Heat, air, and moisture control in slab-on-ground structures: Fid-Bau Portal

Heat, air, and moisture control in slab-on-ground structures

Rantala, Jukka / Leivo, Virpi

The objective of this research is to determine the heat, air, and moisture conditions of slab-on-ground of a heated building. Measurements show that the coarse-grained fill or drainage layers beneath heated slab-on-ground structures are warm and moist throughout the year. Moisture migration: gravitational water, ground water table, capillary action and diffusion, keep these layer moist throughout the year. The water vapor content of the warm and moist fill layer is usually higher than the content of the indoor air. Thus, the diffusion flow is directed into the indoor air, despite the existing heat flow into the opposite direction, and the ground is a moisture source for the structures above. Therefore, thermal and moisture conditions of the fill layers have the major effect on the overall moisture behavior of ground slabs: on the drying times of the fresh in situ cast concrete slab, and on the thermal and moisture behavior of the structure in normal long-term conditions. These conditions are also favorable for microbe growth, which is very common in fill layers. Fungal or bacterial growth was detected in 98 % of the test specimens taken beneath the ground slabs of heated buildings. These conditions: warm fill layers, high moisture level (RH near 100 %) and microbe growth, are the normal conditions for the well functioning slab-on-ground structure. They are not a sign of moisture damage, but the existing boundary conditions for the design of ground slabs. Therefore, it is important to prevent air infiltration between the ground and the indoor air, as effective as possible. The adequate sealing of the joints and lead-ins of the floor assure that the impurities do not penetrate the indoor air.