Modular construction practices are used in many countries as an alternative to conventional on-site construction for residential homes. While modular home construction has certain advantages in terms of material and time efficiency, it requires a different infrastructure than conventional home construction, and the overall environmental trade-offs between the two methods have been unclear. This study uses life cycle assessment to quantify the environmental impacts of constructing a typical residential home using the two methods, based on data from several modular construction companies and conventional homebuilders. The study includes impacts from material production and transport, off-site and on-site energy use, worker transport, and waste management. For all categories considered, the average impacts of building the home are less for modular construction than for conventional construction, although these averages obscure significant variation among the individual projects and companies.