What's the importance
of U-values?

Explaining variance in U-values

Due to the importance of U-values as an indication of energy efficiency, this topic has been explored in-depth and is well understood by industry specialists. Here at Sunsquare, for example, we have the facilities to officially calculate the U-values of any given product using advanced thermal imaging processes. This provides us with invaluable insights into what makes rooflights perform optimally.

The simplest explanation for the variance is that the U-values of comparable products are often calculated differently. For instance: the values provided for skylights on the market are frequently calculated using the U-value of the centre pane, i.e. the glass held in the frame.

In this scenario, an advertised U-value of 0.6W/m² might sound extremely attractive, but it’s missing some key information. That’s because, once installed, the whole skylight’s U-values should be determined using not just the glass, but the frame as well. Factoring in an aluminium frame in the final installation, for example, the whole product U-value of 0.6 is likely to be nearer to 1.5W/m². That’s a lot worse in terms of actual performance.

Aluminium and thermal performance

It may be surprising, but our thermal imaging technology confirms that most industry providers are essentially equally matched in glazing performance. By using the correct insulating gasses and glass specification and up-to-date methods, we would expect to see very little difference in results between competitors.

It follows, therefore, that it’s the type of frame that holds the glass which makes the difference – not just in U-values, but user experience as well.

This is because aluminium is an extremely effective conductor of both heat and cold. Consequently, an aluminium framework can cause unwanted heat loss during winter months, as warm air from inside is transferred directly outside through the metal surface. Additionally, the transfer of cold from outside into the frame can cause condensation within the unit, a loss of energy efficiency and unsightly damage in the long run. This area of heat transfer within the unit is called a cold bridge, and it is something to be avoided at all costs.

Thermal breaks and deceptive marketing practices

The issue can be solved with the inclusion of one or more thermal breaks in the unit. For example: our Skyview skylight illustration and Skyview skylight isometric illustration clearly demonstrates where its polyamide thermal breaks sit within the framework .

Put simply, a thermal break is an insulating section of polyamide fitted directly within the glazed unit. It is also the only proven way of preventing a cold bridge, achieved by keeping the interior and exterior sections of the aluminium framework separated.

However, the performance level of a thermally broken product isn’t easily accomplished. Polyamide is an expensive material, inserting it in the frame is costly, and so is the waste disposal process. Consequently, many businesses in the industry refrain from mentioning thermal breaks in their advertising, sometimes resorting to promoting products with false U-values.

It is fairly easy to estimate U-values by adding together the thermal resistances of each material making up the product in question. However, this calculation does not account for the effects of cold bridging, gaps in insulation or other thermal factors such as mortar joints and wall ties. Similarly, the numerous online U-value calculators which have appeared in recent times tend to offer estimates which are too simplistic to be accurate.

When falsified U-values aren’t on offer, many providers resort to creating an illusion of professionalism using tricky language. “Thermally managed” is a vague marketing term which has seen increasing use in the industry, for instance, even though it means effectively nothing. It certainly doesn’t indicate that a product is thermally broken. A “thermally managed” product with an aluminium frame will still suffer as a result of excessive heat and cold conduction.

As a potential buyer, you should be aware of deceptive marketing strategies such as these. Thermal breaks are the only way to ensure that properties meet standards of the Code for Sustainable Homes as well as Part L Building Regulations regarding the conservation of fuel and power.

During testing at the British Standards Institute (BSI), the highest accredited and most thermally efficient products on the market were proven to be those possessing thermal breaks. Proving our commitment to energy performance, Sunsquare provide the first and only BSI-Kitemarked skylight range manufactured and sold in the UK. At Sunsquare, we believe in better.

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