On this page
- Why people replace windows in the first place
- U-values and WER ratings, explained without the jargon
- The glass matters more than the frame
- Choosing a frame material
- Window styles and where each one works
- Security: what PAS 24 actually means
- What double glazing costs in 2026
- Nine questions to ask any installer
- Five mistakes that cost people money
Replacing the windows on a three-bedroom house is a five-figure decision that most people make once, maybe twice, in their lives. The industry knows that, which is why the buying process is full of numbers that sound authoritative and mean less than they appear to. This guide is written to give you the shorter, more useful version.
Why people replace windows in the first place
In our experience it is almost always one of three complaints, and identifying yours matters because each points to a different specification.
Cold. Rooms that never warm up, cold air falling off the glass, condensation running down the inside on winter mornings. This is a thermal problem, and the fix is in the sealed unit and the frame's thermal break, not in the style of window.
Noise. Traffic, aircraft, a school over the road. This is an acoustic problem, and the answer is laminated glass and asymmetric pane thicknesses. Simply buying "better" double glazing will disappoint you, because two panes of equal thickness resonate at the same frequency.
Failure. Misted units, rotten timber, handles that no longer engage, frames that have discoloured beyond rescue. Here the driver is condition rather than performance, and it is your chance to improve both at once.
If you cannot say which of the three is bothering you most, you are not ready to compare quotes. Two installers can quote wildly different specifications and both be right, depending on the problem you are solving.
U-values and WER ratings, explained without the jargon
Two numbers get quoted constantly. They measure different things.
The U-value measures heat loss, in watts per square metre per degree of temperature difference (W/m²K). Lower is better. Single glazing sits around 5.0. Old 1990s double glazing is roughly 2.8 to 3.2. A current A-rated unit lands near 1.4, and a good triple-glazed unit gets to around 0.8. Building Regulations in England currently require replacement windows in existing dwellings to achieve 1.4 W/m²K or better.
The Window Energy Rating (WER) is a letter grade from G up to A++. It is not just heat loss — it nets off the solar heat the window lets in and the air it leaks. That is why a window can have a good U-value and a mediocre WER, or vice versa.
| Glazing type | Typical U-value | Typical rating |
|---|---|---|
| Single glazing | 4.8 – 5.4 | Unrated |
| Older double glazing (pre-2002) | 2.8 – 3.2 | D – E |
| Standard modern double glazing | 1.4 – 1.6 | B – A |
| High-performance double glazing | 1.2 – 1.4 | A – A+ |
| Triple glazing | 0.8 – 1.1 | A+ – A++ |
A practical point that rarely appears in brochures: the jump from single glazing to A-rated double is enormous, and the jump from A-rated double to triple is small by comparison. If your budget is finite, spending it on getting every window to A-rated will do more than putting triple glazing in three windows and leaving the rest.
The glass matters more than the frame
Most of a window's area is glass, and most of its performance comes from what is inside the sealed unit.
Low-emissivity coating. A microscopically thin metallic layer on one internal face that reflects heat back into the room. Invisible, and responsible for a large share of the improvement over older units. Essentially standard now, but check it is specified.
Cavity gas. Argon between the panes conducts less heat than air. It costs very little and it is worth having. Krypton performs slightly better in narrow cavities but costs considerably more.
Warm-edge spacer. The bar around the perimeter separating the panes. Traditional aluminium spacers conduct heat straight through, which is why old units get cold and damp around the edge of the glass. A composite warm-edge spacer largely solves it.
Cavity width. Around 16mm to 20mm is the sweet spot for argon-filled units. Wider is not automatically better — beyond about 20mm the gas starts convecting inside the cavity and performance drops back.
Acoustic laminate. If noise is your problem, this is the answer: two panes bonded with an acoustic interlayer, ideally paired with a differently sized outer pane. A well-specified acoustic unit can cut perceived noise dramatically where a standard unit barely moves the needle.
Solar control. A coating that reflects a portion of solar heat while letting light through. Essential on large south-facing glazing and on overhead glass, unnecessary on a north elevation.
Choosing a frame material
uPVC is the value option and it is genuinely good. Multi-chambered profiles insulate well, the material needs no painting, and modern foils convincingly imitate timber. The drawback is bulk: uPVC needs a wider frame for the same structural performance, so you see more frame and less glass.
Aluminium is stronger, which means slimmer sightlines and larger panes. Thermally broken profiles — a resin barrier separating inner and outer sections — have removed the cold-frame problem that gave aluminium a bad reputation in the 1980s. It costs more and can be powder-coated to any RAL colour.
Timber suits period and listed properties, performs well thermally, and is the most sympathetic option in a conservation area. It needs redecoration on a cycle, which is a real commitment.
Composite pairs a timber inner face with an aluminium outer, giving the interior warmth of wood and an exterior that needs no maintenance. It is the most expensive of the four.
Window styles and where each one works
- Casement — side or top hung, opening outwards. The British default, widest choice, lowest cost.
- Flush casement — the sash sits level with the frame rather than proud, replicating a traditional timber look. Worth the premium on a period frontage.
- Tilt & turn — tilts inward for ventilation, swings fully open for cleaning. The sensible choice above ground floor.
- Sash — vertical sliders with modern spiral balances. Correct for Georgian and Victorian properties.
- Bay — angled or square projections. Check whether the bay carries structural load above before anyone quotes.
- Fixed — no opening light. Cheaper and better sealed, but every habitable room needs adequate ventilation and, in some cases, escape provision.
Security: what PAS 24 actually means
PAS 24 is a British standard covering enhanced security performance for doors and windows. A PAS 24 window has been tested against a defined set of manual attack methods rather than simply described as "secure" in a brochure.
In practice it means multipoint locking, internally beaded units so the glass cannot be removed from outside, reinforced sashes, and hardware that resists levering. Part Q of the Building Regulations requires PAS 24 in new dwellings; in a replacement job it is optional, and it is one of the cheapest upgrades on the quote. Ground floor and easily accessible windows are the ones worth specifying it on.
What double glazing costs in 2026
Prices vary by region, access, glass specification and frame material. As a broad guide for supplied and fitted work in the London and Essex area:
| Item | uPVC | Aluminium |
|---|---|---|
| Single casement window | £450 – £750 | £700 – £1,150 |
| Larger casement with opener | £650 – £1,000 | £950 – £1,500 |
| Bay window (three-sided) | £1,800 – £3,200 | £2,800 – £4,500 |
| Sash window | £850 – £1,400 | — |
| Whole three-bed semi (8–10 windows) | £5,500 – £9,500 | £9,000 – £15,000 |
These are indicative ranges to help you sanity-check a quote, not a price list. A quote far below the bottom of these ranges usually indicates a thinner profile, a basic sealed unit or subcontracted fitting; a quote far above usually indicates commission built into the price.
Nine questions to ask any installer
- What is the whole-window U-value, not the centre-pane figure?
- Is the sealed unit argon-filled with a warm-edge spacer?
- Are the windows PAS 24 tested, and which ones on my job will be?
- Do you employ your fitters, or subcontract the installation?
- What exactly does the guarantee cover, and is it insurance-backed?
- Who registers the work under the competent person scheme?
- What is the lead time from signed survey to fitting?
- What is not included — scaffolding, making good, disposal?
- Does the price change if I do not order today?
That last one is the most revealing question on the list.
Five mistakes that cost people money
Comparing prices instead of specifications. Three quotes for "double glazing" can describe three genuinely different products. Line the specifications up before the numbers.
Ignoring ventilation. New windows seal a house far better than old ones. Without adequate trickle ventilation, moisture that used to escape through leaky frames stays inside and appears as mould in corners.
Buying triple glazing for the wrong reason. It is excellent for noise and for genuinely cold exposed elevations. On a sheltered wall it adds weight and cost for a marginal thermal gain.
Forgetting the frame-to-glass ratio. Two windows of identical outside dimensions can differ by 20% in glass area. On a small window, a bulky frame noticeably darkens the room.
Signing under time pressure. A discount that expires when the salesperson leaves your house was never a discount. Any reputable company will hold a quote for weeks.
Where to start
Work out which of the three problems you are solving, decide the frame material, then let the glass specification follow from the problem. Get three quotes on a written specification you have set, not on whatever each company chooses to offer, and you will be comparing like with like for the first time.
Frequently asked questions
A well-made uPVC window lasts around 20 to 25 years and aluminium 30 to 40. Sealed units are usually the first component to fail, typically after 15 to 20 years, and they can be replaced without changing the frame.
Yes, if the frame and hardware are sound. Replacing misted units in a good frame is far cheaper than a full replacement and delivers most of the thermal benefit.
Going from single or failed double glazing to A-rated units makes a clear difference. Replacing already-decent modern double glazing rarely pays back on energy alone — comfort, noise and condensation are the honest reasons to do it.
Both are efficient. The rating balances heat lost against solar heat gained and air leaked, so an A+ window gains slightly more than it loses across a heating season.
Replacement windows must comply with Part L and Part K. Using an installer registered under a competent person scheme means the work is self-certified and you receive a certificate for your records.
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