Polyisocyanurate (polyiso) foam insulation with impermeable facers is known for its superior insulating properties in building envelope applications. The impermeable facer on both sides of polyiso foam insulation board is designed to increase and maintain the long-term thermal resistance (LTTR) of the insulation. Currently, the Institute for Research in Construction (IRC)/National Research Council (NRC) of Canada, in association with the Canadian Polyisocyanurate Council, has embarked on a research project to develop a standard test methodology that would help to quantify the design LTTR value of polyiso foam insulation boards with impermeable facers. This paper outlines the research project and presents preliminary test results from experimental work. These preliminary results are discussed with a view to developing a methodology that will be used as the basis for a National Standard in Canada for the determination of LTTR of polyiso foam insulation with impermeable facers.