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Understanding Energy-Efficient Windows

Since 2002, Building Regulations state that all buildings must have energy-efficient windows in order to pass inspection. If you’ve bought windows then you can assume your windows follow the necessary requirements. If your windows are older you may want to consider an upgrade.

Energy efficient windows have many benefits but to fully understand how they work and why should you invest, we’ve put together this helpful guide.

How Do Energy-Efficient Windows Work?

Energy-efficient windows work by insulating your home. They protect your home from heat loss, reduce noise pollution and decrease condensation. They may be more costly at first but the money they can save you over a period of time makes them worth the initial investment.

Although energy-efficient windows are mandatory, as of 2002, some homes still don’t have glazing on their windows. While this wouldn’t be an issue unless you were to sell your home, you’ll be saving yourself a great deal of money by upgrading your windows sooner rather than later.

With added glazing, the heat that would usually be lost through the glass, is retained. This creates a much warmer home. The insulation means your home warms up faster and the heat is retained for longer, meaning you won’t have to have your heating on for as long. The result? A much lower energy bill.

Understanding Window Energy Ratings

When purchasing energy-efficient windows, the energy ratings can be a minefield. What are U-values? What do the letters mean on the energy label? We’re here to answer those questions for you.

A U-value, or thermal transmittance, is a measurement of heat loss. Heat lost through a structure is calculated using a device. The amount of heat loss recorded is then divided by the difference in temperature across that structure.

U-values are measured in watts per square metre per Kelvin. This is then converted to an easy-to-read scale using letters of the alphabet.

This is what gives you the overall U-value rating that you see on windows and doors.

The U-value rating scale is calculated in letters. The scale starts from A++ and goes up to G. Don’t be fooled though, the higher the letter, the worse the insulation. Think of it like grades in school; A++ is extremely energy-efficient.

An A++ window, although will have a higher upfront cost, will mean less energy is required to provide warmth inside the building. This results in much cheaper energy bills; saving money for you in the long-run. To comply with Building Regulations, all external windows and doors must have a U-value of at least C.

The U-value ratings are displayed on windows and doors; usually in the form of a sticker. On this sticker, you’ll have the letters listed down the left-hand side. This is usually from A – G, although some only go to E. On the right-hand side, you’ll have a little arrow with a letter in it. This letter is telling you what the U-value rating of that external structure is. Underneath, you’ll see some text with the watts per square metre per Kelvin figures. This tells you what thermal transmittance and air leakage is lost through that structure.

What Are the Best Energy-efficient Windows?

In terms of the glass, there are 3 types of energy-efficient windows; double-glazed, triple-glazed, and Low-E.

Double-glazed windows are the most common of the 3. There are 2 sheets of glass in one frame. The glass sheets are separated by a gap which is sometimes filled with gas. Whether the gap is filled with gas or not, this gap creates insulation for your window.

Triple-glazed windows are built the same, apart from the extra sheet of glass. There are 3 sheets of glass and 2 gaps for insulation. Triple-glazed windows don’t always provide more insulation than double-glazed windows. Make sure you check the U-value ratings before jumping into purchasing triple-glazed windows.

Low-E, or low emissivity, glass is the most energy-efficient glass to use in your external windows. Low-E glass can be double or triple-paned. One of the panes will have a metal oxide coating which is usually undetectable. This coating lets in light and heat, and prevents any heat from escaping from the inside.

Which Window Frames are Most Energy Efficient?

uPVC window frames are the most energy-efficient, if we simply look at the properties of the material. Timber would be next in line, and aluminium last. However, all modern windows are built to be as energy efficient as possible, which might mean the frames have been manufactured in such a way that the material performs much better than it usually would. For example, aluminium frames are ‘thermally broken’ to increase their thermal efficiency. The most important thing is to look at the U-value for the whole window and use this as a guide.

How Much Money Could Energy-efficient Windows Save You?

Energy-efficient windows are incredibly cost-effective. Although the upfront cost of your new windows may be pretty pricey, the benefits of lower monthly energy-bills should be enough for you to take the plunge.


For an A++-rated window, you’ll be looking to save somewhere between £115 – £120 per year. This is based on a detached house.

For a mid-floor flat, you’d save somewhere around £35-£40 per year.


For an A+-rated window, your yearly savings will be around £110 for a detached property. At the other end of the scale, a mid-floor flat with an A+ window would save you £35 per year.


A-rated windows, in a detached property, will save you £105 – £110 per year. A mid-floor flat with the same A-rated windows, would save £35 per year.


A B-rated window would save you around £100 – £105 per year in a detached house. For a mid-floor flat, you’d be looking at saving somewhere between £30 – £35 per year.


For C-rated windows in a detached property, your savings for the year would sit around £100. C-rated windows in a mid-floor flat would save you around £30 per year.

The above figures are based on monetary savings by installing double-glazed windows into a single-glazed property. The gas-usage is based on average gas consumption.

In Conclusion

U-values can be pretty difficult to understand. Especially when we aren’t sure what the benefits and disadvantages are in the first place. We hope our article provides you with enough information for you to make an informative decision when the time comes.

If you’re looking into purchasing some new windows for your home, always consider the long-term benefits over short-term costs.

Energy-efficient windows are not only better for the environment, but they also help you out too. A reduction in heat loss, draughts and cold spots, improvements to energy bills, noise pollution reduced and condensation kept to a minimum. Need we say more?

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