All properties lose heat through their windows. But energy efficient double glazing, coupled with other energy saving measures keeps your home warmer and quieter as well as helping to reduce your energy bills.
sealed units with feature rectangle leads.
If your heating comes on earlier every autumn, and the seat by the bay window is still the coldest place in the house, your windows are a fair place to start looking. The energy price cap rises again on 1 October 2026, to £1,723 a year for a household on typical use paying by direct debit, and the question we hear most often at our Watford showroom is a simple one. Will new windows bring that down?
Yes, they will, although probably by less than the advertising suggests, and the bill saving is only part of what you get. Here is the honest picture, with the current figures and the current rules.
Did you know?
Nine out of ten homes in England now have full double glazing. If yours is in the tenth, the single glazed panes are letting heat out more than three times as fast as the window that would replace them, and the rules will not let us fit anything less.
How much do energy efficient windows actually save?
The Energy Saving Trust puts the saving at around £85 a year for a semi-detached home in Great Britain that goes from single glazing to A-rated double glazing, along with roughly 290kg less carbon dioxide a year. In Northern Ireland, where more homes run on oil, the same upgrade is put at £140.
Two things are worth understanding about that figure. It measures single glazing against modern double glazing, so if your existing units date from the 1990s the gap will be narrower. It also moves with energy prices, which is why numbers published a few years ago looked larger than the ones published today.
What most of our customers notice first is not the bill at all.
It is the end of the cold draught along the skirting, the loss of that chilly zone you feel standing near the glass, and the disappearance of the morning condensation that used to sit on the inside of the frames. Those changes arrive on day one. The bill saving arrives quietly over the year.
What makes a window energy efficient?
Four things do most of the work, and none of them are visible once the window is in.
1. Low emissivity glass
Low-E glass has a very fine metallic coating that bounces heat back into the room before it can escape through the glass. The high performance version, soft coat, sits on an inner face inside the sealed unit, where the air cannot get at it and tarnish it.
The difference is large: an uncoated double glazed unit sits near 3.0 W/m²K at the centre of the pane, while a soft coat unit filled with argon reaches around 1.0 to 1.2 W/m²K.
2. Argon in the cavity
Double glazing is made using two pieces of glass, with inert gas filling the gap in between. Argon conducts heat at roughly 0.016 W/m·K against 0.026 for dry air, so filling a sealed unit with argon gas cuts the heat crossing the gap by about a third at almost no extra cost. It works best in cavities of 12mm to 16mm, because past about 18mm the gas begins to circulate in convection loops and the benefit cancels out. Wider is not automatically better, which is worth knowing when you compare quotes.
3. Warm edge spacer bars
The spacer holds the two panes apart around the perimeter. Older units used aluminium, which conducts heat well and creates a cold bridge at the edge of the glass, and that is where condensation and mould show up first. Warm edge spacers use thin walled steel, structural foam or polymer instead, keeping the glass edge warmer.
4. The frame and its thermal break
What counts is the whole window U-value, which takes in the frame as well as the centre of the glass. Good uPVC uses multiple sealed chambers to trap still air, while aluminium conducts heat readily and needs a polyamide thermal break separating the inside of the profile from the outside. Aluminium without one, of the sort still found in 1980s conservatories, performs badly enough that it would not satisfy current building regulations.
How to read a window energy rating
Window Energy Ratings run from A++ down to E, and they are issued through schemes including the British Fenestration Rating Council.
The rating is more than a U-value. It combines three measurements:
- heat lost through the window
- useful solar heat gained through it
- and air leaking around it
A window can lose a little more heat than a rival and still rate higher because it lets more free winter sunshine in.
The label is voluntary, so its absence is not a red flag on its own. Ask for the number instead. Most of the window ranges we install achieve an A rating or better, and we can show you the certificate for any product we quote.
What the building regulations require
The U-value or rating you need
For a replacement window in an existing home in England, Approved Document L sets a limit of 1.4 W/m²K, or a Window Energy Rating of band B or better. The current version is the 2021 edition incorporating the 2023 amendments.
The replacement must also not perform worse than the window it replaces.
Trickle vents
Since 15 June 2022, Approved Document F has meant that most replacement windows need trickle vents, the small controllable vents in the head of the frame. Customers rarely want them and we understand why, but they are there to move moisture out of the room, and modern homes are airtight enough to need them.
Certification, and the allowance for period buildings
Because we are registered with both FENSA and CERTASS, we certify that work ourselves and register it with your local authority. You receive a Building Regulations compliance certificate, which is the document your solicitor will ask for when you sell your home.
There is a separate allowance for buildings where character has to be preserved. Where a compliant window would harm that character, the rule shifts to a centre pane U-value of no worse than 1.2 W/m²K, or low emissivity secondary glazing behind the original.
What the Future Homes Standard changes
The Future Homes Standard was published on 24 March 2026 and comes into force on 24 March 2027 for most building work. It tightens fabric standards for new homes, moves compliance onto the Home Energy Model, and requires window U-values to be calculated for the actual size and configuration of each opening, in place of a standard test size.
It applies to new build. The rules for replacing windows in an existing home are unchanged.
If you are wondering whether to wait, there is nothing arriving in 2027 that will make your replacement windows cheaper or better, so the timing is yours.
Is triple glazing worth it here?
For a new home built to heat pump standards, triple glazing increasingly makes sense.
For replacing windows in an existing house in Middlesex, Hertfordshire or Buckinghamshire, good double glazing remains the better value choice, and the reasons are as much practical as financial.
Triple units weigh around half as much again as double, which asks more of hinges and sashes, and they need deeper frames of up to 44mm. That restricts the slim sightlines that make flush casements and heritage aluminium look right.
If traffic noise is the real problem, asymmetric acoustic double glazing, with a thicker laminated outer pane, outperforms standard triple glazing anyway.
Period homes, conservation areas and Crittall replacements
Improving thermal performance without losing the character of the house is the part of this job we enjoy most, and it is what much of Ruislip, Watford, Pinner, Northwood and Rickmansworth needs.
ECOSlide sliding sash windows reproduce the proportions and sill detail of a box sash. Heritage Flush casements and The Residence Collection sit flush in the frame, with the corner detail and glazing bars of painted joinery and none of the repainting. For steel framed Crittall openings, Alitherm Heritage aluminium keeps the slim sightlines those windows need, with a thermal break the originals never had.
Where a conservation area imposes conditions, we can specify to them. Ask your council what they will accept before you shortlist products, and we are happy to talk that through first.
Solar control glass and integral blinds
South and west facing rooms can easily overheat in summer, and solar control glass cuts the heat coming in while keeping the daylight. Integral blinds, sealed inside the glass cavity and worked by a magnetic slider or a motor, give you adjustable shading on bi-folds, sliding doors and roof glazing, with nothing to dust and nothing to damage.
What about grants?
Some lower income households in less efficient homes qualify for help through the Warm Homes: Local Grant, which participating councils in England administer themselves. What each council will fund varies, so if you think you might be eligible, start with your local authority, which holds the funding and sets the eligibility rules.
Where to start
Have a look at your windows on a cold morning.
Condensation between the panes means a failed seal, which cannot be repaired properly, and draughts you can feel, frames that have dropped and locks that need a shove all point the same way.
When you are ready, come to our Watford showroom and see the ranges for yourself. We will go through the ratings on the products you are considering, tell you plainly where the money is worth spending and where it is not, and give you a written quote.
Our lead time from order to installation is normally three to four weeks, the warranty is insurance backed through QANW, and our staff are all employed.
No hard sell. No pressure sales. Ever.