Martin Oxley charts the rise of reflective insulation. Originally featured in the April Issue of Insulate of Magazine.
After running 26.2 miles and receiving your well-earned finisher’s medal, you usually receive a goody bag after completing your marathon. Along with the T-shirt, snack bar and muscle-relieving pain gel, you will find a shiny, highly reflective space blanket. This helps to keep you warm after you have stopped running – so why not use the same technology to keep your house warm as well?
Testing the performance
When the first reflective insulation products hit the shelves of builder’s merchants many years ago, there was limited certified testing, so some of their original performance claims might have been a little optimistic. Testing houses in both the UK and Europe also achieved varying results after analysing the products in the same way as they normally would traditional insulants.
So while the test houses and manufacturers agreed on a standard testing methodology for thermal performance, a process which took a few years, some companies opted for in-situ evaluation.
It soon became apparent that it was extremely difficult to build ‘standard houses’ repeatedly and ensure that thermal performance and weathering conditions remained absolutely identical, so as to evaluate the true performance of these new reflective insulants with 100% accuracy. This caused market confusion and ultimately led to many architects, specifiers and contractors not selecting reflective insulation for their projects.
Thermal Performance Standard
After several rounds of evaluation with some of the best testing houses for insulation’s thermal performance, in 2012 Europe agreed on a standard thermal performance testing standard, BS EN 16012: 2012 +A1: 2015 Thermal insulation for buildings – Reflective insulation products – Determination of the declared thermal performance. (http://bit.ly/2bramb2).
There was now a defined methodology for direct thermal comparison of reflective insulants. This, together with the emergence of the first reflective insulation certificates from the British Board of Agrément (BBA) over 17 years ago, increased the credibility of the industry and provided consumers with the reassurance they required, and sales started to increase significantly during this period.
Radiation of Heat
All insulation products work by reducing heat flow, in one or more of three ways: conduction, convection or radiation. Heat always flows from the hot side to the cold side.
Reflective insulation’s key feature is, of course, to limit heat flow by radiation. The radiated energy is invisible and causes no rise in temperature until it hits a surface, where it is absorbed and causes the object to get warmer. For instance, radiative energy from the sun will pass through double-glazing and once it hits an object inside the conservatory it starts to heat it up.
When comparing the performance of reflective insulations, a key technical characteristic is the emittance of the product, more usually described as emissivity, specifically a surface measurement of the product’s ability to emit radiant energy.
Emissivity is a ratio and is given a value between zero and one. A value of zero indicates that all the energy is reflected and none is absorbed, as for example, with a highly polished silver surface. Conversely, a value of one denotes that none of the energy is reflected and all is absorbed, as by a totally black body such as a black hole in space.
For a typical reflective insulation product, the aluminium foil outer surface would have a declared aged emissivity of around 0.05, meaning that the significant majority of energy is not absorbed. Conversely, for a typical brown house brick, you would usually expect an emissivity value of around 0.90 – that is, most of the energy is absorbed.
As emissivity is a ratio of energy, the temperature and direction of measurement is an important function, so you need to measure the emissivity accurately. The texture of the sample can also affect the true result by scattering the light.
BS EN 16012: 2012 + A1: 2015 refers to BS EN 15976: 2011 Flexible sheets for waterproofing – Determination of emissivity, which details testing and reporting methodology (http://bit.ly/2bCJq5p). With most reflective insulations, you are determining very low emissivity values and are working at the very limits of the instrumentation, so using an experienced independent testing laboratory such as the BBA is vital to obtain accurate, precise and credible results. The results supplied should be ‘aged’ results to take into effect the material’s ability to resist oxidation / corrosion, i.e. how the product will perform after many years as opposed to just initially.
Europe is currently developing a harmonised European Product Standard for reflective insulation which is due to be published shortly. Once officially issued, manufacturers will then be able to CE-mark their product to encourage increased sales across Europe.