Understanding double-glazing’s window energy ratings

Did you know that over 90% of all windows installed at residences in the UK are double-glazed? Yet, not all double-glazed windows are the same when it comes to energy efficiency. Whether or not you can truly call your windows energy-efficient depends on the window energy rating assigned to them.

But what are the differences between window energy ratings, and what level of energy efficiency does each offer? In this guide, we will answer exactly this and more.

What are window energy ratings?

Window energy ratings are the grading system that denotes a window’s level of energy efficiency. Energy efficiency, of course, describes how much heat loss occurs through your windows, but also your doors, walls, ceiling and even floor. The rating system extends from G to A++, although the maximum grade is only available to the highest-quality triple-glazing windows.

The following are what the different levels of energy performance ratings give you:

As you can see, the energy savings can be substantial. The problem for many people is finding out what the energy rating is for your windows. You may be able to find this out from the window energy rating label attached – although this is not always a mandatory feature.

The following is a breakdown of all window energy ratings and the minimum of what is needed to reach each rank. Each rank has a different level of u-value, this being the official metric of how much heat is lost through a material, with lower values meaning higher insulation and placement on the window energy rating system.

G rated windows

• Glass – Single-pane windows.
• Spacer bars – Basic aluminium spacers.
• Gas fill – N/A
• Frame – Typically, if a window is single-glazed, it doesn’t matter the material of the frame.
• Seals – Poor quality.

These windows lose the maximum amount of heat possible from a complete window.

F rated windows

• Glass – Poor double glazing, likely outdated. Possibly single pane windows also, with coating.
• Spacer bars – Aluminium or older metal spacer bars.
• Gas fill – Typically air, if double-glazed.
• Frame – Possible uPVC frames.
• Seals – Basic weather seals.

Slightly better than G. But overall, this is considered quite far below modern standards.

E rated windows

• Glass – Double-glazing, possibly with a standard hard coat of low-E (low emissivity).
• Spacer bars – Aluminium.
• Gas fill – The cavity is likely filled with air, as gas of any other kind would probably push it up.
• Frame – Standard or older multi-chamber uPVC, but none with advanced thermal breaks.
• Seals – Middle-of-the-road quality, still not up to modern standards.

A baseline type of double-glazed windows that simply wouldn’t be worth the money.

D rated windows

• Glass – Double-glazing with a soft coat of low-E glass, perhaps with a slightly wider cavity.
• Spacer bars – Aluminium, most likely. But it’s also possible that basic warm-edge spacers do some heavy lifting for a D rating.
• Gas fill – Air or argon, although the latter is unlikely to have a higher quantity.
• Frame – Standard multi-chambered uPVC.
• Seals – Moderate-quality gaskets.

These are adequate for many homes, but the performance is below average.

C rated windows

• Glass – Modern double-glazed windows, with a soft coating and Low-E glass at least on one pane. The air gap is usually much larger, between 16mm to 20mm.
• Spacer bars – Warm-edge spacers, typically.
• Frame – multi-chamber uPVC frames, or moderately advanced thermally broken aluminium.
• Seals – The sash and frame have improved sealing, blocking more air leakage.

This is considered the “entry-level” of energy-efficient windows. They are modern, and meet many building regulations minimums, making them a feature of new builds and renovations.

B-rated windows

• Glass – Quality double-glazing windows with a well-optimised cavity thickness and soft-coat low-E glass.
• Spacer bars – Warm edge spacers are a standard B-rating, and they kill condensation around the edges.
• Gas fill – Argon gas filled at high percentages (80% to 95%)
• Frame – Can be advanced multi-chamber uPVC, better-insulated timber, or well-executed thermal breaks.
• Seals – Tight and well-fitted gaskets are excellent at weatherproofing.

Highly energy-efficient windows that block more than half of possible heat loss.

A-rated windows

• Glass – Same as B-rating, it could have Low-E soft coatings on both panes or just a single premium variant.
• Spacers bars – Warm-edge spacers of a premium kind, constructed with composite materials – meaning it’s the best of several materials bonded tightly together.
• Gas fill – Maximum argon gas fill, or krypton.
• Frame – Multi-chamber, thermally broken aluminium with advanced polyamide breaks. Sometimes well-insulated timber.
• Seals – High-quality dual or triple gaskets reducing next to all air leakage.

A through to A++ are the premium entries of window energy ratings, so expect a premium price point.

A+-rated windows

• Glass—For glass at an A+ rating, there are several variants, each just as good. You can choose the highest-quality double-glazing available with multiple advanced low-coatings, specialised glass, or standardised triple-glazing. 
• Spacer bars – Warm-edge spacers with high specifications and thermal efficiency.
• Gas fill – Typically krypton for double-glazing windows, and argon for triple.
• Frame—This uPVC has high-performance multi-chamber designs in uPVC or thermally broken aluminium. Timber frames, typically engineered, are rarer and are treated to provide the highest-quality insulation and weatherstripping. 
• Seals – Tight seals, multiple gasket layers and highly robust locking mechanisms.

This is the highest window energy rating you can get with double-glazing.

A++-rated windows

• Glass – Top-tier triple-glazed windows, multiple advanced Low-W coatings across the three panes.
• Spacer bars – Best warm-edge spacers, integrated with insulation.
• Gas fill – Krypton or xenon fill for maximum thermal performance.
• Frame – Frames that are highly insulated with multiple internal chambers, advanced thermal breaks and high-spec window sashes.
• Seals – Multi-layered seals with low heat transfer.

The pinnacle of the window energy ratings, blocking nearly all heat loss through the window.

Will higher window energy ratings increase property value?

Most likely. It is not a definite “yes” because the energy efficiency of an entire home is rated on a similar system to window energy ratings: energy performance certificates (EPCs), ranging from G – to A. This means that if your new windows are not a significant improvement, or they are seriously lacking in other areas, then your home’s energy performance isn’t high.

EPC is important because it shows that a home has good functionality. Its lowered running costs directly translate into lower energy bills, increasing its value on the market substantially.

The EPC rating system concerns all energy performance improvements in the home, including wall, floor, and loft insulation as well as heating appliances. This means that if your replacement windows are a small boost, they may not result in higher EPC.

Nonetheless, consider the following table, which shows how much you can increase EPC through upgrades:

For reference on how many points are needed for what EPC rating, check the table below:

EPC ratings can upgrade property value in the following ways:

• G to F – 1-2% increase in property value.​
• F to E – 2-3% increase in property value.​
• E to D – 3-4% increase in property value.​
• D to C – 4-5% increase in property value.​
• C to B – 5-6% increase in property value.​
• B to A – 6-8% increase in property value.

What do window energy ratings say about solar heat gain?

Solar gain is the metric of how much solar energy passes through the window from the outside. Whilst window energy ratings focus mainly on heat retention, solar heat gain is a trait that appears as a mainstay of ratings A+ and A++.

This means that in cold climates, high solar heat gain will pull in what warmth it can from the cold air outside. However, during hot climates, the window instead will be insulated against the heat for overheating sake.

Conclusion

The British Fenestration Rating Council is aligned with an overall goal to lower the carbon footprint of households in the country. To limit the amount of carbon emissions from your home, you need higher quality, new energy-efficient windows. By minimising heat loss, you minimise heating bills. If you are looking for new windows, check the window energy ratings assigned to each.