Having written an article explaining when just opening a window is enough to dry the air in your home I need to balance that out and explain why just opening a window will not work in unheated spaces.
Opening the window to bring in fresh air will work fine if you are heating the room up because by increasing the air temperature you will decrease the relative humidity of that air. For example if you pull air in from outside at 10°C and 80%rh and then heat it up to 20°C, the relative humidity of that air will fall to around 52%rh.
But what if you need to look after a space that is not heated and contains something that is sensitive to changes in moisture? This is often the case when you look at garages, archive storage, motor homes, caravans, boats or even your garden shed. Often these spaces will contain classic cars, bedding, papers, family photo albums, leather upholstery in the cars, veneer, wooden objects, fabrics and expensive electronics. Lots of things that need protecting from excess moisture and could be damaged beyond repair if they absorb and hold the excess moisture in the air.
If you are not heating the space and air is allowed to come in at our 10°c and 80%rh then it will stay at 80%rh. It could well be that the internal temperature is slightly higher than the outside the conditions thanks to insulation but unless it is (in this example) 5°C warmer, you will not dry the incoming air below 60%rh to get it into what would be seen as the upper end of a ‘safe’ relative humidity zone by conservation experts.
So although a lot of these spaces are ventilated when they are built this only helps to keep the air moving and to stop the air from becoming stale, it will not help to keep the space dry or to protect the artefacts from moisture damage.
This is where a dehumidifier comes in and why we sell so many dehumidifiers to customers to help look after unheated spaces.
