Steildak: “Insulation based on reflection”

steildak- Insulation based on reflection

In December 2018, steilDAK published an article about the insulation foil Isobooster HDS (Sloping Roof System). This article explains the way this material works in more detail.


Insulation foils have been under discussion for some time now: what is the insulating value of the materials and what is the best method to test it? Traditionally, insulation materials are tested using the so-called hot box method, but because of the specific way insulation foils work, these materials require a different test method. According to the ‘Editor’s Note’ included with the article in the December edition, the market considers this alternative method to be ‘unreliable’. That editor’s note is actually correct, because it reflects the general attitude of the market to this type of insulation. The question is whether that is justified: after all, the test method for insulation foils is standardized as well (ISO 9869:2014). The insulating effect of these materials is based on several well-known principles of physics. High time to take a closer look at the way insulation foil works and the methods used to test these materials.


Isobooster is made up of multiple layers of aluminium foil, alternated with air cushions. Its insulating properties are mainly the result of thermal reflection by the aluminium layer. The principle is that the material reflects heat and cold. It is an established fact that aluminium has these reflective properties, but that does not explain the extremely high insulating value it can achieve according to the manufacturer. That explanation lies in a few simple principles of physics and the way the material takes advantage of these.


This invention roughly uses the following principles of physics:


  • Aluminium reflects heat (and infrared radiation);
  • Heat reflection causes water molecules to vibrate;
  • Hot air rises, whereas heavier dry air goes down and stays there;
  • Hot, humid air flows upwards along surfaces (the outer wall) (the Coanda effect);
  • Moisture conducts heat: dry air is a good insulator, conducting up to two times less heat.


Thermal Infrared Radiation

The insulation foil owes its insulating properties to the reflection of thermal infrared radiation. As you may know, thermal infrared radiation is not absorbed by air, but transmitted. The radiation stays intact until it is absorbed by an object or surface. No energy is lost along the way (the radiation is not weakened). A lot of infrared radiation is the result of sunlight heating up surfaces and objects, which changes the frequency of that radiation and creates infrared. Wherever sunlight is absorbed, infrared continues to radiate.


Unlike traditional insulation materials, which absorb heat, this material uses thermal reflection to reflect heat: it sends the heat back into the interior space. This way, the structure actively heats the interior space.

The fact that aluminium reflects heat and therefore acts as an insulator, is widely recognized: virtually every household uses aluminium foil to store food. The same principle is applied on a bigger scale in buildings, with reflective insulation foil. In the construction industry, many producers of traditional insulation materials also use this effect to enhance their insulating values, while keeping their product as thin as possible.




The reflection of heat has two other important effects. Thermal infrared causes water to evaporate. The so-called Coanda effect ensures that this is a continuous process. It creates a layer of dry air along the surface on which the reflective foil has been applied. This enhances the insulating effects of the material even more. Moisture conducts heat; dry air does not, making it a good insulator.


the coanda effect


In a building, the reflective effect is enhanced by the so-called Coanda effect. In the early 20th century, physicist Henri Coanda discovered that hot, rising air always moves upwards along surfaces (e.g. cold outer walls): the Coanda effect. Air flowing along the wall creates low pressure, drawing the rising air towards the surface of the wall. This causes air to flow continuously along the surface. The heat is also continuously reflected, creating a constant layer of dry air along the wall. The repeated reflection of thermal infrared causes a lot more moisture to evaporate than would otherwise have been the case. As long as the water vapour is allowed to escape, the air that remains is dry. The vapour is carried away through the cavity wall: the relative humidity decreases, resulting in a better interior climate. The material itself (stone, wood or plaster) also becomes drier, and dry materials absorb heat more effectively and hold on to that heat for longer (like a soapstone heater).


Dry air expands as it heats up, and contracts when it cools down. For example: 1 kg of dry air takes up 0.85 m³ of space at 27°C, and 0.8 m³ at 9°C. But 1 kg of humid air also takes up more volume than the same weight in dry air. In other words: dry air is heavier than humid air. This is somewhat counter-intuitive, but bear in mind that the molecular mass of water vapour is lower than that of dry air. Air consists of approximately 20% oxygen and 80% nitrogen. The average molecular mass of dry air is 28.8 g/mol. If we add water vapour, the water replaces the heavier dry air and reduces the density of the mixture. That explains why water vapour rises (clouds do not drop) and why dry air does not move, but stays in the same place.


Reflective foil takes advantage of this. When applying the material, however, it is important to keep a cavity wall of at least 8 mm and no more than 30 mm, to ensure its effectiveness.


How can infrared be a source of heat?

Physicists Huib Bakker and Han-Kwang Nienhuys from the Institute for Atomic and Molecular Physics (AMOLF) researched how it is possible that infrared radiation causes water (the water molecule) to fall apart (evaporate). Until recently, physicists thought that water falls apart because the oxygen atoms of two water molecules get so close to one another that one hydrogen proton (H+) ‘jumps’ from one molecule to the other. The tests proved otherwise.


Bakker and Nienhuys ’fired’ infrared light pulses with a wavelength of around 3000 nanometres at liquid water. This wavelength corresponds to a frequency of 1014 Hertz, equal to the vibrational frequency of the hydrogen atoms in the water molecule. They saw that the additional vibration energy of the vibrating atoms weakened and eventually broke the bond between the hydrogen and oxygen atoms, thus making the water molecule ‘fall apart’. This effect occurs with so-called ‘far infrared radiation’, so it does not require any extreme heat or special equipment. Humans also radiate ‘far infrared radiation’.


Test Method

So what is the effect of applying this type of insulation? According to Isobooster, the manufacturer, the traditional measuring methods do not provide any useable figures about this. Like other reflective material manufacturers, they measure the effects in practical settings. In the Netherlands, for example, the manufacturer has insulated various homes in a holiday park, some with traditional materials, others with reflective insulation. The difference in energy consumption gives a clear indication of the effects. However, the fact that no standard can be derived from this, an Rc value which is applicable in any situation, makes it hard to communicate these values.


Within Europe, there is a recognized measuring method for this type of insulation, ISO 9869. This is available in the Netherlands through NEN: NEN-ISO 9869-1:2014 and: Thermal insulation – Building elements – In-situ measurement of thermal resistance and thermal transmittance. This test method uses a heat flow meter. The properties which can be measured are: a) the thermal resistance/ the thermal conductance from surface to surface (the R value), and b) the total thermal resistance and transmittance from environment to environment (the U value, if the environmental temperatures of both environments are well defined). The values communicated by Isobooster have been established by Swiss testing institute greenTEG.

Download orginal article (Dutch, pdf)



  • Sophie De Jonge: Vochttransporteigenschappen van capillaire onderdakmaterialen. (Moisture-transporting properties of capillary building materials) Ghent University, 2006.
  • Frédérique Melma Water trilt zichzelf uit elkaar (Water vibrates itself apart) On:
  • P.A. van Weel, H.F. de Zwart en J.O. Voogt: Vochtbeheersing in kassen en terugwinning van latente energie (Moisture control in greenhouses and recovery of latent energy). Wageningen UR, 2016.
  • P.A.M. Geelen, J.O. Voogt, P.A. van Weel: De basisprincipes van het nieuwe telen. (The basic principles of modern cultivation) LTO Glaskracht Nederland, 2016
  • Various articles on

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