The energy efficiency of US multifamily rental housing is compared with other housing types. A real and growing energy efficiency gap is documented, particularly for lower income households. Findings are based on data from the 2005 and 2009 US Residential Energy Consumption Survey (RECS). Individual energy efficiency features related to HVAC (heating, ventilation and cooling) systems, appliances, and the building envelope are analysed along with weighted and unweighted total energy efficiency indices. Multifamily units occupied by low income renters had 4.1 fewer energy efficiency features in 2005 and 4.7 fewer in 2009 compared with other households. If the number of efficiency features was on par with other housing, the savings could be in the range of US$200–400 per year for most lower-income renters in multifamily buildings. There is an astonishing lack of information on how efficiency retrofits would affect property (real estate) metrics such as cash flow and value. Available evidence suggests that millions of US properties could be good retrofit investment opportunities. Better efficiency would allow renters to increase spending on food, healthcare and other essentials. This is not only an economic issue: it has implications for household health, social equity and environmental problems tied to energy consumption.