5 Things You Need to Know About U-Values

Published on Friday, 4th March 2016

A helpful blog about U-values and what they are

All roof windows come with a U-value rating. It's important to know what this value means, and how choosing the right roof window can benefit your home during its lifespan.

In an effort to help our customers understand the meaning of U-values fully, we've made the following “things to know” guide, with the aim to explain everything about U-values.

1. What is a U-value & why is it important?

U-values measure how well a window stops heat from passing through it. This will be what dictates how much heat is let into your home during the summer and how much heat is kept in during the winter.

U-value ratings are worked out using the equation W/m²K. Wikipedia best describes this as,

"the transfer of heat (in watts) through 1 square metre of glass divided by the difference in temperature across the structure."

Sometimes you'll see Ug- and Uw-values, which represent the pane's energy efficiency and the window's energy efficiency respectively.

The reason this is important is the lower the U-value, the less heat loss the window is going to allow. Having a low U-value on your windows will make your home much more thermally efficient. As well as being more energy efficient, your home will be more comfortable, with fewer hot and cold spots in your rooms.

All of our windows have their U-value stated online, which you can find in the description section on the products page. Remember, the best heat insulators have lower U-values.

2. Over time you'll save money by being more energy efficient

Low U-value windows reduce the amount of heat loss but a low U-value will also save you money. Of course, as with all energy-saving solutions, they cost money to buy and install but you'll see the benefits once you start making savings in fuel costs year after year.

Everyone likes to save money where they can, and with our huge range of low U-value windows from different manufacturers, it’s not too hard. Particularly with our best price guaranteed promise and free delivery option.

A flat roof window installed in snowy conditions. The better the U-value, the more energy you save

3. What Building Regulations say about U-values

Part L of the UK's Building Regs covers Conservation of Fuel and Power and sets a maximum U-value for building work, i.e. the least energy efficient windows that can be installed in a home.

This applies to home extensions and other renovation projects. New builds must prove to be "reasonably energy efficient", which does include windows' U-values but isn't based solely on this figure.

All roof windows and rooflights sold on Sterlingbuild* have good to excellent U-values. There is no need to worry about your replacement window's U-value being higher than the old one, or being an issue when it comes to getting the OK to install. However, it's not only the windows in your home that affect thermal efficiency - your wall and roof insulation also play a part.

When in doubt, we always recommend you talk with your local planning authority or an experienced local roofer, as they should be able to advise you on current regulations that apply to your project.

* Please note, the skylights sold on Sterlingbuild aren't designed to be installed in inhabited spaces, such as loft conversions. They're suitable for adding daylight and fresh air to attics, sheds and similar places.

The lower U-value, the more energy you save

4. What's the best glazing for an energy efficient window?

For the most energy efficient glazing, choose from our selection of triple glazedquadruple-glazed or quintuple-glazed windows. 

These glazing options will provide you with the very best insulation, however, they are generally the most expensive option so be sure the choice you make offers the perfect balance between budget and comfort.

You shouldn't rule out double glazing. Higher spec, double glazed windows also have excellent energy performance that will help keep your home warmer. For comparison, your typical double glazed uPVC casement window has a U-value of 2.8 W/m2K, whilst a triple glazed uPVC window scores 1.0.

Window Double glazed U-value (W/m2K) Triple glazed U-value (W/m2K)
FAKRO white PU centre pivot roof windows with laminated glazing 1.1 0.97
VELUX white PU top-hung roof windows with laminated glazing 1.3 1.0
Sterlingbuild white PVC top hung windows 1.2 0.78
Sterlingbuild fixed flat glass rooflights 1.7 0.6
Sterlingbuild roof lanterns 1.1 0.6

5. Knowing what else to look for

Now you know what a U-value means and the effect it has on your home and energy bills, it's time to look at what makes a window as energy efficient as possible.

What can you look for to ensure your window's glazing is as energy efficient as your budget allows?

  • Glazing treatments - A great alternative to triple glazing is double glazing with a low emissivity coating. Low-E coatings cut the amount of heat that can escape your room.
  • Gas - The space(s) between panes is filled with an inert gas. Each gas has different properties, letting heat transfer across the gap at different rates. An energy efficient window will be filled with argon, whilst a superior window will use xenon.
  • Pane spacers - Used internally between panes, energy efficient spacers contain little or no metal and are called "warm edge spacers".

When it comes to regulations, it can be daunting. If you have a question or any doubts, please don't hesitate to contact our customer services team on our local rate number 01304 219 922 Monday to Friday between 08:30 and 17:30.