Moisture, Condensation, Dampness and Mould
Condensation happens when warm moisture laden air is cooled down, the air can no longer hold as much moisture and excess moisture is left as water on colder surfaces. We all see this whilst showering, the colder surface, glass, tiles and tap wear fog up first as they are generally colder than other surfaces in a bathroom, occasionally you may see the toilet cistern have condensation on the outside up to the inside water level, for exactly the same reason its surface being cooler.
The point at which moist air condenses is called the Dew point.
In our homes there can be dew points in different areas of the house, generally they will be found on the cold side of the house (Southern side, and particularly when shaded), near areas of large amounts of glass, or where an un-insulated ceiling is close to the roof of the house.
Ventilation plays a role in controlling condensation, as generally moving air won't have a chance to settle and cool down to produce condensation. By allowing air from outside into our homes we will naturally be equalising our inside humidity with outside.
What you must remember with regards to RH (Relative Humidity) is that the reading takes into account the Temperature. Without trying to get to technical at 0 degrees air can't hold moisture, at 20 degrees it can hold 20 grams/m3 and at 30 degrees it can hold 30 grams/m3. Assuming the air in your house is at 20 degrees and 70% RH it has about 14 grams of water per m3, and overnight the temperature dropped to 8 degrees and 100% RH then there would be 14grams - 8 grams = 6 grams of water per m3 of air. If your house is 10mtr x 20 mtr and 2.7mtr = 540 m3 so a total of 540 x 6= 3,240 grams or 3.24 litres of water deposited on fabrics furnishings, windows walls ceilings throughout the house, but the majority will be in the lowest dew points, the coldest part of each space.
The above example may be a little extreme, but if you get condensation in a particular area of your home you can now see why. We will deal with what can be done over the next few paragraphs.
Why is there excess moisture?
Each of us live differently, below is a list of moisture makers
1. long hot showers, coupled with not using or having ineffective bathroom fans
2. Cooking particular Boiling, simmering for long periods, coupled with range hoods that aren't ducted outside, or not used.
3. Having a shower, opening door turning off fan and locking up and leaving for the day.
4. Having driers in internal laundries, not enough exhaust fan,
5. Humidifies putting moisture into an area
6. Gas heating unflued
7. Keeping house locked up, windows closed
8. Sub-floor moisture if applicable
How Can Moisture be Reduced
1. Better Bathroom Fans, and using correctly
2. Ducted Range Hoods
3. Using Reverse cycle Air Conditioning
5. Opening Windows, airing out house
6. Sub-floor ventilation