Our previous research showed that increasing the thickness of a polyolefin (PO) film increases the longwave radiation absorptance (a), which in turn reduces the overall heat transfer coefficient (k). Moreover, the effect of the covering thickness on longwave radiation absorptance varies depending on the material type. Thus, the relationship between covering thickness (other than that of PO films) and heat insulation properties must be clarified to develop coverings with better heat insulation performance. This study aimed to clarify the relationship between covering thickness and k for several fluororesin films and polyethylene (PE) films and identify any differences between the materials. Additionally, comparisons were performed with PO data obtained in previous studies. Thickness was measured using a micrometer, a was measured using an emissivity meter, and k was calculated using an approximate formula based on our previous research. Thickness and k can be approximated by a linear function for all the test materials. The absolute value of the slope of the approximation equation for PE was smaller than the values for PO and fluororesin films. This implies that an increase in PE thickness is less likely to improve the heat insulation performance compared with increases in PO and fluororesin films.
Ohashi et al. (Thu,) studied this question.