Measuring the thermal resistance of double and triple layer pneumatic cushions for textile architectures

Lightweight architecture, implementing coated fabric and membrane materials, increasingly adopts multiple layer pneumatic cushions in order to guarantee adequate thermal insulation and better indoor comfort, especially in permanent buildings. The cushion thermal transmittance decreases as the number of layers increases. However, the U-value is generally assessed by simple calculations assuming the cushion layers are parallel planes. This way, any effect of the curved geometry of the cushion on the heat transfer is disregarded. In this paper, an experimental approach to evaluate the overall U-value of multiple layer cushions is proposed. Two sample cushions of about 1 m2 made up of PVC-coated polyester are built, one double layer and one triple layer. They are tested in vertical position in a double thermal chamber laboratory apparatus, establishing a constant temperature difference equal to 25°C across them. The cushion surfaces are divided into thermally homogeneous portions where temperature and heat flow density probes are centrally placed. In parallel, the U-values are calculated using literature correlations for free convection in rectangular cavities and assuming radiative heat transfer between parallel grey surfaces. The experimental results show that, passing from two to three layers, the overall thermal resistance of the cushion almost doubles (+ 91%) and the thermal transmittance reduces by about 30%. At the same time, simple calculations of the U-value underestimate the insulation capacity of the cushions, especially for the double layer one. Further investigations are necessary to understand the impact of the cushion shape on free convection in the air cavity.