Double glazing guarantee: what it covers and how to compare it

What does a double glazing guarantee cover?

A double glazing guarantee covers the parts its written statement names, and Which? splits a good one into frames, glass and other parts and hardware. Each part carries its own length, so read the guarantee line by line.

What the guarantee document must say

A guarantee given at no extra charge takes effect as a contractual obligation when the windows are delivered, under section 30 of the Consumer Rights Act 2015. The same section sets four things the document must contain:

• the contents of the guarantee and how to claim, in plain and intelligible language
• a statement that your statutory rights are not affected
• the name and address of the guarantor
• the duration and territorial scope of the guarantee

The installer must give you the guarantee in writing within a reasonable time when you ask (section 30(6)).

How long should double glazing be guaranteed for?

Double glazing should be guaranteed for 10 years on frames, 5 to 10 years on glass and at least 2 years on other parts and hardware, all backed by a 10-year insurance-backed guarantee. That is the combined advice of nine Which? Trusted Trader installers, not a legal minimum.

Use the benchmark on every quote. A line that falls short is a question for the installer: why it is shorter, and whether the installer can match it in writing.

Guarantee or warranty: is there a difference?

Double glazing paperwork uses guarantee and warranty for the same written promise, so read what the document covers rather than what it is called. The Consumer Rights Act 2015 says "guarantee", Which? says "warranty" for the same 10-year frame cover, and FENSA notes that insurance-backed guarantees are "sometimes also called warranty insurance or guarantee insurance policies".

Company guarantee or insurance-backed guarantee?

An insurance-backed guarantee keeps your guarantee valid if the installer stops trading, Which? explains. Without it, the written guarantee is a promise from the company alone. Four facts decide what the insurance gives you:

• What it is: FENSA describes it as "a type of indemnity insurance purchased by your installer on your behalf", covering faults the installer's warranty would have covered.
• When it is mandatory: work by a FENSA Approved Installer within FENSA's remit comes with a FENSA certificate and an insurance-backed guarantee. FENSA says the policies were not mandatory until 2014, so work from 2013 or earlier may have none.
• Who issues it: an insurance broker, not FENSA. FENSA "does not issue guarantees".
• How long it runs: DGCOS says the policies it provides run 2 to 10 years and can differ from the written guarantee. They apply once the installation is complete with no faults and every amount is paid.

Claiming after the installer has closed is covered in our guide on what happens if your double glazing company goes bust.

Your guarantee sits on top of your legal rights

A double glazing guarantee adds to your rights under the Consumer Rights Act 2015 and never replaces them, which is why section 30 makes every guarantee say so. Three rights apply whatever the guarantee says:

• Windows the installer fits incorrectly do not conform to the contract (section 15).
• The fitting must be done with reasonable care and skill (section 49).
• A fault that appears within six months of delivery is presumed to have been there on delivery (section 19(14)).

A contract claim must start within six years in England and Wales (Limitation Act 1980, section 5) and five years in Scotland (Prescription and Limitation (Scotland) Act 1973, section 6). Our guide to badly fitted windows sets out what to ask the installer to do about a fitting fault.

How to compare the guarantee on three quotes

Compare the guarantee line by line, asking every installer the same nine questions. Copy the table and fill in one column per quote:

Our guide on how to compare double glazing quotes lines up price, glass and registration the same way.

Questions to ask about the guarantee before you sign

Ask every installer these five questions before you sign:

1. How long is the guarantee on frames, glass and hardware?
2. Does the guarantee name the fitting as well as the products?
3. Is the guarantee insurance-backed, and which insurer issues the policy?
4. What conditions must I meet before the insurance applies?
5. Who is the guarantor, and at what address?

Write the answers into the contract: our double glazing contract checklist shows where they go. Our list of questions to ask a double glazing salesman covers the rest of the visit.

How to claim on a double glazing guarantee

Claim on a double glazing guarantee by writing to the installer with the fault and the repair the guarantee promises. Four steps take a claim from first letter to escalation:

1. Find the written guarantee. Ask the installer for a copy if you do not hold one: section 30(6) requires it in writing within a reasonable time.
2. Write to the installer. Describe the fault, add dated photos and say when it appeared. One Which? Trusted Trader puts it plainly: "Contact the installer in writing and expect a timely response that respects the guarantee given."
3. Claim on the insurance if the installer has stopped trading. Contact the insurer named on the policy. FENSA advises asking the installer for the insurer's name and asking the insurer for a copy of the policy, and its IBG team (FENSAIBG@fensa.org.uk) helps with FENSA-registered work.
4. Escalate a refused claim. FENSA's complaints service does not handle guarantee disputes, so use the routes in our guide on how to complain about a double glazing company.

Questions people ask