Polyurethane/polyisocyanurate (PU/PIR) foam is effective thermal insulation material which is broadly used in construction industry. PU/PIR foam must have good mechanical properties, low density, low thermal conductivity, high fire resistance and thermal stability. Fire retardants are added to fulfil demanding fire resistance tests. Additive halogenated fire retardant agents are most commonly used in PU/PIR industry. Many of these chemicals are now recognized as global contaminants and are associated with adverse health effects in animals and humans, including endocrine and thyroid disruption, immunotoxicity, reproductive toxicity, cancer, and adverse effects on fetal and child development and neurologic function1. It is required to replace halogenated fire retardant agents for a less human and animal health hazardous and environment-friendly fire retardant chemicals. Red phosphorous dust could provide such alternative. The present study describes Latvian State Institute of Wood Chemistry (IWC) investigation into red phosphorus influence the rigid PU/PIR foam flammability and thermal stability.