What are the insulating varnishes: a guide on the technical characteristics and operation
Many companies offer special insulating coatings or additives to be mixed with more conventional paints promising miraculous results in terms of energy savings. They argue that the application of these paints on the walls of the building to ensure a 25% reduction in heating costs and even 40% savings on summer cooling.
To make it more convincing often use words like secret ingredients, nanotechnology and ceramic powders. But is it true? It 'really possible with a simple coat of paint and what is special, earn one or two Class Energy Efficiency?
The idea, I must admit, it's pretty tempting. Upgrading the energy efficiency of a building with current techniques require rather complex work, such as the interposition of insulating materials for roofing and insulation. How easier it would solve everything with a painted!
Then we do the math to figure out how much you can expect from an insulating paint.
The flow of specific heat (per 1 square meter of surface and 1 degree kelvin thermal gradient) that crosses an opaque element (wall, etc) coincides with the value dellatrasmittanza (U) equal to the reciprocal of the sum of the resistances of the various layers of materials (Ri) and the coefficients liminal Interior (Rsi) and external (Rse).
U = 1 / (Rsi + R1 + R2 + R3 + Ri + .... + Rse)
For a wall made of plaster exterior (3 cm), bricks (30 cm) and plaster for interior (2 cm) supponiamoi ottenre for the transmittance of the following value:
U = 1 / (Rsi + R1 + R2 + R3 + Rse) = 1.16 m K / W
Now suppose we add an additional layer, that constituted by the magic paint. The problem is to determine its thermal resistance Rv also because this information is never available for this category of products. We have to make some assumptions, a value of the conductivity of the material, the other on the thickness of the layer of paint.
Indeed, we know that:
R = s / λ [W / m K]
The best values of insulating materials have very low thermal conductivity λ. Their effectiveness is related to the presence of trapped air in the form of micro-bubbles in the foam products and as air pockets in those fiber. Commercial products with the best insulating properties, and also the most expensive, are the materials in silica airgel with very low values of thermal conductivity of just 0.016 W / m K.
For the uninitiated, the Aerogel is a synthetic porous material derived from a gel in which the liquid component of the gel is replaced by a gas. The result is a solid light as air and with exceptional insulating properties.
Suppose, then, that even these paints have extraordinary values of conductivity, such as those based on Aerogel. So λv = 0.016 W / m K
Regarding the thickness, we could hypothesize that this is of some tenths of a millimeter (generous assumptions since it has a paint thicknesses well below!).
Suppose, however, that the thickness is sv = 0.0005 meters (0.5 mm). It thus appears that:
Rv = sv / λv = 0.03 W / m K
and consequently the transmittance painted wall:
U = 1 / (Rsi + R1 + R2 + R3 + Rv + Rse) = 1.12 m K / W
This equates to a performance improvement of about 3.5%, which is far from the values promised.
In fairness it should tender account of solar gains, which, however, does not change the absorption coefficient of the outer surface will be slightly lower. Solar gains specific (per unit area and irradiation), in fact, are proportional to:
Rse α U
where α is the absorption coefficient of own and other terms already know them.
The solar gains are then hampered further by the presence of the paint. This is bad during the heating season, but does not affect much because the radiation is very intense at that time and energy performance of the building are dominated by the transmission of heat. During the summer season, however, is in favor and also takes on a more important role because the intensity of the radiation is high, but the benefit is still very modest.
To ensure energy saving promised, insulating varnishes should have a thickness of about 4 millimeters (which is not possible), and thermal conductivity values extraordinarily low (which is unlikely). This suggests that, unfortunately, are not so miraculous.
It should be remembered that, always talking about Aerogel, you can reach conductivity values even lower, up to 0,004 W / m K. These values, at present, are difficult to obtain in commercial applications, but who knows if in the near future, further refinements of the technology can not make even the paint a good solution for the insulation of buildings.
For now the paints have proved effective to modify the absorption coefficient α of the external surfaces. Light colors, for example, are suitable to avoid the heat in the summer season (White roof). But for this (to change color), is certainly not enough to spend a fortune, do not you think?
28/02/2012
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Translated via software
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Source:
Italian version of CercaGeometra.it