What is Condensation?
Empty houses don’t have condensation. It is us humans that cause condensation as we introduce moisture into the house. So it is very difficult to completely stop condensation as we produce it. We just have to deal with the condensation when it occurs (mostly in winter). https://www.energywise.govt.nz/at-home/dampness/
When warm moist air touches a cold surface the air cools down and can no longer hold the moisture in the air, so it effectively rains (condensates) on the cold surface.
This is why you get condensation on the outside of your glass of cold drink on a warm sunny day (see in the picture how the air around the glass is cooled).
The easiest place to see the effect of condensation in your bedroom is where warm moist air condenses on your windows. Glass and aluminium window frames get cooled by the outside temperature so the warmer moist air in your bedroom condensates as it touches these cold surfaces.
The same thing happens inside your house. Condensation occurs in the rooms where we spend most of our time and where we produce the most moisture (eg Bathrooms, Kitchens, Bedrooms etc).
Wiping down your windows and window sills to soak up the condensation is really important. If you don’t remove the condensation then as your room warms up some of that moisture will evaporate back into the air, then condense on the window again when the room cools down (meaning condensation will continue to build up over time). If the condensation is not removed it will wet the curtains and it will create mould.
High humidity is caused by adding moisture into the air and then not changing the moist air in the room for dryer air (ventilating). Adding moisture to the bedroom comes from human activity like drying towels on the back of the door and even from breathing (we have to breath but even when we sleep we exhale a lot of moisture into the air each night).
Ventilating by opening windows and doors is essential in winter to dry out your bedroom. The cold air entering from outside is dry (cold air can’t hold much moisture) so you have to swap cold dry air from outside for the moist air in your bedroom to keep you healthy and to keep your room free of mould. https://www.energywise.govt.nz/at-home/ventilation/ventilation-checklist/
If you have very high humidity (lots of moisture in the air) and you don’t ventilate then moisture will condense on other things in your room, like a pair of shoes under your bed, or on the wall behind a mirror (areas where air movement is minimal). We see examples every year where a tenant has not ventilated and they end up with mould on clothes, shoes etc. This is nothing to do with the bedroom as one year a tenant will have mould issues and the next years tenant has no mould issues (in the same room).
Heating your bedroom will raise the dew point meaning the air will hold more moisture. Heating is a good healthy thing to do and will reduce condensation if you keep your room warm. But don’t add extra moisture to the air by drying clothes or having wet towels in your room as the air will become very humid (warm air will hold more moisture) and when the room cools down the air cannot hold all that moisture and it will condense on cold surfaces making things feel damp.