10 things to check before buying new windows
Buying new windows is a major investment that affects your home’s energy bills, security, and comfort for years. With dozens of options on the market, it is easy to focus on price and overlook the details that determine long-term performance.
Before buying new windows, the single most important check is the product’s energy rating, aim for at least A++ (or a U-value of 1.2 W/m²K or lower) to maximise heat retention and reduce your heating bills. You should also verify the installer’s certification (FENSA or equivalent) and check the warranty length, as these directly affect long-term performance and legal compliance.
What matters when you choose: the product’s energy efficiency is the biggest factor in long-term savings, but the installation quality and the warranty are just as critical. You also need to consider security features, noise reduction, and whether the frame material suits your home’s style and maintenance needs, each of these affects the final cost and your comfort.
| Energy rating | Typical U-value (W/m²K) | Annual heat saving vs. single glazing (typical 3-bed semi) | Typical extra cost over rating D |
|---|---|---|---|
| A++ | ≤ 1.2 | £195–£285 (Energy Saving Trust, 2026) | £200–£350 |
| A+ | 1.2–1.4 | £160–£240 | £100–£200 |
| A | 1.4–1.6 | £130–£200 | £50–£100 |
| B | 1.6–1.8 | £100–£160 | Base cost |
1. Check the energy rating or U-value
The energy rating tells you how well a window keeps heat inside your home. The Energy Saving Trust states an A++ rating (U-value ≤ 1.2 W/m²K) can save a typical semi-detached house £195–£285 per year on heating (Energy Saving Trust, 2026). Lower U-values mean less heat escapes; always ask the supplier for the certified rating label.
- Look for the BFRC (British Fenestration Rating Council) label on the product.
- Compare the U-value against your current windows to estimate savings.
- Remember that frame material affects the overall U-value, not just the glass.
2. Verify FENSA certification for the installer
FENSA-registered installers self-certify compliance with Building Regulations; GOV.UK states this is a legal requirement for replacement glazing (GOV.UK, 2026). Without FENSA (or equivalent like CERTASS), you may need a separate Building Regulations application, costing £200–£400.
- Check the installer’s FENSA number on the FENSA website before signing a contract.
- Ask for a certificate of compliance after installation, keep it with your property documents.
- Uncertified work can cause problems when you sell your home, as solicitors often request proof of compliance.
3. Confirm the warranty length and coverage
Most reputable manufacturers offer a 10-year guarantee on the sealed unit and a 5–10 year warranty on the frame and hardware (FENSA, 2026). Check what is excluded, e.g., accidental damage or condensation between panes, and get the terms in writing.
- Ask whether the warranty covers both parts and labour, or just the materials.
- Check if the warranty is transferable if you sell your home within the guarantee period.
- Keep the original invoice and warranty documents in a safe place.
4. Measure the noise reduction (acoustic performance)
Double or triple glazing with a 4mm–6mm air gap can reduce outside noise by 30–40 dB (Energy Saving Trust, 2026). For homes near busy roads, ask for a certified Rw (weighted sound reduction) value of at least 40 dB.
- Thicker glass (e.g., 6mm outer pane) improves sound insulation without much extra cost.
- Acoustic laminated glass can reduce noise further, but adds roughly 20–30% to the window price.
- Test the noise reduction by visiting a showroom with similar glazing specifications.
5. Look for Secured by Design accreditation
Windows with Secured by Design (SBD) certification meet police-approved security standards, including multi-point locking and laminated glass. SBD windows can reduce the risk of burglary; check the SBD website for a list of approved products (Secured by Design, 2026).
- Multi-point locking systems are standard on SBD-certified windows, avoid single-point locks.
- Laminated glass (at least 6.8mm thick) is harder to break than standard float glass.
- SBD accreditation adds around 5–10% to the window cost but may reduce home insurance premiums.
6. Compare frame materials, uPVC, timber, aluminium
uPVC is the cheapest (approx. £200–£400 per window) and low-maintenance, while timber costs more (£500–£800) but offers better insulation. Aluminium frames (£400–£700) are strong and slim but can be less energy-efficient unless thermally broken (Energy Saving Trust, 2026).
- uPVC frames need occasional cleaning but do not require painting.
- Timber frames require repainting every 3–5 years to prevent rot.
- Aluminium frames with thermal breaks achieve U-values close to uPVC, but cost more.
Compare frame material costs and maintenance in our guide on window frame types
7. Check for trickle vents or background ventilation
Building Regulations require adequate ventilation to prevent condensation and mould; trickle vents in window frames meet this requirement (GOV.UK, 2026). Without them, you may need to install separate wall vents, adding £50–£150 per room.
- Trickle vents are small slots at the top of the frame that allow background airflow.
- Some modern windows have integrated vents that are less visible than older designs.
- If you have allergies, check whether the vents include insect mesh to keep pollen out.
8. Assess the glass type, double vs. triple glazing
Triple glazing offers a U-value as low as 0.8 W/m²K, but costs 30–50% more than double glazing (Energy Saving Trust, 2026). For most UK homes, double glazing with a low-E coating and argon fill is the best value-for-money option.
- Low-E (low emissivity) coating reflects heat back into the room, improving efficiency.
- Argon gas fill between panes reduces heat loss compared to air-filled units.
- Triple glazing is heavier and may require stronger frames, adding to installation costs.
9. Confirm the opening style and safety features
Casement windows are the most energy-efficient, while sliding sash windows may have higher air leakage (FENSA, 2026). For upstairs rooms, ensure windows have restrictors or safety catches to prevent falls (Building Regulations Part K).
- Casement windows open outward and seal tightly against a gasket, good for energy efficiency.
- Tilt-and-turn windows offer flexibility but can be harder to fit blinds or curtains.
- Restrictors limit how far a window opens, typically to 100mm, for child safety.
10. Get at least three quotes and compare total costs
The average cost for a standard uPVC double-glazed window (including fitting) is £250–£500 (TrustMark, 2026). Always ask for a breakdown of materials, labour, and VAT, and check that the quote includes disposal of old windows.
- Request itemised quotes so you can compare like-for-like specifications.
- Check whether the quote includes scaffolding if you live in a flat or have hard-to-reach windows.
- Ask about payment schedules, avoid paying the full amount upfront.
Read our guide on getting quotes for home improvements to avoid common pitfalls
Choosing the right windows comes down to checking the energy rating, installer certification, and warranty before you commit. Focus on these 10 points and you will avoid costly mistakes and enjoy lower bills, better security, and a more comfortable home.
See our full guide on window replacement costs and savings for more detail
Frequently Asked Questions
Aim for at least A++ (U-value ≤ 1.2 W/m²K). The Energy Saving Trust states this can save a typical semi-detached house £195–£285 per year on heating.
Yes, FENSA certification (or equivalent) is required for building regulations compliance. A FENSA-registered installer ensures your windows meet legal standards.
Most reputable manufacturers offer a 10-year warranty. Check the terms cover both glass and frame defects.
A U-value of 1.2 W/m²K or lower is excellent. Lower values mean better heat retention and lower heating bills.
Typically £200–£350 more than B-rated windows (Energy Saving Trust, 2026). The annual savings of £195–£285 offset the cost within a few years.
More Windows & Glazing guides
- 10 signs your windows need replacing
- 10 ways to stop condensation on windows
- 10 ways to cut heat loss through windows
- 10 reasons double glazing units fail
- 10 ways to soundproof your windows
- 10 questions to ask a window installer
- 10 mistakes to avoid when buying windows
- 10 ways to maintain uPVC windows
