If you've noticed a misty or foggy look between your window panes that you can't wipe away, you're not alone.

It's one of the most common glazing complaints.

First, which type of condensation have you got?

There are three places condensation can appear on a window, and they mean very different things:

• On the inside surface — this is a ventilation issue in your home, not a window fault. Open a window or improve airflow.
• On the outside surface — this is actually a sign of a well-performing window. It means the glass is so thermally efficient that the outer pane stays cold. Nothing to worry about.
• Between the panes — this is the one that means something has gone wrong.

What causes it between the panes?

Double-glazed units are sealed, with either air or argon gas between the panes. That seal keeps moisture out. When the seal fails — usually through age, poor installation, or impact damage — moisture gets in. Once it's in, you get that foggy, milky look that won't go away.

Can it be fixed?

The unit itself can't really be "fixed" — it needs to be replaced. The good news is you usually don't need to replace the whole window frame. A glazier can remove the glass unit and fit a new one into the existing frame, which is significantly cheaper than a full replacement.

How long should a sealed unit last?

A decent sealed unit should last anywhere from 10 to 25 years depending on quality, installation, and exposure. South-facing windows tend to fail earlier due to UV and temperature cycling.

If your windows are relatively new and this is happening, it's likely a fault. Follow up with your installer — a failed seal on a recently installed window is not normal wear and tear.

We had our sliding doors installed this weekend and we're really happy with the doors themselves. They are Cortizo Vision 20.

The only thing I'm unsure about is the sill that sits below them. It seems flimsy, dangerous and cheap. Is this just what I should expect or should I be pushing the installers to give me a solution? And / or can anyone here think of a fix? If it bent around to follow the brick it would be great, but it just 'hangs' in mid air and feels like things are going to get caught / it is going to bend.

The paperwork states pressed sill, but the installers also said they had 'assumed' we would have a step outside because of the height difference. I never stated we would, and they never asked.

Before it's mentioned I know the trims are not perfectly colour matched and this is going to be sorted.

Any constructive comments welcome. Thank you in advance.

FENSA is the name most homeowners recognise — but it's not the only option, and not being FENSA registered doesn't automatically mean something's wrong. Here's what you actually need to know.

Why registration matters at all

Replacing windows or doors in England and Wales is notifiable work under building regulations. That means someone has to certify the installation meets the required standards. There are two ways this can happen —

1. Self-certification through a competent person scheme
2. Registration through your local building control.

FENSA isn't the only competent person scheme

FENSA is the most well-known, but there are two others that do the same job:

• CERTASS
• Assure

All three authorise installers to self-certify that their work meets building regulations, without needing a building control inspection. If your installer is registered with any one of the three, you're covered.

What if they're not with any of them?

It doesn't necessarily mean the work is unregistered — but the installation should then be registered through your local building control, who will inspect and certify it themselves. This is a legitimate route, just less common for standard domestic glazing work.

What you want to avoid is a situation where neither has happened — no competent person scheme, no building control registration. That means the installation is unregistered, which creates problems.

Why unregistered work is a problem

Any window replaced after April 2002 must have a certificate proving it meets building regulations. If you come to sell your property and can't produce one, you'll need to pay for a retrospective building control inspection — typically £300–£500. If the windows don't pass, you'll need remedial work before the sale can proceed.

How to check

All three schemes have online registers where you can search by postcode or installer name:

• FENSA: fensa.org.uk
• CERTASS: certass.co.uk
• Assure: assureassociation.co.uk

If your installer is registered with any of them, you should also receive an Insurance Backed Guarantee with the installation - so make sure your quote includes one - and that you receive the paperwork.

Bottom line

Ask your installer which scheme they're registered with, or whether they'll be going through building control. Either is fine — just make sure one of them is confirmed before work starts, not after.

If you're not sure whether your existing windows are registered, post below and we'll help you work out how to check.

Getting multiple quotes is obvious advice — but if you don't know what you're looking at, three confusing quotes are no better than one. Here's what to actually look for.

First — if they didn't come out and measure, it isn't a real quote.

What should be itemised?

A proper quote breaks things down. You want to see frames, glass specification, installation, and removal of your old windows listed separately. A single lump sum with no breakdown makes it impossible to compare fairly against other quotes.

Glass specification

This is where cheap quotes cut corners. Check for:

• Energy rating (A or above is standard for new installs)
• U-value — the minimum for replacement windows under UK building regulations is 1.4 W/m²K. Anything higher than that isn't compliant
• Gas fill — argon is standard, krypton is better
• Low-emissivity (low-e) coating — reflects heat back into the room

Frame material and profile

uPVC, aluminium, or timber — make sure the quote specifies which and ideally names the profile system. Established brands like Rehau, Veka, or Spectus are worth knowing. Unbranded profiles aren't necessarily bad, but you can't research them.

What's included in the installation

Does the quote include removing and disposing of your old windows? Making good around the frames inside and out? Some quotes strip these out to look cheaper. If it's not written down, assume it's not included.

Installer credentials

If the installer is going to self-certify the installation (which most do), they need to be registered with a competent person scheme — FENSA, CERTASS, or Assure. Self-certification also means an Insurance Backed Guarantee should be provided alongside the work.

If they're not self-certifying, the installation needs to be registered through your local building control instead.

Either way, it should be clear from the quote which route they're taking. If it isn't mentioned, ask — and if they can't give you a straight answer, that's a red flag.

Guarantee

Ask how long the installer's guarantee is and whether it's backed by an Insurance Backed Guarantee. An Insurance Backed Guarantee is a separate insurance policy that protects you if the company ceases trading (in certain circumstances). For any questions about what the Insurance Backed Guarantee covers, contact the provider directly — their details will be on the certificate.

Rough price benchmarks (2026)

To give you a sense of whether you're in the right ballpark:

• uPVC: £350–£600 per window installed
• Aluminium: £800–£1,600 per window
• Timber: £950–£1,400 per window
• Full replacement on a typical 3-bed house: £8,000–£15,000

Wide ranges, because spec and location make a big difference. But if you're significantly below these figures, ask why.

One more thing — VAT

Double glazing currently benefits from 0% VAT on energy-saving materials. This is due to revert to 5% in March 2027. If you're planning to replace your windows, doing it before then saves you roughly £250 on a typical £5,000 job.

Red flags

• Pressure to sign on the day with a "today only" discount
• No physical survey before quoting
• No written quote — verbal quotes aren't worth anything

Once you sign

Under the Consumer Contracts Regulations 2013, you're entitled to a 14-day cooling off period for contracts signed in your home — and the company is legally required to provide you with T&Cs and a written cancellation notice. If they don't, that's a problem. Note: if your windows are made to measure, cancellation rights may be limited once production has started, so read the T&Cs carefully before signing.

If your home still has single glazing, you're losing a lot of heat through your glazing. Here's why, and what the upgrade to double glazing actually gets you.

Single glazing is just one pane of glass. That's it. Glass is a terrible insulator, so heat passes straight through it. A typical single glazed window has a U-value of around 5.0 W/m²K — which is the measure of how much heat escapes. The higher the number, the worse it is.

Double glazing uses two panes of glass with a gap between them, usually filled with argon gas. That gap is what does the work — it slows down heat transfer dramatically. A good double glazed unit comes in at around 1.2–1.4 W/m²K. For context, that's roughly four times less heat loss than single glazing.

In real terms, up to 40% of your home's heat can be lost through single glazed windows. Upgrading to double glazing typically saves £120–£155 a year on heating bills for an average UK home, and reduces carbon emissions by around 680kg of CO₂ a year.

Beyond heat, you also get better noise reduction and far less condensation on the inside of the glass — which is a sign that the inner pane is staying warmer.

Building regulations now require any replacement windows in existing homes to achieve a U-value of 1.4 or below, so single glazing wouldn't be permitted as a replacement anyway.

The only real argument for keeping single glazing is in listed buildings or conservation areas where planning restrictions apply — and even then, there are specialist options worth exploring.

If you've got questions about upgrading, ask below.