The aim of this paper was to assess the effect of the microorganism development on the thermal and mechanical properties of building insulation materials (rapeseed straw and lime (RL) and sunflower bark and pith and lime (SL)) and three plant aggregates (rapeseed straw (RS), sunflower bark (SB) and pith (SP)). The samples were exposed to accelerated ageing, in a static environment with a temperature of 30°C and a high humidity of 90 % (±3 %), to favor the proliferation of microorganisms. The paper presents the biological study of the cultivated bacteria and fungi at four ageing times (0, 3, 6, and 12 months) using a counting test. The results showed that these materials were more vulnerable to fungi than to bacteria. Cryo-HRSEM observations were carried out to follow the development of microorganisms at micro scales, at 0 and 6 months of exposure to microbial ageing. The observations showed that the aggregates (RS, SB, and SP) were colonized by microorganisms prior to microbial conditioning for 6 months, but not with the same tendency on the biocomposites (SL and RL) studied which presented weaker development of microorganisms by comparison to the aggregates. The lime has an immediate anti-microbial effect on the aggregates after storage conditions. After ageing, the microorganisms continued to grow on the aggregates, as did the biocomposites, which showed microbial growth of microorganisms of the same type as those found initially on the aggregates. Physical characterization was conducted in order to evaluate the effect of the microbial development. The thermal conductivity was measured with hot wire method for plant aggregates and heat flow meter for the biocomposites. Mechanical characterization was performed by uniaxial compressive test. The studied functional properties (thermal conductivity and compressive strength) of RS, SB, RL, and SL showed a decrease after 3 months, and after 6 months for SP.