Thermal paints: just a joke?

With a short investigation, we add our contribution to what has been recently published by some Italian consumers and professionals associations.

Although the example described in this article is purely Italian (and deeply so), we believe that the same situation with thermal paints is happening in other countries as well. Here we try and evaluate how solid these products are, and whether or not they can be helpful, with an example of a thermal bridge.

We started to collect data on these thermal paints at the KlimaHouse expo in Bolzano, at the end of January. The event is a yearly milestone for the Italian landscape of energy efficiency. At the expo, one of these companies was advertising their thermal paint, promising great results in energy savings and mold mitigation.

The KlimaHouse expo in Bolzano (from the internet)

ANIT, the Italian Association for Thermal and Acoustic Insulation, recently issued a report on thermal paints, explaining the physics of the product, and how ineffective it is compared to traditional insulation materials.

AltroConsumo, an Italian consumers association, tested four different thermal paints, none of which delivered the claimed performance.

Nonetheless, other media have been writing positively about these thermal paints, asking no question whether or not the data given by the producers is true. It is the case of La Stampa (a national newspaper), and the website of the Association of Italian Building Designers (Geometri).

We asked ourselves some questions and contacted two producers. We’re going to name them “Franco” and “Alberto.”

Please note: while writing this article, we’ve been trying to keep a joking attitude. The data here shown comes directly from the two companies: we’re not mentioning their actual names (the names “Franco” and “Alberto” have nothing to do with them); calculations have been done according to the norms mentioned. Our spirit remains free.


Franco knows that optimism makes the sunshine.

Franco is one of those companies that took part in the 2016 edition of the KlimaHouse expo in Bolzano. His brochure impresses us, so we contact him and asked him for the thermal “certification” of their product.

Franco sends us their certificate.

The document we received is not a certificate. Instead, it is a calculation of the thermal transmittance of a standard wall (brick masonry with plaster), in the hypothesis of using a thermal paint.

00 calcolo
A passage in the document, describing the purely theoretical calculation of the improvement of the thermal resistance of the assembly thanks to the thermal paint.

The document quotes scientific “literature” as the source for the conductivity value of the thermal paint, without providing any source for the data. So we decide to call the Certifier that issued the document.

The Certifier turns out to be very well prepared and helpful:

The calculation in the document is purely theoretical: what would be the thermal performance in the hypothesis that the conductivity of the paint was…

Unfortunately, this company (and many other ones) took this theoretical calculation, and now is running around telling everyone how great their paint is.

The conductivity data for the paint – purely theoretical – has been provided by Franco, the producer of the paint.

The Certifier gave us also a broader look into the world of thermal paints:

In our lab, we test many of the so called thermal paints. The conductivity value is hardly ever below 0,10 W/mK (0.058 BTU/h*ft*F). We only had one case, where the value was 0,08 W/mK (0.046 BTU/h*ft*F). As far as emissivity, the value for a traditional paint is about 0,9; these thermal paints float somewhere between 0,5 and 0,3.

Now we ask Franco about the source of the “literature”, to see where the data came from.

Possibly by mistake, Franco sends us the result of lab testing on their paint.

Lab testing results of Franco's thermal paint
The results of lab testing on Franco’s thermal paint: the real values are about ten times worse than the ones used in the theoretical calculation.

The results shown in the report are about ten times worse than the ones used in the theoretical calculation.

Emu: Why did you not use these results in the calculation?

Franco: – no answer –

spider pork
Mid-article break. A scene from The Simpsons Movie: Homer Simpson and Spider Pork – no correlation to the content of the article whatsoever.


The second producer, Alberto, loves the days of yore.

In the data sheet, Alberto declares a reduction of the thermal conductance above 60%, thanks to his product. Furthermore, the temperature on the internal surface would rise by 4 °C (7,2°F). All of this has supposedly been certified by the University of Bologna.

Emu: Can you please send us a copy of the certificate issued by the University of Bologna?

Alberto: The numbers on the data sheet are the same as in the certificate.

Emu: We are not questioning that, but can you please send us copy of the certificate?

After nagging a little bit, we finally receive a copy of the certificate.

Intestazione certificato
The header of the certificate, issued by the University of Bologna in 1985. The tests were done on a board of asbestos.

It is all true.

The University of Bologna ran tests on a board of asbestos, with and without a coating of Alberto’s thermal paint. The data in the technical sheet is exactly the same as in the certificate:

  • Conductance without thermal paint: 26,8 W/m2K (4,720 BTU*h*ft2*F); with paint: 9,8 W/m2K (1,726 BTU/h*ft2*F);
  • The temperature of the surface without thermal paint: 4,8°C (41,6°F); with paint: 8,5°C (47,3°F).
In 1986, I used to collect stickers of the soccer players of the World Cup. The certificate of Alberto was already one year old.

After reading the one-page certificate dated 1985, we have the following doubts:

  1. after more than thirty years, isn’t it time to revise your documentation (*)?
  2. the test was done on an asbestos board coated with 6 mm (0,24 in) of paint, while the technical sheet lets you understand a standard thickness of 2 mm (0,08 in): why?
  3. glorifying the conductance value is highly misleading. Although this value is highly reduced (-60%) by the thermal paint, the actual heat flow  – expressed as thermal transmittance (U value) – is reduced by less than 14% (see later);
  4. more than two-thirds of the thermal resistance provided by the paint is due to its low emissivity: how long is this going to last? How long until a thin layer of dust is going to deposit on the surface of the wall, inhibiting this effect?

(*) In Italy, we are all nostalgic at heart. When I was preparing my state exam to become a registered architect, ten years ago, I had to study the Royal decrees that would allow me to be an architect “in the Kingdom and in the Colonies” although, at that time, Italy had been a republic for about sixty years.


Let’s assume Alberto’s data is correct.

From the conductance value certified by the University of Bologna, you can calculate the conductivity value of the paint: 0,10 W/mK (0.058 BTU/h*ft*F). This is in line with what was discussed with the Certifier. It is still more than two times worse than an average insulation material (about 0,04 W/mK, 0.023 BTU/h*ft*F)).

On top of this, we still need the value of emissivity of the paint. We asked Alberto, but we never received an answer. Let’s take the most optimistic value: 0,3.

With an analytic calculation according to ISO 6946, an internal surface at 20°C (68°F), with an emissivity of 0,3, has an internal resistance of 0,237 m2K/W (instead of the usual 0,13 m2K/W).

For the ISO 13788 calculation of interior temperature and the risk of mold, we kept the following boundary conditions:

  • inside: 20°C (68°F); humidity 54,3% for C2, and 61,2% for C3 (according to the National appendix to the norm), internal resistance 0,25 m2K/W (from the norm);
  • outside: 1,0 °C (33,8°F), humidity according to climate zone (Reggio Emilia), external resistance 0,04 m2K/W (from the norm).

As far as the thickness of the paint, we considered the following values:

  • 0 mm – existing wall, without thermal paint;
  • 2 mm (0,08 in)- wall with two coats of thermal paint, as described in the data sheet;
  • 6 mm (0,24 in)- wall with the same thickness of thermal paint as in the certificate of the University of Bologna.

The node we studied is a typical corner column in a masonry construction, with hollow bricks of baked clay, 25 cm thick (0,82 ft), and lime plaster on both sides, with no thermal insulation.

01 nodo
The geometry of the corner column of a masonry structure, without any thermal insulation.
02 iso
Isotherms in the structure.

The isotherms show how the corner is a weak point of the thermal envelope, both from a heat flow point of view and for the internal surface temperatures.

03 muffa
Mold forming on the interior surface of the corner, as simulated according to ISO13788

Even with a relatively low internal humidity level, the corner allows for mold to form because of the very low surface temperatures.

11 dettaglio
The same corner, enlarged, in the hypothesis of using thermal paint on the internal surface (marked in green).

An enlargement of the corner, with the 6 mm (0,24 in) thermal paint highlighted in green.

ISO6946 risultati
ISO 6946 calculation of the thermal transmittance of the wall, and the equivalent thickness of standard insulation to achieve the same goal.

A simple ISO 6946 calculation shows how the combined effect of the thermal paint (conductive+radiative insulation) is anyway quite scarce. To achieve the same result, you’d need less than one centimeter of standard insulation.

12 iso
Isotherms in the corner, with a 6 mm coating of thermal paint.

From the finite element analysis of the node, you can see that the thermal paint allows for an improvement in terms of heat flow. The PSI value of the thermal bridge of the corner is substantially reduced (as it would also happen, with 1 cm of standard insulation).

ISO10211 psi
Analysis of the heat flow according to ISO 6946 (thermal transmittance) and ISO 10211 (thermal bridge).

This effect is nothing special: the very first layer of insulation is always the most effective one. In this, thermal paint is no exception.

U - R graph
The decrease of the thermal transmittance (U-value), with the increase of the thickness of insulation, according to ISO 6946.

From the point of view of internal surface temperatures, the result is not as positive. This is shown in the ISO 13788 analysis for the risk of mold.

13 muffa
The finite elements analysis shows that even in the case of a 6 mm coating with thermal paint, the corner will be covered in mold.

The analysis clearly shows that 6 mm of thermal paint are not going to make a difference for mold. In this case, the internal temperature is going to be only 0,8 °C (1,4°F) higher than in the case of the the untreated wall.

ISO13788 temperature
fRsi and temperature calculation according to ISO 13788.

The improvement of the fRsi temperature factor of the corner, even with 6 mm of thermal paint, remains not good enough.

ISO13788 muffa
Even if using thermal paint, with a standard internal humidity level, the temperature in the corner would remain too low, allowing for mold to form.

From the point of view of internal temperatures, mold would form even in the case of a 6 mm coating with thermal paint. Even if we accept the data provided by the producer, the thermal paint does not perform well enough to raise the temperature above the threshold of mold.

This leads us to a double conclusion:

  • if a thermal paint allows the prevention of mold to grow, this is not thanks to its thermal performance, but due to its anti-mold additives instead;
  • thermal paint seems to be a very smart way to sell anti-mold paint at a much higher price.
Krusty from The Simpsons
Mondays are hard for everyone. (Krusty the Clown, from the Simpson. Vector Junky).


This article is a summary of personal research in the field of thermal paints. The data declared by the producers seemed to be far too optimistic to be true: we had multiple confirmations of the data being unrealistic, by asking the producers to show us their data.

Even if you don’t question the data itself, the example described above shows that thermal paint is not capable of raising the internal temperature enough to be above the threshold for mold. Once the walls are treated with this paint, if mold does not for is because of the anti-mold additives included in the paint.

That being said, we believe that thermal paints are nothing but another case of barbaric marketing.


The finite element calculations have been carried out with Dartwin software.


  1. Interesting take down of a product category and a warning for all product manufacturers to get their technical documentation right (and indeed for specifiers to up their scientific literacy).

    One question: if the critical surface temperature in a house was 15degC and thermal modelling determined the surface temperature in a given corner to be 14.3degC resulting in an fRsi failure, would remediation with 6mm of “thermal” paint suffice to achieve the critical temperature? (Given that the test is the production of a theoretical model in accordance with ISO 13788). In that case, might it be the difference between compliance and non-compliance?

    I know there are materials scientists working to produce paint-on nano coatings with very advanced properties which will have combinations of insulative, radiative, hydrphobic, anti-static, anti-bacterial and anti-fungal properties at the nano / molecular level. The ability to bond gas molecules (from fresh air) at the surface is critical to achieving some of these properties. Such bonding is measurable, permanent and represented in either an increased surface resistance factor or a reduced thermal conductivity (deciding which can be difficult given the exponentially greater surface area of nano materials over naturally occurring materials). Already, these materials are being used in hospitals with good results. They are even available on standard components like window blinds. They can be seen in the strong (and transferable) hydrophobic effect in aerogel insulation and contribute greatly to its thermal resistance in some of its quilt forms.

    It would therefore be wrong to suggest that paint-on coatings as a whole are irrelevant, thus depriving such coatings of a future market. In fact, I anticipate great advances in this field in the coming decade with particular application to building retrofits.

    Your findings really only apply to “Alberto” and “Franco” and their snake-oil counter-parts around the world. The sooner they are put out of business, the better. But lets not prejudice the use of thermally, hygrothermally or environmentally beneficial coatings which happen to be best applied using a paint brush.

    1. Our finite elements calculations showed that even in the 6 mm coating case, the surface temperature was not high enough to avoid mold at the simulated boundary conditions.
      Had the 6 mm paint been enough to pass ISO13788 mold analysis (but it wasn’t), then yes: it would have been the difference between compliance and non compliance.

      I understand your point in defending product research, however low emissivity/high reflectance is an overrated field (in my opinion). In the article, we used 0,3 as surface emissivity, which is already optimistic. I also calculated the same corner with emissivity 0,1 (not realistic), but the result was the same: mold.

      We now have received documents on a “thermally reflective plaster”, that claims an equivalent lambda value of 0,00019 W/mK. I am not kidding. 0,00019 W/mK, supposedly thanks to its high reflectivity (and low emissivity). This is bullshit – I find no other words to describe that, sorry. They also provide you with the standard lambda and emissivity values: 0,06 W/mK and 0,72 respectively, which allow anyone to calculate the equivalent lambda to 0,05 W/mK, 54 times worse (fifty-four times) than their claimed value.

      Low emissivity / high reflectance is an overrated field for opaque components, in my opinion.

      Try and ask yourself: your surface has extremely low emissivity today. How long is that going to last? One month? One year? If dust deposits on it, that is gone. If you put a picture up on the wall, that is gone. If you paint over it, that is gone. What are we talking about?

      My intention is not to deprive products of a market: I am just poking my nose into “revolutionary” new products, and this is what I have found in the field of thermal paint. The certifier I spoke to confirmed that NO paint they ever tested, performed better than Alberto and Franco.

      I repeat: even in the theoretical case of an emissivity of 0,1, the result does not make a difference in the basic case I tested.

      The future is open for endless new good products and heaps of new bullshit.

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