This project focussed on improving the performance of polystyrene insulated panels (PIP) in New Zealand. An experimental programme addressed the areas of concern in PIP performance identified in an industry workshop, an international literature survey, and steering committee meetings. The value of flame retardant treated expanded polystyrene (EPS) was demonstrated in both cone calorimeter testing and in inhibiting fire spread within panel cavities. The performance of interlocking joints between panels and corner joints was demonstrated to be a prime determinant of panel performance in fire, where better performing joints delayed the involvement of the EPS cores in a fire and ultimately the rate of fire spread. Current industry practice of suspending ceilings was demonstrated to be satisfactory with no detachment of ceiling panels when subjected to onerous fire testing from below. The hanger systems also performed satisfactorily when totally immersed in fire conditions to simulate a fire in a ceiling space. This paper recommends joint detailing as an area for future improvement and makes further recommendations on additional aspects of PIP performance not originally addressed by this project.