The accelerating depiction of natural resources has brought environmental sustainability to the forefront of engineering and therefore, design educators must integrate sustainability into the engineering design curriculum. Several researchers have proposed educational interventions and design tools for sustainable design education. The timing of introducing such interventions, particularly in project-based courses, could influence the effectiveness of these interventions, and these effects remain largely unexplored. Our aim in this research is to investigate this research gap through a mixed-methods experiment conducted with first-year engineering design students. Specifically, we introduced a two-day module on sustainable design either in the first or sixth week of an 8-week long design project. The effects of this variation were compared by analyzing (1) changes in students' trait empathy, (2) changes iii their beliefs, attitudes, and intentions towards sustainabilith and (3) their responses to a reflection assignment collected at the end of the semester/design project. From the results, we see that the timing of the sustainable design intervention did not relate to changes in students' trait empathy or their beliefs, attitudes, and intentions towards sustainability. However, students from both timing conditions reported significant increases in their attitudes and intentions towards sustainable action. Fitially~ students who received the intervention later more frequently mentioned the use of sustainable design heuristics in their reflection responses. Taken together, these findings suggest the need for educators to consider the timing of sustainable design interventions, especially when integrating them into longer project-based courses. [ABSTRACT FROM AUTHOR]