Assessing the impact of climate change on the building sector has many challenges and has gained attention among scholars and designers in recent years.To quantify the effect of climate on the building energy demand and to adequately size the HVAC equipment, the use of design condition (design day) isnecessary. The current design day files available for the United States are updated to 2013 and few descriptions regarding the use of future scenarios fordesign day files are available. In this article, future design day files are developed using weather generators applying the IPCC emissions scenarios.Challenges associated with building current and future design day files and their discrepancies are identified, and a process for addressing the uncertaintiesis described. The future design day files are implemented into a practice study of DOE large office buildings in Philadelphia to assess the impact of thechanging climate on the energy consumption and equipment sizing. The results are compared with outcomes obtained using the ASHRAE 2013 climaticdesign conditions as the base. Buildings in the city of Philadelphia, on average, are more than 60 years old and most were built before standard building regulations existed and are highly vulnerable to the changing climate. Using the most conservative design day strategy, results show very small reduction inthe mean value of boiler capacity and high increase (up to 15 percent) in the mean value of chiller capacity by the end of the century for the cases underexamination. This study aims to guide the development of design day files for future climate scenarios and to address a gap in the literature for assessingbuilding energy performance while considering equipment updates in the future using climate design conditions. This is intended to promote improvedmitigation strategies towards climate change for buildings.