Many natural building materials are hygroscopic, that is they can absorb (water condenses) and release water (water evaporates). The process is thus:
*Condensation (gas to liquid) ΔH<0; enthalpy decreases (exothermic process) gives off heat.
*Vaporization (liquid to gas) ΔH>0; enthalpy increases (endothermic process) absorbs heat (or cools).
While this process liberates a small quantity of energy, large surfaces area allows significant (1–2 °C) heating or cooling in buildings. The corresponding materials are wool insulation and earth/clay render finishes.