 This paper explored the use of recycled polyethylene terephthalate, PET, to create eco-friendly engineering materials for construction projects. The recycled PET was obtained from waste bottles and used to replace sand in cement mortar mixtures and as fibers in premixed screed mixtures. The effects of PET treatment, chemical or mechanical, on the thermal performance of these materials were evaluated through non-destructive physical testing. The results showed that both types of PET treatments reduced the thermal conductivity of the cementitious materials without significantly affecting their compressive strength. This suggests that recycled PET could be used in non-structural applications such as flooring, wall cladding, and roofing. This article was authored by Mikayla Lerna, Dora Fodi, Andrea Petrella, and others.