Within the Brazilian National Policy for Saving and Rational Use of Energy, the Brazilian Regulation of Technical Quality Level Energy Efficiency of Residential Buildings (RTQ-R), is a legal instrument for obtaining the rating the energy efficiency of dwellings.This paper focuses on Rio de Janeiro city, one of the hottest in Brazil, and aims to verify to what extent some architectural variables (such as openings) used in the evaluation process can be more decisive than others for classification of dwellings about their efficiency in terms of electric energy consumption.The research took place first in a virtual model, and then in the design of recent built dwellings at the city; the parameters examined were how much opening was possible in different kinds of windows; if there was shading on them; orientation of facades and orientation of openings.These elements were crossed each other in various combinations, using appropriate spreadsheets for its calculation.Results show that the shading of openings is the most significant parameter influencing the energy efficiency of envelope.This result is significant for those existing building who intend to increase their energy efficiency, but don't have many possibilities in the