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A U value tells us how much heat is lost from a building through different elements, such as windows, walls and floors.

It is a measurement of heat transference, also known as the “overall heat transfer co-efficient”, meaning the higher the number value, the more heat is being lost, and the less environmentally friendly the building is. A lower value therefore indicates greater insulating properties and is more energy efficient.

Calculating the U Value

The U value of a building element is calculated by working out heat loss for each component of the building. This is determined by the material used, its thickness, and its positioning.

Rooflights are made up of a number of different component materials that each have different properties when it comes to heat transference. Therefore, to find out the U value of this type of element, a number of factors need to be determined.

Firstly, the ability of each component material to conduct heat must be identified. This is shown as a ‘K value’. This value is used along with the thickness of the given material (d) in the following equation to give the resistance of the component material:

R = (1/k) x d

A resistance value (R) must then be found for each component material.

Next, the internal and external faces of the building element must be taken into account. These are known as external and internal resistances and are fixed values. This information is then combined in the following equation to discover the U value (U):

U = 1 / (Rso + Rsi + R1 + R2 …)

Rso is the fixed external resistance and Rsi is the fixed internal resistance, whilst R1, R2… give the sum of the resistances previously worked out for the different building materials in the building element.

Current building regulations in England and Wales allow a maximum U value of 2.2 W/m2K for skylights, covering 20% of the roof surface.

Green Energy Rooflights


All rooflights from EOS are energy efficient and feature U values as low as 0.97 W/m2K.

The EOS Eco roof lights, which are triple-glazed and injected with Krypton Gas, have an even lower U value of 0.65 W/m2K and include a centre pane U value of 0.6 W/m2K.

The U values of these skylights are so low that official building standards permit them to make up an additional 10% of the roof area than other, less energy efficient designs – taking total potential coverage to 30%.

So, as well as flooding a building with daylight to naturally enhance the indoor environment, EOS rooflights ensure green energy efficiency by reducing the need for CO2-emitting artificial lighting, whilst guaranteeing that heat isn’t lost unnecessarily.

For green efficient skylights with low U Values, speak to the EOS Rooflights team today.


Image Sources

Green house courtesy of Pixabay via CreativeCommons

4.83 Average

162 Reviews

Kevin Dean

Very good product and service from the team. Would recommend

Posted 1 week ago

Sam Phillips

We require 3 windows for 2 projects and they where very well priced and also had them in stock. will be using again.

Posted 1 week ago

James Payne Cook

For all the reasons I have rated them highly

Posted 1 week ago

Sunny Tiwana

Awsome company to deal with, item was out of stock online and thier guys found me what i needed and sorted delivery quickly and efficiently. Will defiantly recommend to others.

Posted 1 week ago

Stuart Herring

Good service and good communications on queries and time scales

Posted 2 weeks ago

Fiona Whyte

Their customer service was good. I was able to speak with someone quickly and easily by telephone or email to clarify points before I ordered my items. The ordering process was very easy and the items were delivered on time. Upon delivery there was a scratch on one of my rooflights. I contacted EOS immediately and they were happy to send a repair kit to me, free of charge, which arrived in the post within a few days of me speaking with them. I'm very pleased with the service I've received and the product looks great in my new extension!

Posted 2 weeks ago

Dawn Thomas

Beautiful roof lights - exceeded our expectations

Posted 2 weeks ago

Piero Realmuto

Good product and service. Have never let me down

Posted 2 weeks ago

Steve Orr

The roof lights arrived on time, were well protected in transit and all queries were answered to our satisfaction.

Posted 2 weeks ago

Leanne Wallser

Fantastic service, super helpful getting it all to Jersey and product is perfect for us. Thanks

Posted 1 month ago

Dr J Trinder

Swift. Delivery. Arrived as described. Straight forward payment arrangement. Great advice and support when planning project. Motorised rooflight works remarkably quietly. A little costly however compared to some competition.

Posted 1 month ago

Richard Simmons

They have provided us with a quality product

Posted 1 month ago


Product and delivery as expected.

Posted 1 month ago

Alan Oliver

Good product, delivered on time, and looks great in our new flat roof extension.

Posted 2 months ago

Mike Guirguis

I actually bought my roof lights on the back of EOS’s reviews section, and now my extension is nearing completion, (finally) I wanted to pass on my experience. I couldn’t be happier with my decision, its clearly a very high quality product and they look amazing in our new kitchen. They are heavy so make sure you have lots of strong guys on site. The staff are all very helpful and seem very knowledgable. Highly recommended them.

Posted 2 months ago

Nicole Gray

Excellent customer service. Really took the time to listen to our needs and help us find exactly what we wanted, within the time schedule from our builders. They were also flexible as the schedule shifted and always available for any questions. And beyond that, our roof lights look so amazing! So, I'm a very happy customer.

Posted 2 months ago

Ben Jennings

EOS were very helpful during the sales process, liaising directly with my builder to confirm requirements (when I was way out of my depth). My builder’s feedback post installation is that the windows are heavy duty and top quality, even when compared with more expensive options he’s fitted in the past. He’ll be recommending EOS for future builds.

Posted 2 months ago

Bill Greenaway

Product was just as promised and delivery and information excellent. We are very pleased with it.

Posted 2 months ago

Jason Berry

Really helpful when I called to order my roof lights explaining delivery info, weights, fitting instructions etc.., Really accommodating in facilitating a specific delivery time and date (at relatively short notice); finally the finished product, robust, elegant and fantastic. First class.

Posted 2 months ago


Very competitively priced!! Went the extra mile to help me out and gave all the support, advice and technical details that I needed for our project (Triple glazed - PassivHaus project) Will be sure to use EOS on any further Rooflight orders!!

Posted 2 months ago


Lovely skylight... I bought the fixed 1500*1000 and opening 1500*1000, both with upstands. Thanks to Chris for all his help. Tex - DA8

Posted 2 months ago