Straw bale construction offers a renewable, sustainable and proven alternative to mainstream building methods; still, little is known about its airflow characteristics. To this end, the intent of this paper is to evaluate airtightness of fully constructed and plastered straw bale walls as well as individual plain straw bales. The first experiment entailed measuring the influence of straw bale orientation on airflow characteristics with the finding that straw bale considered alone has poor air flow-retarding characteristics and that plaster is the primary air barrier. A second experiment involved thirty plastered straw bale specimens using three different plaster types. From this experiment, a crack grading system was developed and is herein proposed as a tool to evaluate plaster performance as an air barrier. A third experiment validated the crack grade system through application on four fully constructed straw bale walls. Practical use of the crack grading system was demonstrated on a case study straw bale house in Radomlje, Slovenia, where the predicted air tightness results were validated through comparison to results of blower door tests.