PUR and PIR foams are currently one of the most powerful thermal insulating materials used in the construction. These are flammable materials therefore, it is important to deal with them also in terms of fire protection and security. The polyurethane foam, as well as the polyisocyanurate foam, are not dangerous on their own, but their danger lies in combustion when they can contribute to the development of the fire or can cause damage to the release of the toxic gaseous products. The aim of the diploma thesis was to evaluate the flammability of PUR and PIR foams used in construction based on the determination of combustion heat according to the standard STN ISO 1928:2003 and the reaction to fire according to the standard STN EN ISO 11925-2. Tested were selected types of PUR and PIR foams, which are used in construction mainly as an insulation. In the reaction to fire test, where the samples were tested to surface and edge, the worst results in both cases achieved the PUR foam. In terms of combustion heat, PIR foam had better results than the PUR foam. The surface treatment had an impact on the reaction to the fire test - flammability, but also on the calorific values.