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U-What?!

Updated: Oct 5

Paul Trace from Stella Rooflight offers some advice on rooflights and thermal performance


Thermal performance is measured in terms of heat loss, and is commonly expressed in the construction industry as a U-value or even sometimes an R-value. U-value calculations will invariably be required when establishing construction strategies as it provides an indication as to how much heat loss a building is likely to suffer upon completion. The products used in the build are normally required to be tested and a figure for each component given. One example of where this is an essential requirement is rooflights.

Thermal transmittance (U-value) is measured in units of W/m²K which stands for Watts/meter square Kelvin. The lower the u-value the more efficient the construction is at keeping heat flow through the structure to a minimum. It is not just the building materials and products that have an impact on the thermal performance of a building as both workmanship and installation standards can strongly affect the thermal transmittance. If insulation is fitted poorly, with gaps and cold bridges, then the thermal transmittance can be considerably higher than desired, no matter how good the individual products are.

Thermal transmittance takes heat loss due to conduction, convection and radiation into account. The amount of heat conducted through a material of a given volume, in a unit of time i.e. the rate of conduction is why the units are measured as W/K.

There are guidelines in the UK, set out in Building Regulations Approved Document Part L (Conservation of Fuel and Power), that give the maximum U-value that materials and structures are allowed to have in a range of buildings, including domestic properties. It sets a national standard to ensure that homes must be built to a certain performance level of energy efficiency for both the reduction of carbon emissions and the reduction of residents heating bills.

U-Value is one of the most difficult thermal measurements to calculate and so it is important that any figures are produced using reliable software from a bona fide source. When it comes to roof light suppliers providing U-value figures for their products, we all want the lowest possible number to prove that our rooflites give the best thermal performance which ultimately reduces heat loss for our customers. In the rush to be the best, it is not inconceivable that figures get a little massaged so it is always best to ask for a copy of the test performance report to ensure that a) the figures are genuine and b) that the figures were produced in the correct way.

In the case of rooflights you might be surprised to find out that despite them being used in their millions across the country, there is no specific test for a rooflight. Instead rooflights/skylights/roof windows are tested to BS EN ISO 10077-1:2017 which is a thermal performance test for windows, doors and shutters.

So does this actually matter? Well, actually yes it does because most roof skylights cannot be used in the same way as windows and this will result in a change to the U-Value.

The pitch of the roof will change the thermal performance of your rooflight. However, the testing of rooflite performance is based on either a vertical (above 60 degrees) or horizontal (0 degree) pitch. Both of these positions are usually outside the maximum and minimum pitch that roof light manufacturers recommend for their rooflight products. There is quite a bit of difference in the U-values given to skylights at both ends of the scale. For example, our own Stella rooflight can achieve a U-value of 1.5 W/m²K in the horizontal position but this improves to 1.1 W/m²K when used above 60 degrees. Now when you ask most companies what the U-value is for their rooflight which figure do you think you will be given? This is why it is important to either ask for both figures or to request data to support the figures quoted.

The design of a roof window is also critical to the thermal performance. Most modern roof skylights such as Velux, Keylite and Fakro are produced from thermally broken aluminium which is then clad internally with wood to provide very good U-values. Whilst making the frames more thermally efficient, this approach also makes the roof lites chunky which often results in them sticking several inches above the roofline and providing poor frame to glass ratios.

One of the easiest ways to lower a u-value on a window or rooflight is to increase the thickness of the glazing. A decent double glazed unit will give a centre pane value of 1.0W/m²K and a triple around 0.5W/m²K. Then of course there are some that offer quad glazing which reduces this figure further. However, it should also be noted that by reducing the U-value in this way, you will be significantly increasing the weight of the product, the cost and also the depth of the roof light profile.

If you are working on a Listed Building or in a Conservation area then you will require a conservation rooflight design, which should sit completely flush with your roofline. If you are looking to use quad glazing or modern bulky rooflights to improve thermal performance then this will most certainly be to the detriment of the buildings aesthetics.

There are a few exceptions relating to the thermal requirements in certain buildings and it is always a good idea to take professional advice should your project fall into one of the following categories:

i) Where the replacement roof windows are unable to meet the requirements because of the need to maintain the external appearance of the façade or the character of the building, replacement windows should meet a centre pane U-value of 1.2 W/m2K.

ii) Buildings which are; Listed in accordance with section 1 of the Planning (Listed Buildings and Conservation Areas) Act 1990; In a conservation area designated in accordance with section 69 of that Act; or Included in the schedule of monuments maintained under section 1 of the Ancient Monuments and Archaeological Areas Act 1979; Carports.

iii) Buildings which are; Listed in accordance with section 1 of the Planning (Listed Buildings and Conservation Areas) Act 1990; In a conservation area designated in accordance with section 69 of that Act; or Included in the schedule of monuments maintained under section 1 of the Ancient Monuments and Archaeological Areas Act 1979; Used primarily or solely used as places of worship; Stand-alone buildings with floor area less than 50m2; Carports; Temporary building with a planned time of use less than 2 years.

In conclusion, a U-value figure is very important when both choosing products and in the overall construction of your property. The lower the figure the more thermally efficient a product or building is and this can contribute to reducing heating costs and ultimately helps in the fight against global warming.

It is also important that the figures provided are genuine and it is advisable to request proof that U-values being quoted are not only correct but are applicable for the application that you are using that product. Nowhere is this more important than rooflights because of the way in which these products are tested.

If you have any questions or require further technical information concerning the use of rooflights in your own project, please contact one of our team on 01794 745445.

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