Choosing a double glazing installer is a big decision that affects your home’s energy bills, security, and value for decades. The wrong choice can lead to draughty windows, failed seals, and expensive remedial work.
The most reliable sign of a good double glazing installer is full certification with FENSA or CERTASS, because this proves they are registered to self-certify their work under Building Regulations. Always check a company’s registration number on the official register before agreeing to any quote.
What matters when you choose is the price, the paperwork, the guarantees, and the standards behind the installation. The ten signs below will help you separate a professional from a cowboy before you sign anything.
| Accreditation | What it covers | How to verify | Why it matters |
|---|---|---|---|
| FENSA | Double glazing installation and replacement | Check on FENSA.org.uk | Legal self-certification under Building Regulations Part L |
| CERTASS | Double glazing and energy-efficiency work | Check on CERTASS.co.uk | Alternative to FENSA for compliance |
| TrustMark | Overall quality and consumer protection | Check on TrustMark.org.uk | Government-endorsed, covers complaints and insurance |
| Glass and Glazing Federation (GGF) | Code of practice and dispute resolution | Check on GGF.org.uk | Industry standards and consumer protection |
| British Fenestration Rating Council (BFRC) | Window energy ratings | Check on BFRC.org | Independent energy performance certification |
| Which? Trusted Traders | Customer reviews and vetting | Check on Which.co.uk | Verified customer feedback and trading standards check |
| Checkatrade | Customer reviews and background checks | Check on Checkatrade.com | Verified reviews and insurance checks |
| Public Liability Insurance | Cover for damage or injury on your property | Request a copy of the certificate | Minimum £2 million cover is standard |
| Insurance-backed guarantee | Protection if the company stops trading | Confirm the policy provider and terms | Typically 10 years cover |
| Building Regulations compliance | Legal requirement for replacement windows | Check FENSA/CERTASS registration | Part L (energy) and Part F (ventilation) compliance |
1. Holds FENSA or CERTASS registration
Under Building Regulations, replacement windows must meet energy-efficiency standards under Part L. FENSA and CERTASS are the two main schemes that allow installers to self-certify their work, meaning you do not need a separate building control inspection. A registered installer will give you a certificate of compliance after the job is done. Without this registration, you may face legal issues when selling your home, as the installation will not be recorded on the local authority database. Always check the company’s registration number on the official register before agreeing to any quote.
- Check the FENSA register at FENSA.org.uk
- Check the CERTASS register at CERTASS.co.uk
- Ask for a Building Regulations compliance certificate after installation
2. Provides a 10-year insurance-backed guarantee
A good installer will offer a guarantee that is backed by an insurance policy, not just the company’s own promise. This means if the company goes out of business during the guarantee period, the insurer steps in to cover any defects. The guarantee should cover the sealed units, frames, and installation work. TrustMark requires its members to offer this level of protection. Without an insurance-backed guarantee, you could be left with no recourse if the installer disappears.
- Confirm the guarantee is insured by a third-party provider
- Read the terms to see what is covered and for how long
- Keep a copy of the guarantee certificate with your home documents
3. Uses only BFRC-rated windows
The British Fenestration Rating Council (BFRC) rates windows from A++ to G for energy efficiency. An A-rated window can save around £100 to £130 a year on energy bills compared to a single-glazed window (Energy Saving Trust, 2026). A good installer will show you the BFRC label on every window they propose. Avoid any installer who cannot provide a clear energy rating or who dismisses the rating system as unimportant.
- Look for the BFRC label on the window frame
- Aim for A-rated or higher for best energy savings
- Ask how the rating affects your home’s overall energy performance
4. Offers a detailed written quote with no hidden fees
A transparent quote itemises the product cost, labour, disposal of old windows, and VAT. It should also state whether scaffolding or skip hire is included. TrustMark and Trading Standards both advise that a clear quote reduces the risk of disputes later. A good installer will not add surprise charges partway through the job. If the quote is vague or contains clauses like “additional costs may apply,” ask for clarification before you accept.
- Request a written quote before any work starts
- Check whether scaffolding, skip hire, and disposal are included
- Compare at least three quotes to spot outliers
5. Shows proof of public liability insurance
Public liability insurance covers damage to your property or injury to a worker while on your premises. The standard minimum cover is £2 million. A reputable installer will provide a copy of their insurance certificate on request. If they cannot or will not show it, that is a clear warning sign. Without this cover, you could be liable for any accidents that happen during the installation.
- Ask to see the insurance certificate before the quote is finalised
- Check the expiry date and the level of cover
- Verify the certificate is in the company’s name
6. Has positive reviews on a verified third-party platform
Reviews on the company’s own website can be cherry-picked. Instead, look for recent feedback on TrustMark, Checkatrade, or Which? Trusted Traders. These platforms verify that the reviewer is a genuine customer. A good installer will have a consistent pattern of positive reviews over at least 12 months. Be wary of companies with only a handful of reviews or an unusually high number of five-star ratings posted in a short period.
- Check TrustMark at TrustMark.org.uk
- Check Checkatrade at Checkatrade.com
- Look for reviews that mention workmanship, timeliness, and communication
7. Uses factory-fitted trickle vents or meets ventilation requirements
Building Regulations Part F require adequate ventilation in homes, and modern double glazing can reduce natural airflow. A good installer will assess whether trickle vents are needed and explain their purpose. They will not simply install windows without vents if your home requires them. Ignoring ventilation can lead to condensation, mould, and poor indoor air quality. The installer should also provide a ventilation compliance certificate if required.
- Ask whether your home needs trickle vents under Part F
- Check that vents are factory-fitted, not added on site
- Request a ventilation compliance statement with the quote
8. Gives a clear timeframe and sticks to it
A reliable installer provides a start and finish date in writing and gives at least 48 hours’ notice if any delay occurs. Trading Standards considers a written schedule a basic consumer right. A good installer will also explain how long each phase of the work will take, from removal of old windows to final sealing. If the installer cannot commit to a timeline or is vague about start dates, that is a red flag.
- Get the start and finish dates in the contract
- Ask about the process for notifying you of delays
- Check whether the timeline includes weekends or public holidays
9. Conducts a proper survey before quoting
A good installer will visit your home to take measurements, inspect the frame condition, and check for any structural issues. Quoting over the phone or from photos alone is not reliable. The survey should also note any access restrictions, such as narrow paths or parking limitations. Without a proper survey, the final quote may be inaccurate, and the installation may not fit correctly. FENSA and TrustMark both recommend a site survey before any quote is given.
- Refuse quotes given without a site visit
- Ask the surveyor to explain their measurements
- Check whether the survey includes checking for damp or rot in existing frames
10. Belongs to a recognised consumer code of practice
Membership in the Glass and Glazing Federation (GGF) or a similar body means the installer follows a code of conduct and offers dispute resolution. The GGF also provides a deposit protection scheme for members. TrustMark endorses this approach as part of its government-backed standards. A company that is a member of such a scheme is more likely to resolve complaints fairly without involving trading standards or legal action.
- Check the GGF membership at GGF.org.uk
- Ask whether the installer has a formal complaints procedure
- Confirm that the code of practice covers the work you are having done
By checking these ten signs, you can choose a double glazing installer who will deliver a compliant, energy-efficient installation backed by proper guarantees and consumer protection. Always verify accreditations and insurance before paying a deposit, and keep all paperwork for your records.
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Frequently Asked Questions
Full certification with FENSA or CERTASS is the most reliable sign. This proves the installer is registered to self-certify their work under Building Regulations, as required by GOV.UK.
Visit FENSA.org.uk and enter the company's registration number. You can also search by postcode or company name to confirm their status on the official register.
Both are government-approved schemes for self-certifying double glazing installations under Building Regulations. FENSA is more common, while CERTASS offers an alternative route to compliance, as explained on CERTASS.co.uk.
Yes, TrustMark is a government-endorsed scheme that covers quality and consumer protection. It ensures the installer follows a code of conduct and has complaints insurance, as verified on TrustMark.org.uk.
A good installer offers an insurance-backed guarantee that covers windows for at least 10 years. This protects you if the company stops trading, and you should confirm the policy provider and terms before signing.
