Will it rain inside my house?

As you probably know by now our house will be more or less airtight with ventilation provided by a MVHR (mechanical ventilation and heat recovery) system. Essentially this pumps in air from outside and chucks out the stale, stinky air once it has had the heat removed.

So far so good, but if it’s raining outside, won’t it suck nasty damp air into the house and make everything wet?

Spoiler alert …. apparently not but I still struggle to get my head round this one particularly as it seems to rely on the particularly incomprehensible phenomenon that are psychrometric charts and the mythical science that is thermo-dynamics.

The scientists and engineers in the room tell me that if you take cold, wet air and make it warmer by bringing it inside, it can then hold more water vapour, changing the relative humidity in the room. Lower humidity equals no rain.

Exactly how the humidity varies by temperature is set out in these psychrometric charts which let you work out for a given temperature change how the relative humidity and saturation of the air changes.

At this point I would love to be able to explain how to read these fascinating collection of lines and curves but the internet does it far better than I can: here and here will get you started. If you want to know more then check out the easy to read (???) CIBSE Knowledge Series: KS20 – Practical Psychrometry

… or alternatively put the whole thing down to magic and forget about it.

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