Due to the high energy consumption associated with the building sector, several legal documents with the aim of promoting energy efficiency have emerged in recent years, e.g. the Directive 2002/91/EC on the energy performance of buildings (EPBD) and its recast on 19 May 2010, Directive 2010/31/EU. The EPBD led many countries to adjust their own building energy-related regulations. The Portuguese thermal regulation for residential buildings, “Regulamento das Características do Comportamento Térmico de Edifícios (RCCTE)”, is a part of the transposition of the EPBD to Portuguese law. The main objective of this paper is to analyse the RCCTE in order to study the influence of some parameters considered important to the energy efficiency of buildings on energy performance and on the final energy certification rating.