NB: sorry for the long post....
Humidity changes all the time, even indoors, (unless you live in a very highly controlled environment).
What you really want to avoid is condensation.
What is Condensation?
Condensation occurs where moist air comes into contact with air, or a surface, which is at a lower temperature.
Air contains water vapour in varying quantities, its capacity to do so is related to its temperature - warm air holds more moisture than cold air. When moist air comes into contact with either colder air or a colder surface, the air is unable to retain the same amount of moisture and the water is released to form condensation in the air or on the surface.
Condensation is generally noticeable where it forms on non-absorbent surfaces (i.e. windows or tiles, metal surfaces, etc) but it can form on any surface and it may not be noticed until mould growth or rotting or rusting of material occurs.
Conditions for Condensation
In Britain, condensation in houses is mainly a winter problem particularly where warm moist air is generated in living areas and then penetrates to the colder parts of the building.
The moisture in the air comes from a number of sources within the house. Water vapour is produced in relatively large quantities from normal day to day activities - a 5 person household puts about 10 kg of water into the air every day (without taking into account any heating), see examples below.
breathing (asleep) 0.3 kg
breathing (awake) 0.85 kg
cooking 3 kg
personal washing 1.0 kg
washing and drying clothes 5.5 kg
heating - especially paraffin and flueless gas heaters. For every litre of paraffin burnt over one litre of moisture vaporises into air. Every carbon fuel produces some amount of water from combustion.
(1 kg of water equates to 1 litre)
Moisture can also be drawn from the structure of the building into the internal air; from below the floor or through the walls/ceilings. Obviously outside spaces, eg: sheds, garages, can be worse affected by condensation due to lack of heating/insulation, etc.
Problems with the structure of the building can mean that its moisture content is unnecessary high.
The effect of moisture generation is made worse by keeping the moist air in the house - it is theoretically possible to avoid condensation by adequate ventilation. Usually in certain areas of a house (such as bathrooms and kitchens) the warm air contains a lot of moisture, if that air then spreads to cooler parts of the house, it will condense on any colder surface.
Up until the middle/late part of the twentieth century, most house had high natural ventilation as the level of home insulation was low. Conservation then became popular and natural ventilation was greatly reduced by the introduction of double glazing, draught excluders, fitted carpets (which prevent air movement up through suspended wooden floors) and the removal of open fire places with the introduction of central heating.
Houses have become more effectively sealed, keeping any moisture produced within the house and providing better conditions for condensation to occur. Ventilation is only effective if consistent throughout the whole envelope of the house. Condensation is encouraged by poor air circulation where stagnant air pockets form (behind furniture and in cupboards) and the first evidence is often the appearance of mould growth.
Modern life styles mean that many houses remain unoccupied and unheated throughout the greater part of the day, allowing the fabric of the building to cool down. The moisture producing activities are then concentrated into a relatively short periods (morning and evening) when the structure is relatively cold while the building is still warming up.
Hope the info helps.
