Thermal Conductivity of Concrete

Concrete conducts about 70 times better than fibreglass batt and 40 times better than still air, which makes an uninsulated concrete wall or slab a thermal bridge, not an insulator. Its value in a building is thermal mass — high volumetric heat capacity, around 2 MJ/(m³·K) — which shifts and flattens temperature swings rather than resisting heat flow. That is why radiant floor slabs work well and why massive walls need the insulation on the outside.

The spread is wide and driven by aggregate and water. Siliceous aggregate mixes conduct better than limestone; structural lightweight concrete falls to 0.6–0.9 W/(m·K), and autoclaved aerated concrete to about 0.15. Moisture is the big swing: a saturated slab conducts far better than a dry one because water fills the pores, so a below-grade wall or a slab on wet ground performs worse than the datasheet. Quote conductivity with a density and a moisture state or it means very little.