Energy bills are rising continuously, and saving money by using energy effectively has never been more vital, which is exactly why more homeowners are asking the same question: which windows are the most energy efficient?
The right combination of glazing, frame and style can cut heat loss dramatically, lower your bills, and make your home more comfortable all year round.
What Makes A Window Energy Efficient?
An energy efficient window keeps warmth inside during winter and reduces heat gain in summer, without letting draughts or condensation build up. A few things determine how well a window performs:
U-value: measures how much heat escapes through the window. The lower the U-value, the better the insulation.
Low-emissivity (low-E) glass: a microscopically thin coating that reflects heat back into the room while still letting light through.
Gas-filled cavities: most modern double and triple glazed units are filled with argon or krypton gas between the panes, which conducts heat far more slowly than ordinary air.
Warm edge spacer bars: these sit around the edge of the glass unit and reduce the “cold bridge” that older aluminium spacer bars used to create.
Frame construction: multi-chambered frames with thermal breaks stop heat travelling straight through the frame itself.
What Type Of Window Is Most Energy-Efficient?
There isn’t a single “most energy efficient window” that suits every home. It depends on your glazing, your frame material, and how well the window is installed. Here’s how each element affects performance.
Glazing
Double Glazing: two panes of glass with a gas-filled cavity between them is still the UK’s most common glazing choice. It fits any window style, suits most budgets, and typically starts from around £150 per window depending on size and style. Look for a unit with a Window Energy Rating (WER) of at least C, and ideally A or A+, to get genuine heat retention benefits.
Triple Glazing: adding a third pane pushes insulation further. Triple glazed units typically cost 30–40% more than double glazing, but can cut heat loss enough to reduce energy bills by up to 50% in well-insulated homes. The trade-off is a small reduction in natural light and, in some cases, a heavier frame to support the extra weight.
Secondary Glazing: for period or listed properties where the original frames can’t be replaced, fitting a secondary pane on the inside of the existing window is a practical way to improve efficiency without altering the building’s character. It won’t perform quite as well as double or triple glazing, but it’s a significant upgrade on single glazing.
Frame Material
The frame accounts for a large part of a window’s total surface area, so its material matters just as much as the glass.
uPVC: the most popular frame material in the UK. Multi-chambered uPVC profiles trap air within the frame itself, giving strong thermal performance at a relatively low cost, with minimal maintenance.
Aluminium: slim and strong, but a poor natural insulator unless it includes a thermal break. Thermally broken aluminium performs well, but standard aluminium does not.
Timber: a naturally good insulator with a lower embodied carbon footprint than uPVC or aluminium, though it needs more upkeep to stay weathertight over time.
Composite: combines a timber core with an aluminium or uPVC outer skin, aiming to offer the insulation of timber with the low maintenance of uPVC or aluminium.
For most homeowners looking for the best windows for energy efficiency on a reasonable budget, a multi-chambered frame paired with low-E double or triple glazing offers the strongest balance of performance, cost and maintenance.
Best Windows For Heat Retention In UK Homes
Given the UK’s wet winters and long heating season, heat retention matters more here than in many other climates. To get the best windows for heat retention in UK homes, look for:
A Window Energy Rating of A or A+ (or a U-value of 1.2 W/m²K or lower)
Low-E glass with an argon- or krypton-filled cavity
Warm edge spacer bars rather than older aluminium spacers
A multi-chambered, thermally broken frame
Installation by a FENSA or CERTASS-registered installer
Which Window Styles Offer The Best Energy Efficiency?
Glazing and frame material matter most, but the style of window also affects how well it seals and performs.
Casement Windows
The most budget-friendly option, starting from around £150.
Casement windows use a simple hinge-and-seal design that closes tightly against the frame, making them one of the most consistently energy efficient window styles available. Flush casement versions sit level with the frame for an even tighter seal and less condensation.
Tilt And Turn Windows
Priced from around £450, tilt and turn windows open inward for easy cleaning or tilt from the top for secure ventilation.
Tilting inward simplifies cleaning while reducing maintenance time, and because each window can be adjusted independently, you can heat and ventilate room by room, which can help reduce overall energy use.
Sash Windows
Available as flush sash (mimicking traditional timber joinery) or vertical sliding sash. Historically less airtight than casement designs, but modern designs feature durable frames protecting the internal springs and hinges, ensuring seamless opening through thousands of uses while still suiting period properties.
Bi-Folding Windows
These fold back neatly into the frame, opening up a wide space when needed while sealing tightly when closed. Pricing is broadly in line with tilt and turn and sash windows, rising for larger installations.
Bow And Bay Windows
Bow windows curve gently outward with more glass and fewer frame joints; bay windows use an angled design with more frame sections for added security. Both typically start from around £1,000 and, because of their larger glazed area, benefit the most from high-performance glazing.
Energy Efficient Window Prices
Exact pricing depends on your chosen glazing, frame material, window style, and the size and number of windows you’re replacing. As a rough guide, casement and standard double glazed windows start from around £150, tilt and turn from around £450, and bow or bay windows from £1,000 upwards.
The best way to find the best energy efficient windows for your budget is to compare quotes from more than one installer. Homeowners who get multiple quotes typically save up to 40% compared with accepting the first price offered.
Compare Companies matches your design with your budget and refers you to trusted local installers holding approvals from FENSA, Checkatrade and Which?, so you can compare like-for-like quotes with no obligation to buy.
Frequently Asked Questions
What type of window is the most energy efficient?
There’s no single winner, it comes down to the combination of glazing and frame. Triple glazed windows with low-E glass, argon-filled cavities and a thermally broken or multi-chambered frame currently offer the best heat retention available, though double glazing with a high WER rating is a close, more affordable second.
How do window energy ratings from A++ to G work?
The Window Energy Rating (WER) scale, set by the British Fenestration Rating Council (BFRC), runs from A++ (most efficient) down to G (least efficient).
It’s based on three factors: the U-value (heat loss), the effective solar heat gain (free heat from sunlight), and air leakage. Building Regulations require replacement windows in England and Wales to meet at least Band C, though A or A+ rated windows perform noticeably better.
Do energy efficient windows really lower heating bills?
Yes. Upgrading from single glazing to A-rated double glazing can cut heat loss through your windows by roughly half, and moving to triple glazing can reduce it further still.
The exact saving depends on your home’s size, current windows, and how well the rest of the property is insulated, but the reduction in draughts and cold spots is usually noticeable straight away.
Which window frame material is best for energy efficiency?
uPVC and thermally broken aluminium both perform well and suit most homes. Timber is naturally insulating but needs more maintenance. Composite frames aim to combine timber’s insulation with lower upkeep.
The best choice depends on your budget, your home’s style, and how much maintenance you’re willing to take on, but the frame’s construction (multi-chambered or thermally broken) matters more than the material alone.
What is the best glass for energy efficient windows?
Low-emissivity (low-E) glass is the standard for energy efficient windows. It has a transparent coating that reflects heat back into the room while still letting daylight through. Paired with an argon- or krypton-filled cavity and a warm edge spacer bar, low-E glass gives the best balance of light, insulation and cost.
Are energy efficient windows worth the extra cost?
For most UK homes, yes. The upfront cost is higher than basic double glazing, but the ongoing savings on heating bills, the added comfort, and the boost to your home’s EPC rating and resale value tend to outweigh the difference over time.
