Water vapor diffusion
Pathology of humidity. Simple wall. Phenomenon of condensation.

We now consider a wall separating two environments whose temperatures T₁ and T₂ are equal (T₁ ≥ T₂), but whose relative humidities HR₁ and HR ₂ are different, with HR₁ > HR ₂ .

To each relative humidity of the air, HR₁ and HR ₂ , corresponds a partial pressure P_p1 and P_p2 of water vapour, with of course: P_p1 > P_p2 .

A diffusion flow, g, of water vapour is established through the wall according to Fick's law (figure 3):