We supply and fit energy efficient windows in Clerkenwell, and this guide explains how they keep heat in. It is for homeowners who want to read a window quote with confidence. You will learn what the ratings mean, which parts of a window do the work, and what the law asks of a new window.
This page is about how a window performs, not how it looks. For styles such as casement, sash and tilt and turn, see our double glazing windows range. For a wider look at the service, start on our double glazing home page.
Window Energy Ratings Explained
A Window Energy Rating, or WER, grades a whole window from A++ down to E. It is a bit like the label on a fridge. The rating weighs up three things:
- Heat loss: how much warmth leaks out through the glass and frame.
- Solar gain: how much free warmth the sun adds through the glass.
- Air leakage: how much air slips through the seals.
The result is a single band. A window rated A or higher keeps more heat in than it lets out over a year in a typical UK home. The rating applies to a set frame and glass make-up, so ask for the rating of the exact window you are buying, not the best one in the range.
The rating is tested and listed by bodies such as the British Fenestration Rating Council. You can ask your installer for the licence number of the rated product.
U-Values and Part L of the Building Regulations
A U-value shows how fast heat passes through a window. It is given in W/m²K. The lower the number, the better the window holds heat.
Part L of the Building Regulations sets the rules for heat loss. In England, a replacement window must reach a whole-window U-value of 1.4 W/m²K or better, or a WER of band B or better. Wales and Scotland set their own limits, which are close to these.
As a rough guide:
- Single glazing: around 4.8 to 5.0 W/m²K.
- Double glazing from the 1990s: around 2.8 W/m²K.
- Modern A-rated double glazing: around 1.2 to 1.4 W/m²K.
- Triple glazing: around 0.8 to 1.0 W/m²K.
A registered installer must meet these limits and certify the job. We register every job with FENSA, and you get a Building Regulations certificate once it is logged. Not sure if a third pane is worth it? Our double vs triple glazing guide compares the two.
The Glass: Low-E Coatings and Gas Fills
Most of a window is glass, so the glass does most of the work. Two parts matter most.
Low-E coatings. Low-E means low emissivity. It is a thin, clear layer of metal oxide on one face of the glass. It lets daylight in but bounces long-wave heat back into the room. You cannot see it, but it makes a big difference. A modern unit without a low-E coat would not meet Part L.
Gas fills. The gap between panes is filled with a heavy, inert gas instead of air. Argon is the most common. It is cheap, safe and slows heat flow. Krypton works better in thin gaps but costs more, so it is mostly used in slim or triple units.
The width of the gap matters too. A 16mm argon-filled cavity is a common sweet spot in double glazing. Go much wider, and the gas starts to move and carry heat across. Go much narrower, and heat passes too easily.
The Edges: Warm-Edge Spacers and Frames
The edge of a sealed unit is its weak spot. The spacer bar that holds the panes apart used to be aluminium, which carries heat well. That made the edge of the glass cold, and that is where condensation formed.
Warm-edge spacers use plastic, steel or foam that carry far less heat. They keep the edge of the glass warmer and cut the risk of mould on the seals. Ask for warm-edge spacers on every quote. They add little to the cost.
The frame matters as well:
- uPVC: multi-chamber profiles trap air and slow heat loss.
- Aluminium: needs a thermal break, a strip of low-conducting material that splits the inner and outer frame.
- Timber: a natural insulator, but it needs care to stay sealed and sound.
Our uPVC vs aluminium guide compares the two most common frames in more detail.
Draught-Proofing and Air Tightness
A window with great glass can still feel cold if air leaks round it. Draughts come from three places:
- The seals on the sash. Gaskets press the sash tight to the frame when shut. Worn or flat gaskets let air through.
- The locking points. Multi-point locks pull the sash in at several places. A loose or worn lock leaves a gap.
- The joint round the frame. The gap between the frame and the wall must be packed and sealed, inside and out.
Fitting matters as much as the product. A rated window fitted with gaps round the edge will not perform as tested. We pack, foam and seal each frame, then check that every sash closes tight.
Air tightness has a flip side. Your home still needs fresh air. That brings us to condensation.
Condensation and Ventilation
Condensation forms when warm, damp air meets a cold surface. With old single glazing, that surface was the glass. With good new windows, the glass stays warmer, so you see less of it.
But a tighter home can trap moisture from cooking, washing and breathing. If it has nowhere to go, it may show up on walls or in corners instead. The answer is controlled fresh air:
- Trickle vents in the frame let a small, steady flow of air in. Part F of the Building Regulations may require them when you replace windows.
- Extractor fans in the kitchen and bathroom remove damp air at the source.
- Daily habits help too: lids on pans, and drying clothes outside where you can.
Mist between the two panes is different. That means the seal on the unit has failed. You do not need a new window. A new sealed unit fixes it. See our page on misted double glazing.
Choosing and Comparing Quotes
When you compare quotes for energy efficient windows, check these points:
- The WER band or whole-window U-value, for the exact frame and glass.
- Low-E glass and argon fill in every unit.
- Warm-edge spacers.
- A thermal break, if the frames are aluminium.
- Trickle vents where the rules call for them.
- Registration with FENSA or Certass, or a similar scheme, so the job is certified.
What drives the cost? Mostly the frame material, the size and number of windows, the glass make-up, and access. Every job gets a written, fixed quote after a free survey. You can request your quote here. Our how we work page sets out each step, from survey to aftercare. Every install is backed by a 10-year guarantee.
Frequently Asked Questions
How long does it take to fit energy efficient windows?
Most homes take one to three days on site. A fitter can often replace several windows a day, as each opening is done one at a time and sealed before the next. Larger homes or tricky access can add time. Lead time for made-to-measure frames comes first, and we confirm both dates when you accept the quote.
What does an A-rated window actually mean?
It means the whole window, frame and glass together, has been tested and rated in band A on the Window Energy Rating scale. An A-rated window keeps in more heat than it loses over a typical year. The rating covers heat loss, solar gain and air leakage. Always ask for the rating of the exact product on your quote.
Can energy efficient windows reduce outside noise?
Yes, to a point. A well sealed modern window cuts noise far better than old single glazing. For busy roads, ask for acoustic laminated glass, which damps sound inside the pane. Mixing pane thickness helps too. Good seals matter as much as the glass, because sound leaks through the same gaps as draughts.
Are there options for heritage or conservation area homes?
Yes. Slim double glazed units can go into sash and timber-look frames that keep the original lines. In a listed building, you may need consent, and secondary glazing is often the better route. It sits inside the old window and leaves it in place. Check with your council before you order.
How should A-rated double glazing be cleaned and cared for?
Clean the glass with warm, soapy water and a soft cloth. Avoid harsh or gritty products, which can mark the coating or the seals. Wipe the frames, clear the drain holes at the base, and oil the hinges and locks once a year. Check the gaskets for splits, as a worn seal lets in draughts.
