Best double glazing companies in the UK: how to compare in 2026

Why we don't rank double glazing companies

You will find plenty of pages listing "the top 10 double glazing companies" with scores beside each name. Those scores are almost always either paid placements or invented, and in this industry that is more than a harmless bit of fluff — it steers people towards long, high-pressure sales appointments and away from perfectly good local installers who quote in twenty minutes without theatre.

The useful thing we can give you is the specification vocabulary. Once you can read a window quote properly, the differences between companies become obvious without anyone needing to rank them.

FENSA and CERTASS: the compliance point

Replacing windows in England and Wales is notifiable work under the Building Regulations, mainly because of thermal performance, ventilation, safety glazing and means of escape. There are two routes to compliance:

• Use a FENSA or CERTASS registered installer, who self-certifies the work and registers it. You receive a certificate, usually within a few weeks.
• Or apply to Building Control yourself before the work starts and pay for inspection.

The first route is what almost everyone uses, and it matters beyond compliance: your conveyancing solicitor will ask for those certificates when you sell. Windows fitted by an unregistered installer with no Building Control application can become an awkward and expensive problem years later, typically resolved with a regularisation application or indemnity insurance. Scotland and Northern Ireland operate their own building standards regimes, so check the local requirement if you are outside England and Wales.

Separately, the Glass and Glazing Federation (GGF) is a trade body whose members sign up to a consumer code. GGF membership is a reasonable additional signal, but it is not a substitute for FENSA or CERTASS registration.

How to read the glass spec: U-values and WER

This is where most quotes become comparable. Two numbers do the work:

• U-value measures heat loss in watts per square metre per kelvin. Lower is better. Modern replacement windows in England and Wales must meet the standard set in the Building Regulations, and a good 2026 double-glazed unit typically achieves a whole-window U-value around 1.2 to 1.4. Triple glazing can reach roughly 0.8 to 1.0.
• Window Energy Rating (WER) is a banded A to G scale (with A+ and A++ above it) that balances heat loss against solar gain. It is easier to compare at a glance, but the U-value is the more precise figure. Most quality 2026 windows are rated A or A+.

Also ask about the things that sit behind those numbers: the spacer bar (a warm-edge spacer reduces condensation at the perimeter), the gas fill (argon is standard, krypton in narrower cavities), the low-emissivity coating, and the cavity width. A quote that names these is from someone who knows their product.

Frame materials compared

Costs are supplied and fitted per window for a typical casement, 2026, including removal and disposal of the old unit. Bay windows, sash windows, arched heads, obscure or toughened glass, trickle vents and awkward access all add to the figure. A whole-house uPVC replacement on a three-bedroom semi commonly lands between £5,500 and £9,500.

Guarantees and deposit protection

This is the part that costs people money when it goes wrong.

• Insurance-backed guarantee (IBG): a company guarantee is only worth anything while the company exists. An IBG is a separate policy that stands behind the guarantee if the installer ceases trading. Ask whether one is included, who underwrites it and for how many years.
• Deposit protection: avoid large up-front deposits. Where a deposit is genuinely needed, ask for it to be protected, and pay by credit card where possible — Section 75 of the Consumer Credit Act can give you additional protection on qualifying transactions.
• Separate the guarantees: the frames, the sealed units, the hardware and the installation workmanship are often covered for different periods. Get all four in writing.
• Cancellation rights: for a contract agreed in your home you normally have a 14-day cooling-off period. A salesperson pushing you to waive it is a warning sign.

Handling the sales appointment

Some parts of this industry still run long in-home appointments built around discount pressure. You are entitled to be brisk about it.

• Set a time limit at the start and stick to it
• Never sign on the first visit, whatever the discount is said to expire
• Ask for a written itemised quote to consider in your own time — a genuine price does not evaporate overnight
• Get at least three quotes and compare U-value, WER, frame, glass spec and guarantee side by side
• Be wary of a headline price that only applies to a minimum order of several windows

FAQs: double glazing companies (UK, 2026)