The effect of chemical modification of oil palm mesocarp fiber is considered.• The effect of the sample's thickness on heat flow time is investigated. • The properties of the POP composites depend on the proportion of the fiber.This study was designed to investigate how some important physical properties of Plaster of Paris (POP) ceilings are affected by the utilization of untreated oil palm mesocarp fiber (UOPMF) and treated oil palm mesocarp fiber (TOPMF) as a modifier. Both fibers were utilized separately at various weight proportions (0, 10, 20, 30, and 40%) to fabricate the ceiling samples. The samples were dried completely and then tested for water absorption, porosity, bulk density, thermal conductivity, specific heat capacity, thermal diffusivity, heat flow time, and flakiness. All the samples could improve thermal insulation performance better than conventional ceilings like asbestos and polyvinyl chloride (PVC). However, the UOPMF enhanced a greater potential than the TOPMF over the control sample. Loading up to 30% of the fiber was optimum for a good blend, considering the resistance to wear offered by the samples.