In short, myths about thermal insulation are based on an insufficient understanding of the respective physics, which has been very well-understood for centuries. Building physics has brought clarity to this, and actually everything is also quite easy to understand once fundamentally wrong ideas have been rectified. Incidentally, everything that we have described here corresponds to the latest developments in science that are generally accepted among experts; in this regard, the science has not changed fundamentally even in the past 50 years ¹⁾. Most of this is recorded in the relevant international norms just as we have described here; none of this is “disputed” from the scientific point of view ²⁾.

This is based on the (incorrect) assumption that heat is somehow transferred towards the outside, carried by the warm air through the exterior building assemblies, and that in contrast “thermal insulation” must make the building assemblies “completely airtight”.

Correction: a major part of the heat is not carried towards the outside by air currents, instead this happens due to thermal conduction. In thermal conduction, the movement of heat is passed on from one molecule to the next (by it “giving a push” so to speak); no material flows at all in the process, it is only the violent state of motion of the microscopic heat vibration which is passed on and then given off into the surrounding air on the outside in the end. It is very important to understand this correctly because it will help us avoid various other mistakes:

• Potential error 1: “sealing everything”, e.g. by using joint sealant etc. – generally doesn't achieve much at all in the case of conventional exterior building components in existing buildings (except that air exchange ³⁾ is reduced). A major part of the heat continues to be conducted towards the outside due to e.g. walls with a very high thermal conductivity.
• Potential error 2: If this kind of sealing is also carried out at the wrong place without any expert knowledge, then damage to the building component may even occur. One example of this is “sealing paint” which is often applied on exterior plaster with good intent, and then frequently leads to moisture saturation and peels off. Note: vapour-impermeable layers must NEVER be applied on the cold side (external side).

What is true:

• Thermally insulating materials are those materials that pass on heat movement only to a greatly reduced extent. In general, they do this because they consist mostly of air, “packed air” in other words. They put up a resistance against the heat, and they do not even have to be perfectly airtight for this. Most insulation materials are even less airtight ⁴⁾ than conventional building materials like brick, concrete or wood.
• In the case of materials such as cotton, wool, straw, hemp fibre matting, mineral wool etc. this is immediately apparent. However, these materials are generally actually “too non-airtight” to have a full insulating effect on their own – if the wind blows through it, heat will actually be transported by air currents in this case. These materials therefore work only in combination with the other components of an exterior building component: for example the interior plaster of a plastered wall is airtight enough, so that an overall functional effect results in the interaction of the existing plaster and an insulation layer. The insulation does not make the building assembly thicker at all in the process. With insulation on the inside, these points must especially be taken into account, see here: Interior insulation must actually be airtight on the room side!
• In buildings there are components which are almost perfectly airtight e.g. window panes or metal sheets. That they really are “airtight” ⁵⁾ is easy to see due to the fact that these materials are also suitable for the exterior envelope surfaces of submarines or spaceships. Such components can certainly also be used in constructions without causing problems (as shown by the example of glazing; this can be done in a “good” way or a “bad” way, see Glazing). The materials mentioned here even exhibit high thermal conductivities – so they are anything but thermally insulating materials. Exterior components which contain such layers must therefore ensure thermal insulation in some other way: in the case of glazing this is done by means of the spaces between the panes which are filled with gases that are less heat-conducting.

Of course, this is related to the incorrect idea in Myth 1. The idea that walls are able, or must be able, to breathe is misleading. Generally, the amount of air that passes through exterior components that are commonly used today is extremely small – unless cracks and open joints are present. Often there is a certain amount of material exchange/mass transfer that is very slow and entirely insignificant for the energy balance. We do not have to change this at all for the thermal protection measure, unless there are other reasons for this.

What is true: …e.g. the moisture generated by the occupants inside the house must be removed. However, walls and other building assemblies contribute almost nothing to this and the thermal insulation changes nothing in this respect. Through window ventilation on a regular basis, fresh air can enter the house in winter. A ventilation system is even better. On the topic of humidity we have provided several information sheets on Passipedia: Humid air and Avoiding problems with moisture also in case of interior insulation.(German only)

What is correct: with proper execution, exactly the opposite is true: with thermal insulation, the interior surfaces become warmer. Warm surfaces are automatically also drier. Mould cannot grow on dry surfaces. This is true for both thermal insulation on the outside and for correctly executed insulation on the inside (but there are some points to be kept in mind for this: Correctly executed interior insulation).
Let us see which places are actually affected by mould and increased humidity:

• Most conspicuous: old window panes, or in more extreme cases, old unseparated aluminium profiles as window frames. The condensation water often runs down the interior surfaces here; if this cannot be drained towards the outside (some old windows actually have drainage channels built into them) then this water must regularly be wiped away by the occupants. Mould growth is virtually guaranteed here if the window is not cleaned thoroughly.
• Also visible: cold interior surfaces e.g. underside of projecting balcony slabs. The thermal bridge effect is often visible as a 'biological thermographic image' here: the black mould grows in the places where the building assembly is the coldest and therefore the most damp. The situation is similar near roller shutters, window reveals (where the effective exterior wall is very thin) and at the inner edges/corners of uninsulated walls, especially if there is a piece of furniture placed here.
  

Uninsulated and poorly insulated building assemblies are at risk of mould growth. Ultimately it is improved thermal insulation which solves this problem; in an existing building which has undergone an overall EnerPHit retrofit/insulation measure, there are no longer any damp interior surfaces of building components ⁶⁾.

What is true: there are a number of different insulation materials which differ with regard to the energy expenditure for manufacture (embodied energy), among other things ⁷⁾. But even with synthetic insulation materials such as widely available EPS ⁸⁾ the energy required for manufacturing these is usually already compensated again in the first winter season ⁹⁾. Further information on the topic of embodied energy can be found here (German only). Embodied energy particularly for subsequent insulation of building assemblies in usually very poorly insulated existing buildings is negligible compared to the savings achieved with it; the savings are achieved year after year and with proper execution, the thermal insulation will last for decades. This also applies for the carbon footprint¹⁰⁾.

Most thermal insulation materials are durable: 50 years and more of service life are regularly achievable (see e.g. [Feist 2020]). For this reason, presently there are only small amounts of old insulation materials. Practically demonstrated recycling processes exist for many insulation materials; however, one thing is often hardly considered: 95 to 99% of insulation materials consist of air, so they can be compressed to a small fraction of their volume (usually by a factor of 20). Loose-fill insulation materials, such as cellulose flakes or installed panels can also be re-used straightaway. All this does not relieve us of the responsibility to produce the materials used here in a non-toxic and also in a non-ecotoxic way; this is quite easily possible in the case of insulation materials – and is obviously the case for materials like straw, hemp, cellulose, foam glass and mineral wool. Other insulation materials are more difficult, but even so, today all insulation materials that have been approved for the European market are safe for humans and the environment.

What is true: A 'climate like that in a shack' may result if a building can only store very little heat and also has inadequate thermal protection in addition. In sunlight such a building can heat up quickly and also cools down quickly later on. The indoor climate is often improved with properly planned thermal protection. This is easy to understand because the thermal storage capacity of the interior building assemblies can be utilised in a much better way thus. However, this has also been proved through practical experience with built Passive House buildings and EnerPHit retrofits.

What is correct: both form part of the description of heat transport processes in physics, not only in buildings but also in all fields relating to nature and technology. The associated processes are very well-understood today, we have provided an easy-to-understand description here: "Insulation versus thermal mass" (German only). The short description: in buildings in Europe, the most important influence on the energy consumption by far comes from thermal insulation. Heat storage in a building usually isn't a bad thing – its positive effect is often intensified by improving the thermal protection.

What is correct: Improved thermal protection of a single building component always reduces the heat losses through this building component by the same absolute amount in a good approximation, whether this is a new building or an existing building; the saving hardly depends on the condition of the other building components ¹¹⁾. It is also correct that the total losses of an existing building might be so high that for example, an absolute saving of 20 kWh/(m²a) after a measure is undertaken (e.g. a good standard of roof insulation) is only 10% relative to the total consumption. But because we are also improving the other building components in stages whenever the opportunity arises, the individual contributions will add up (e.g. roof 20 + exterior walls 65 + windows 25 + basement ceiling 10 kWh/(m²a), that already brings us to 60 % below the starting value). Building owners are often annoyed that they did not execute the initial measures to a good enough extent and they are then left with the avoidably high heat losses. “If you're doing it, do it properly'” is therefore always the right approach. And the quality of the work should be based on the target values for the house, not on the “still bad” existing state.

One can find a lot of more or less correct statements about all kinds of questions, especially on the Internet. Most people today have already realised this and exercise the necessary caution – not all sources are reputable and misconceptions are common, such as those which we have refuted above.

Many people therefore rightly ask themselves: “What can I actually believe?” or “Who can I trust?” Even these questions don't really lead anywhere because they are asking about “believing” and “blindly trusting”. In the case of issues such as the relationships in physics being discussed here, neither is expedient; this namely concerns facts and issues which exist in nature (and therefore also in technology, because this also obeys the laws of nature of course), no more and no less. It does not matter by who, when and where these facts of nature are “inquired about”, the answer is always the same each time. Incidentally, this can even be verified very easily by anyone especially in the case of the questions being dealt with here relating to heat conduction, air flow, and radiation exchange. “Blind trust” is not at all necessary here, see e.g. "Examples of processes relating to humid air".

The laws of physics which apply in the area of the heat balance of buildings have been known and proven for more than 150 years in respect of their essential features. Even if a well-presented “report of success” about entirely new insights in this area is made year after year, in the end all of these always proves to be in full agreement with the fundamental laws of heat and mass/material transport, often in a particularly “smart” application of precisely these laws. Before modern computer technology became available, it was often only possible to evaluate these laws in simple approximations, because real buildings are assembled in a complex manner in terms of geometry and details, but now the most complex of processes can be broken down into elementary cells, calculated here according to the applicable laws and then reassembled numerically. Since then, these procedures have also been repeatedly compared to measurements (in the laboratory and in actual buildings), and these comparisons have been confirmed time and again, both with regard to the numerical procedure and the applied laws of physics.

We attach great importance to following the generally accepted findings of building physics and the associated disciplines in all our work. The statements you see here might therefore be perceived by some as being 'boring' or 'nothing new'. We are following the established findings - and we also will not hesitate to adopt new findings if these are substantiated. Unfortunately, today some 'spectacular innovations' even in actually substantiated technical fields also follow the pursuit of something new at any cost ¹²⁾ or even worse, the interests of individuals or groups commissioning research work. These things are especially likely to undermine trust in scientific research. That is why we place great emphasis on the independence of our research.

Interestingly, the field of energy efficient construction is one in which there has been huge progress and innovation in practice in the last 60 years, simply due to the consistent application of known physical principles. However, even today all these innovations can be understood by all those who wish to do so, from their own experience on the basis of familiar general rules. One example is the (enormous!) improvement in the window panes in general use today, which now have heat losses that are lower by a factor of 8 (eight!) compared to old single glazing which often is still found today in some places¹³⁾. It is these innovations which have resulted in our buildings becoming much more comfortable than they used to be 60 years ago, and in their consuming (significantly!) less energy based on the living area in spite of this. 10,000 litre oil tanks were installed in the single-family houses that were built in the 1960s, and a consumption of more than 5,000 litres per year was no exception in those days (that's more than 300 kWh/(m²a)). That such a gigantic oil consumption can be drastically reduced if just the top floor ceiling is insulated and the windows replaced, has saved many homeowners from extremely high heating costs with constantly increasing energy prices. Based on extensive experiences, applying the established principles of building physics to construction practice has obviously been successful. The average heating energy demand of our residential buildings still lies around 128 kWh/(m²a), because we thought that by using seemingly cheap natural gas, we would be able to interrupt the recently initiated modernisation of buildings that were built in the days of “cheap oil”. But: the innovation of applying those very same laws still continued, e.g. through large-scale testing of the Passive House standard in practice in what are now millions of new builds worldwide. Quite a few of these entire housing developments were metrologically monitored, which enabled further empirical results to be obtained regarding the validity of building physics in this sector as well; [Johnston 2020] documents one such analysis. According to this, the measured average heat consumption in these buildings is less than 15 kWh/(m²a) and is thus less than a tenth of the average value today in existing buildings in Germany. These results are all based on measures implemented in line with generally accepted building physics findings.