Eco-Niwas Samhita 2018 (ENS) is the new Energy Conservation Building Code for Residential Buildings (ECBC-R). Among different code provisions, a maximum Residential Envelope Transmittance Value (RETV) is defined for cooling dominated climates. RETV gives a quantitative measure of heat gains through the building envelope (excluding roof), calculated by a simple formula. This paper describes detailed methodology followed for the development of the RETV formula.