Vapour pressure

Vapour pressure is like the “push” force that vapour molecules exert. Imagine a bunch of tiny water particles in the air, bouncing around, and pushing against surfaces. When it’s warmer, these particles move faster and push harder.

Calculation and Example: Now, vapour pressure can be calculated, but it’s a bit complex. It involves temperature and the amount of moisture in the air. For example, indoors at 20°C and 70% Relative Humidity (RH), the vapour pressure might be around 1.4 kPa (kilopascal), while outdoors at 5°C and 95% RH during a rainy storm, it might drop to around 0.7 kPa.

Dew Points and Dew Point Differentials: The dew point is the temperature at which air becomes saturated and water vapour turns into liquid (dew). It’s easier to grasp since it’s a temperature. For example, with indoor conditions at 20°C and 70% RH, the dew point is around 14°C. Outdoors at 5°C and 95% RH, the dew point is also around 4°C.

In your scenario, the wall’s temperature is 14°C, which is right at the indoor dew point. This means moisture could condense on the wall.

Vapour Migration: Vapour likes to move from areas of high vapour pressure to low, trying to balance things out. So, in theory, indoor vapour might want to move outdoors. However, if your house is well-sealed (draught-proofed), not much vapour can escape, and it might end up condensing on cooler surfaces inside, like your wall.

So, understanding vapour pressure and dew points helps to manage where moisture goes in your house, helping to prevent damp issues, especially on colder days when there’s a big difference between indoor and outdoor conditions.

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