Biochar to improve the thermal performance of living wall systems: laboratory assessment of three planting substrates

Living wall systems (LWSs) help to alleviate the climate and biodiversity harms associated with buildings and bring benefits to building occupants. Their performance can be variable and existing research points to the planting substrate as a key design factor. This study provides quantitative evidence on the physical, thermal and moisture performance of three planting substrates that vary according to the proportion of biochar added to green waste compost (GWC). Thermal conductivity (Wm−1 K−1), thermal resistivity (mK W−1), volumetric moisture content (%) and mass (g) are measured for each fraction, replicated six times. Controlled drying procedures were employed, measuring these properties at a range of moisture levels. Data analysis finds that volumetric moisture content and biochar fraction have a statistically significant (p ≤ 0.05) effect on thermal conductivity. Added biochar is associated with non-linear reductions in thermal conductivity at low moisture levels. This suggests increasing the biochar fraction while reducing moisture in the substrate of a LWS will reduce its thermal conductivity, with a 100 mm planting substrate with 30% biochar and 30%vol moisture content providing 0.82 m2 KW−1 of thermal resistance, compared to 0.46 m2 KW−1 without added biochar. The methods build on previous work to assess the properties of different planting substrates for LWSs, providing a practical, lab-based assessment of biochar. The data produced are useful for researchers and professionals seeking to understand how biochar additions impact irrigation and thermal performance when specifying and designing LWSs and underline the potential value of biochar for improving the thermal performance of green infrastructure more widely.