Hemp concrete walls: evaluation of the relationship between CO2 and TVOC

Climate change is driving the construction sector to use of more environmentally friendly and sustainable materials. Hemp concrete has been recently adopted as an innovative solution by the building industry to reduce emissions, as this material stores more CO2 than the emitted during its production. Part of this storage occurs during its service life leading to a reduction of indoor CO2 levels. CO2 has been widely used as a proxy for evaluating indoor air quality (IAQ). However, these assessments do not consider the features of carbon sequestration materials where the use of CO2 as the sole indicator might lead to a poor IAQ. This paper describes the results of an IAQ monitoring campaign conducted in a CO2-negative prototype house built with hemp concrete and wood. During the campaign, temperature, relative humidity, total volatile organic compounds (TVOC), and carbon dioxide (CO2) levels were continuously monitored inside and outside the test house for three months. Due to the low occupancy of the prototype house, the indoor CO2 levels were generally low. These levels ranged between 400 and 1100 ppm when people was present and decreased down to 30 ppm once the visitors were gone and the house was closed. Such low indoor CO2 levels suggest that hemp concrete walls could sequestrate a significant amount of CO2. During the same period, TVOC concentration varied between 0.19 and 3.62 ppm and was negatively correlated to CO2 levels. The highest TVOC concentrations were recorded in the absence of visitors when the prototype house was closed, indicating that building materials or furniture could potentially emit such compounds. The comparison of CO2 and TVOC levels showed that, while the CO2 levels were very low, the levels of TVOC were at their maximum. Consequently, in buildings made of hemp concrete, the use of CO2 as a single indicator for ventilation might be misleading and could result in the occupants exposure to high pollution levels.  Despite the importance of CO2 monitoring, the findings of this study indicate that in hemp concrete buildings additional measurements may be needed to assess IAQ and identify potential sources of pollution. In some cases, adapting ventilation strategies to CO2 levels could be sufficient to ensure acceptable IAQ, however, a preliminary assessment of IAQ should be conducted before making any decisions.