Reflective insulation materials
Beautiful they look so, this mirror-like, film-coated, thin mats and bubble wrap, making lately from time to time as a cheap super-insulation of itself speeches. Most recently in specialist journals such as building physics. After all advertised on the part of various manufacturers so that this reflective thermal insulation materials building protect just as well against energy losses as conventional 200 mm mineral wool. Independent laboratory tests, however, contradict these statements. So what’s really up to the products?
But how do reflective insulation actually work? Here, too, the opinions of manufacturers and physicists diverge. Both groups agree that heat is principally transmitted through conduction, convection and radiation . Since convection is by and large independent of the material and insulating materials with reflective surfaces heat conduction can hardly prevent due to their small thickness and the materials used, so advertised effect must be justified with the heat radiation. There are, however, three basic prerequisites for making this effect possible:
- the temperature difference between air and surface must be (very) large
- the surface must adjoin an air space, so that heat can be radiated and
- The surface must have a high reflectance , ie be spotless
In order to meet the first requirements, the use of insulating materials with reflective surfaces is conceivable, in particular in roof areas of pitched roofs, whose roof tiles are frequently heated by the sun. If you use conventional roof structures, can be met by the normal roof battens and the second condition. However, it is more difficult with the last condition, since the ventilated cavity under roofing not only of air but also of dust and dirt flows through, which settles on the reflective surfaces and thus their efficiency reduced. However, it is precisely this aging behavior of reflective insulating materials that has hitherto been largely ignored and, in particular, hardly considered by manufacturers of the products. Finally, manufacturers of insulating materials with reflective surfaces usually testify their statements about the energy efficiency of their products with a handful of theoretical calculations or self-made experiments, the meaningfulness of which is regularly denied by experts in the field of construction.
In order to check the advertising claims of the manufacturers of reflective insulation material once and for all, the IBP scientists therefore carried out both on-site practical tests and theoretical simulation calculations. For the practical investigations, the roof surfaces of two identical buildings were insulated in one case only with said 200 mm mineral wool and in the other case only with reflective insulation materials. The result of this unusual because very realistic investigations planners and builders to be given security in terms of the applicability of the latter products.
And the results of all investigations of the IBP were clear. So it says there that the roof construction with the reflective insulation shows typical features of a structure with a lower insulation effect. In all measurement periods, the energy losses over the roof areas with the reflective insulation materials were about two and a half times as high as with the mineral wool-insulated roof space. Even bigger was the difference in leaky attics. In other words, the reflective insulation only replaces about 80 mm of conventional insulation. The theoretical calculations and laboratory measurements confirm these results. When determining the refrigeration requirement In the summer, energy was detected for the reflective insulated roof, which was about 10% higher than that of the mineral wool-insulated roof.
So it’s one thing that manufacturers promise on product packaging and in advertising brochures. It can be something completely different, which stands at the end on the heating billing. Therefore my recommendation at this point:
Required thermal insulation standards must be adhered to by traditional thermal insulation materials alone. If this is guaranteed, you can certainly save a few kilowatt hours with reflective insulation materials. However, to rely solely on the effect of the radiation reflection, is useful neither for the purse nor for the building fabric.