Approximately 24% of the total source energy is used by buildings and residential buildings consumed 56% of building energies by 2006 yearbook of energy statistics in Korea. Especially, 0.12EJ of the total 0.4EJ for residential heating and cooling is attributable to heat loss through windows. This amounts to 1.6% of total energy consumption in Korea. Accordingly, Korean government is trying to encourage energy savings by strengthening of building energy code. Also, the government reinforced U-factor for windows which will be implemented from January 1st, 2011 and is planning to reinforce the U-factor of windows. In terms of thermal effect, windows decrease the heating demand by solar heat gain and increase heating demand by conduction heat loss. Recent the importance of thermal environment control through windows is catching concerns about building energy savings according to increasing window-to-wall ratio. The purpose of this research is to analyze the energy use and cost by using RESFEN program to four widows for detached house in Seoul and Busan.