Labor productivity impacts the overall cost and schedule performance. Thus, an accurate prediction of labor productivity is required to estimate project costs and schedule. Many researchers have measured labor productivity and identified factors affecting labor productivity losses, but only a few have explored labor productivity losses in cleanroom conditions. This paper presents the impact of cleanroom conditions on construction labor productivity by comparing two data sets that were derived from work on a semiconductor fabrication facility (FAB) construction project. Both data sets involved the same crew working on the same FAB construction project. The results indicate that, in cleanroom conditions, construction labor productivity fell by 28.85% compared to general conditions. Additionally, interviews with project managers revealed that the difference was affected by additional factors, including access to tools, rework, and overcrowded work areas. The factors cause significant inefficiency under the cleanroom conditions; even though the factors affect both general and cleanroom conditions. Factoring in the labor productivity decrease when budgeting or scheduling allows cleanroom project stakeholders to estimate project duration or cost more accurately. The primary contributions of this paper are the identification of the labor productivity difference and the factors that cause the difference when working in cleanroom conditions.