Transparent thermal insulation (TWD) - Energy savings through innovative insulation

A transparent thermal insulation (TWD) fulfills two functions: On the one hand, it acts as an insulating layer, and it also uses the sun's energy to support the heating of the building.

Transparent thermal insulation consists of materials that combine good thermal insulation with high light transmission. These properties are caused by honeycomb, hollow chamber or capillary structures as well as translucent granules (airgel). TWD materials are able to scatter the incident light rays and convert the light energy into an underlying absorption layer into heat. Transparent thermal insulation is therefore a passive form of use for solar thermal energy. TWD is suitable for both new construction and energy renovation.

Table 1: An overview of the properties of transparent thermal insulation

thermal conductivity0.066 - 1.40 W / mK
Building material classAlt: A1, New: A1 (non-combustible)
Price per m2200 - 700 EUR

The specifications for the minimum insulation thicknesses of the Energy Saving Ordinance (EnEV) 2014 do not take into account transparent thermal insulation - the thickness of the insulation layer depends on the respective material structure. The same applies to the weight or bulk density of a TWD.

How does a transparent thermal insulation work?

A TWD usually consists of very thin (capillary) tubes made of plastic or glass, which are arranged ("stacked") between two glass panes. Such materials are not completely transparent, but cause scattered light, which is evenly distributed. In aerogels, the granule beads have the same mode of action. How much light is absorbed by the insulation depends on the angle of incidence of the solar radiation. Behind the transparent insulation layer is a solid wall construction material with good thermal conductivity, which absorbs solar energy and transmits it to the interior of the building. At the same time, the solid walls are able to store part of the heat energy and release it into the interior with a time delay.

Price and manufacturer of TWDs

Transparent thermal insulation is used in the context of thermal insulation composite systems (ETICS) and an energetically defined overall concept. Their m2 price ranges between 200 and 700 euros. With the costs for conventional thermal insulation - for example mineral wool, polystyrene (EPS, XPS) or natural insulating materials - the prices of the systems are not comparable. TWDs are produced by manufacturers of construction glass and innovative building materials, and in part the development of the systems is accompanied by technology research projects. Well-known names in German-speaking countries include Wacotech, Hedde and isobouw.

What are the physical properties of a transparent thermal insulation?

The thermal conductivity (? - lambda) of transparent thermal insulation is between 0.066 - 1.40 W / mK (watts per meter x Kelvin). As a result, their thermal insulation performance is very moderate compared to conventional insulation materials, which is more than compensated for by the energy savings that a TWD brings about. In addition, this insulation form provides excellent sound insulation.

Open to diffusion and capillary activity

TWD devices need to be diffusion-open: the air in a TWD heats up during the day, expands and expels out of the element-at night, it cools, shrinks volume, and draws outside air into the TWD. Depending on the material and construction, however, the diffusion openness of the material may be very different.

Table 2: Comparison of TWD and conventional thermal insulation materials

insulation materialsThermal conductivity (W / mK)Minimum insulation thickness according to EnEV (cm)Cost per m2 (Euro)
TWD0,066 – 1,40n / A.200 - 700 EUR
rock wool0,035 – 0,0451410 - 20 EUR
EPS / Styrofoam0,035 – 0,045145 - 20 EUR
hemp0,04 – 0,0451610 - 27 EUR

Applications of transparent thermal insulation

The basic application of TWDs is facade insulation - transparent thermal insulation is exterior insulation systems for walls and facades. As a rule, they are combined with a conventional ETICS. The Transparent Thermal Insulation Association states that the systems are particularly suitable for use on solid walls with high bulk density.

How much energy can be saved through transparent thermal insulation?

There is no overall value for energy savings by a TWD. It depends on the materials used, weather conditions and hours of sunshine. On southern facades the annual energy gain can amount to about 120 kWh / m2 per year, on the other facades the energy gains range between 30 and 80 kWh / m2.

TWDs in the warm season

In order to minimize too much sunshine and corresponding heat effects in the warm season, the installation of active shading systems is recommended.

Advantages and disadvantages of transparent thermal insulation

  • Advantages: The biggest advantage of TWDs is the coupling of solar thermal energy - ie air conditioning effects from solar energy - and thermal insulation.
  • Disadvantage: TWDs are expensive compared to conventional thermal insulation. Before deciding on such an insulation system, a cost-benefit calculation should therefore be carried out by an energy expert or a specialist company.

Tips & Tricks

Transparent thermal insulation couples thermal insulation and solar thermal energy. They pass on the heat energy of solar radiation to the interior of the building and thus contribute to noticeable energy savings. TWDs are exterior insulation systems for facade insulation. Usually they are used in combination with a conventional ETICS. In addition to their insulation effect and their conditioning effect TWSs cause a very efficient sound insulation.

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