Reflective thermal insulation in non-ventilated air-gaps: experimental and theoretical evaluations on the global heat transfer coefficient

The refurbishment of existing buildings represents a priority target to reduce global energy consumption. An interesting solution is represented by multilayer systems made by insulating panels mounted on a suitable frame to form non-ventilated air-gaps with the existing envelopes. This solution is attractive because it allows for renewal and energy requalification of building façades by interventions addressed exclusively to the external surfaces, without interfering with occupants. However, it produces an increase of the building volume that could be in contrast with local construction regulations. In this context, the application of reflective (low-ε) thermal insulation panels inside the air–gap seems appropriate to reduce the system thickness without penalizing the thermal transmittance of the renovated envelope. However, addressed investigations conducted on the combined convective and radiative heat transfer coefficients inside enclosures equipped with low-ε materials, are lacking. In this paper, an experimental campaign conducted in a climatic chamber by the heat flux meter method on three different samples of commercial reflective panels inside a non-ventilated air–gap has shown, for the same sample thickness, an increase of the air–gap thermal resistance up to seven times. In order to evaluate the performances on a real scale, the experimental results were employed to tune a model developed in the COMSOL environment to determine the attainable global heat transfer coefficient in real scale air cavities. The results showed that thermo-reflective panels can produce the same effect of at least 6 cm of traditional insulating materials by avoiding additional space. Moreover, useful design information concerning the attainable thermal resistance growth related to the air–gap thickness and the thermo-reflective panel emissivity, were introduced. Results showed that by setting the vertical wall height, an optimal air–gap thickness that allows for minimizing the global heat transfer coefficient, can be identified, however, this is mainly when the emissivity coefficient of the panel surfaces is lower than 0.5. Furthermore, by setting the optimal thickness, the increase of the transferred thermal flux by quadruplicating the wall height is moderate being slightly greater than 16%. By monetizing the saved volume growth, the multilayer system equipped with thermo-reflective panels is economically profitable, especially with the insulating panel cost increase and in zones where the average building value is high.