Dehumidifier advice

[Phil L]

I do believe in physics dennis and if you are correct with your formulae and predictions re condensation and evaporation, then you'd better tell the bigwigs down at 'physics house' that they've got it all wrong, they are totally way off.

No, my plaster was 70% dry today, I put it on yesterday, that's the day before today...tomorrow will see today's plaster similarly dried out while yesterday's will be completely dry, and all by magic too, considering there's no electricity or gas to the property and it's only a degree or two warmer than outdoors, although all the windows are open....you don't honestly believe that housebuilders wait six months after plastering before painting?

And you still haven't explained where all the condensation comes from, which is running down the inside of the greenhouse windows every morning when I get up..

[Andrew Gabriel]

Guys, you don't need to argue, because you're both right.

Moisture does require heat (or rather energy) to evaporate. So consider the wet wall. A bit of moisture evaporates from the wall and in doing do it steals some heat from the wall, so the wall gets colder. Now that the wall is colder than the surrounding air, it will extract heat from the air, which continues to provide the energy to evaporate more water. This is the same principle as a wet bulb thermometer, which will be a few degrees colder than a dry bulb thermometer because the moisture is stealing heat from it in order to evaporate.

However, the example in this case wasn't simply a wet wall, it was a newly plastered wall. This changes things because plaster gives off a lot of energy when it sets, which heats the wall up and considerably speeds up the drying process. The most obvious case of this is if you mix up some plaster which was too old and sets in a few minutes in the bucket. All that energy is given off much quicker in a confined space, which is why that bucket of plaster gets hot.

[dennis]

Well plaster setting is exothermic. It doesn't need to dry much for the surface to appear to be dry. Walls are painted with emultion which is water + oil so it wets the wall anyway.

The same place as the fog and dew.

[Phil L]

Thanks for the explanation...although I knew that plaster gave off a small amount of heat...also there's the added heat of people being in there during the day, don't people give off around 200watts each or summat?

Either way, a ventilated hoose is a dry hoose. (providing it's not raining in!)