This paper aims to present the elaboration of a device able to modify the airtightness of a test House. This project has been conducted with three Master student groups in the framework of their research projects from 2015 to 2017. The TIPEE IEQ test House, comprising of two floors and eight rooms, is dedicated to the study of Indoor Air Quality (IAQ), thermal comfort and energy consumption. Its envelope has been designed to reach an airtightness target slightly lower than the French Energy Efficiency Standard RT2012 requirement for dwellings i.e. Q4 Pa value equals to 0.5 m3/h.m2 (n50 = 1.2 ACH). The role of the new device, called Building Airtightness Modifier (BAM) in this paper, is to allow the experimental researchers to adapt the envelope airtightness of the test house (or part of it) in a gradually fashion from the lower value of 0.5 to about 2 m3/h.m2. The main objective here is to evaluate the influence of envelope airtightness on IAQ and heating/cooling/ventilation system efficiency. In this paper, the description of the TIPEE IEQ test House is given in a first part. Then, the design process of the BAM is presented. Tests performed by the students on both experimental bench and in situ show a good agreement with the initial design objectives.