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The importance of rooflight U-values

16 May 2017
So, what are U-values? Put simply, U-values measure how effectively a window stops heat from passing through it. As such, it will tell you how much heat will be let into the building during the different seasons.

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The measurement for U-values is defined as “the transfer of heat (in watts) through 1 square metre of glass divided by the difference in temperature across the structure”. So, when checking out different specifications you’re looking for the figure followed by: W/m2K.

For example: A wall with the U-Value of 0.5 W/m2 K will lose heat at half the rate of a wall with a U-value of 0.6 W/m2 K.

(You may also see Ug- and Uw- values in some situations: these represent the energy efficiency of the glass pane itself and the window as a whole respectively).

The lower the U-value of the window, the less heat it will allow through.

What are the benefits of low U-value?

A low U-value will mean that the property is more thermally efficient, which is ideal in any build. The building will be more comfortable and level in terms of temperature, remaining stable throughout the year: less chance of freezing winters and boiling summers.

This is particularly ideal for larger buildings such as offices: a pleasant, level working environment will benefit employees and nearly always lead to increased productivity.

The other major benefit is, of course, that a lower U-value will nearly always save the building occupants money. The more stable a property is in terms of temperature, the less necessary it’ll be to constantly have the central heating working in the winter, or the air-con on in the summer

In larger properties, the saving on bills can be substantial.

Building Regulations and U-values

U-values are covered by Part L of the U.K.’s building regulations, which sets out a maximum U-value for modern building work. (In other words, the lowest energy-efficient ratings windows are allowed to have)

The laws apply to home extensions and other renovation projects – upgrades to older properties – and also to new builds. However, in new builds U values are part of the overall goal for energy efficiency: all new projects must prove themselves to be “reasonably energy-efficient”.

The current legal limits under Building Regulations Part L are as follows:

  • Roofs – 0.20 W/m2 K
  • Walls – 0.30 W/m2 K
  • Floor – 0.25 W/m2 K
  • Party Wall: 0.20 W/m2 K

For roof windows, curtain walls and pedestrian doors, the limit is: 2.00 W/m2K.

The BREEAM ‘Excellent Rating’ is considered the water mark for buildings wanting to achieve better ratings, and the minimum standards for this are as follows:

  • Roofs – 0.16 W/m2 K
  • Walls – 0.2 W/m2 K
  • Floors – W/m2 K

The aim, of course, should be to over-achieve and fall well within the limits.

Heat loss through Sunsquare skylights and roof windows is not even an issue as every rooflight that leaves our manufacturing facility is thermally broken, ensuring homes stay warmer whilst benefitting hugely from the vast amounts of natural light flooding in to living areas below.

For a rooflight to be thermally broken to today’s standards you should expect to see polyamide sections dividing all materials that sit across the internal to external parts of a building. This is the only effective way to maintain insulation and the climatic control of the property.

Also bear in mind

For architects especially, there are a number of other things that need to be considered when managing the energy efficiency of the overall project. Roof and wall installations can both impact the overall performance of a building.

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