As school systems are faced with overcrowding, portable classrooms are a common resolution when time and funding for new construction are not available. This study focused on differences between the portable classroom environment and the in-building classroom environment. Documentation of the differences between student academic achievement in portable classrooms as compared to in-building classrooms could not be found in the literature. As portable classrooms are being used in greater numbers, pupil density and acoustical quality of these environments must be examined to determine relationships to student learning. Eight elementary schools from a suburban school system agreed to allow site visits for data collection. The researcher gathered test data, measured square footage of the classrooms, measured space taken by permanent objects, and measured background noise in decibels. Useable square footage was calculated by subtracting space taken by permanent objects from the total square footage of the classroom. Teachers of these classrooms responded to a questionnaire. Of the 43 distributed, 38 were returned. No significant difference was found between teachers perceptions of the two environments. Independent variable t-tests found no significant differences between the two environments in the areas of standardized test scores and pupil density. While there was no significant difference in pupil density between the two environments, a negative correlation between pupil density and standardized test scores was found. Thus, from this data set, as pupil density increased, standardized test scores decreased. Additionally, when the two environments were compared, a significant difference was found in background noise. Portable classrooms had a significantly higher level of background noise than in-building classrooms leading to the hypothesis that students in portable classrooms have difficulty in clearly hearing teacher presentations and discussion items throughout the day; therefore according to the results of this study, students in the portable classroom environment should score lower on standardized tests than those in a school building. In addition, a negative correlation between background noise and standardized test scores was found.