This is the fourth in a series of technical briefs exploring emerging opportunities for energy efficiency in residential and commercial buildings. This brief looks at residential window attachments, including storm windows, interior shades and blinds, and window films. These devices save energy by mitigating heat transfer during summer and winter. Two new platforms have been launched in the past year that may spur the use of attachments as energy efficiency measures: the ENERGY STAR Storm Windows program and the Attachments Energy Rating Council (AERC) certification and label. We explore these ratings, present data on energy and cost savings, highlight efficiency program pilots, and make recommendations for full-fledged window attachment programs.