An eight-page building simulation paper. A simulation methodology has been created for establishing the impact of increasing the insulation of the building envelope upon its global thermal performance and annual energy consumption (heating plus cooling). A particular emphasis is placed upon the consequences in terms of increased temperatures in summer leading to needs for installation of air conditioning. This will provide an important input for the revision of the Portuguese thermal regulations for buildings. This paper presents this methodology, the models of buildings chosen for the first phase of simulation and the results that have been obtained. It also discusses future developments planned for these studies. Refers to specific software tools that may not be available.