laminated acoustic pane and one standard pane. Triple glazing can be built the same way, and then it does perform well, but you are paying for the acoustic glass, not the third pane. If noise is the reason for the upgrade, say so at the survey and ask for the sound reduction figure in decibels for the whole window. For noise through a shared wall, see soundproofing a party wall.
Weight, frames and period windows
A triple glazed unit is roughly half as heavy again as a double glazed unit of the same size. Frames need deeper profiles, hinges and friction stays must be rated for the load, and large opening sashes can be hard to handle. On a new window designed for triple glazing this is all built in. Fitting triple glazed units into existing frames rarely works.
- Box sash windows in Victorian and Edwardian homes usually cannot take triple glazed units; the weights and pulleys are not designed for them and the glazing bars are too slim. Slimline double glazing or secondary glazing is the usual route.
- Conservation areas and listed homes may restrict the frame style and sightlines. Triple glazing means thicker frames, which can be refused. See insulating a period or listed home.
- Cold bridging at the reveal. A good window in an uninsulated reveal still runs cold at the edges. Insulating the reveals and sills when walls are insulated matters more than the third pane; see window reveals and sills.
When is triple glazing worth it in London?
Triple glazing makes sense when the rest of the fabric is already good and the windows have become the weak point. It is worth the extra when:
- The walls, loft and floor are insulated, or are being done in the same project, so the windows are now a large share of what is left.
- You are planning a heat pump. Lower heat loss means a smaller unit, smaller radiators and lower running costs; see insulate before a heat pump.
- A room has large north facing or exposed glazing, or a bedroom feels cold near the window even with the heating on. The warmer inside pane makes a noticeable comfort difference.
- You are doing a whole house retrofit to a low energy standard, where every element is specified to a target, and you intend to stay for the long term.
Stick with A-rated double glazing when the walls and roof are still uninsulated, when the windows are period sashes, or when noise is the main driver and acoustic double glazing does the job for less.
How Stay Warm handles windows
Windows and doors are arranged and project-managed by Stay Warm and installed by accredited FENSA or CERTASS registered installers, who issue the certificate that proves the work meets Building Regulations. We plan the glazing alongside any wall insulation so that reveals, sills and the window position in the wall are done once. Start with a free heat loss survey and we will tell you whether the windows are the next job or the last one.
Frequently asked questions
What is the U-value of double glazing compared with triple glazing?
A-rated double glazing has a whole window U-value of around 1.4 W/m²K. Triple glazing is around 0.8 W/m²K. Lower means less heat lost. Compare the whole window figure on the quote rather than the centre pane figure.
How much more expensive is triple glazing than double?
Indicative prices for A-rated uPVC double glazing are £500 to £1,200 per window, and £900 to £2,000 in aluminium. Triple glazing costs more per window because of the third pane, the heavier unit and stronger hardware. Ask for both priced on the same quote so the premium is clear for your house.
Does triple glazing reduce noise more than double glazing?
Not necessarily. Noise reduction depends on glass mass, using panes of different thicknesses and a laminated acoustic interlayer, not on the number of panes. A double glazed unit with acoustic laminated glass often outperforms a standard triple glazed unit for traffic noise.
Is triple glazing worth it in a Victorian terrace?
Usually not as the first step. A solid wall Victorian terrace loses most of its heat through the walls, and original sash windows generally cannot take triple glazed units. Insulate the walls and loft first, then look at slimline double glazing or secondary glazing for the sashes. See our guide to insulating a Victorian terrace.
Do I need triple glazing for a heat pump?
No, but lower heat loss helps. A heat pump runs at low flow temperatures, so a well insulated home needs a smaller unit and smaller radiators. Walls and roof matter most; triple glazing is worth considering once those are done and the windows are due for replacement anyway.
Is there any VAT relief on triple glazing?
No. Windows and doors are charged at 20% VAT whether double or triple glazed. The 0% VAT rate until 31 March 2027 applies to energy-saving materials such as insulation, solar panels, batteries and heat pumps, not to glazing.