Sub-floor ventilation fans are very effective for removing mould, damp smells and drying excessively damp earth below your building, but there are a few things you should know before installing them.
Installing Sub floor fans sound like a great idea to cure rising damp. After all, the walls are wet so why not just dry them out with fresh air?
If you have rising damp, installing fans will likely accelerate the process that will deteriorate the bricks and mortar below your floors, turning them to dust.
Here’s a short video showing what fans can do when used incorrectly:
Before we explain why sometimes sub floor ventilation fans aren’t a good idea, lets look at the mechanics of rising damp.
Rising damp is most often happens when the damp course that was installed at the time of construction fails in some way, allowing water to pass higher in the walls via capillary action to an evaporation point, usually 30cm – 1M above the floors in your house.
The damage from rising damp is not so much the water but the salt that comes with the water. When water is in the earth it dissolves the minerals (salts) in the soil and transports them as the water seeps through soil and porous building structures.
As this salty water dries, the water evaporates into the atmosphere and the salt stays behind and crystalises inside the pores of the bricks and mortar.
The scientific explanation is “a hypothetical tendency for the universe to attain a state of maximum homogeneity in which all matter is at a uniform temperature“.
An example is when you put an ice cube in a glass of water. The heat energy from the water will warm the ice, this heat energy from the water is transferred to the ice and as the ice warms it melts. In the end the water from the ice and the water from the glass are the same temperature.
..Lets change this logic to apply to the masonry walls of a building.
The tendency for water to attain a maximum state of homogeneity (equal saturation) until all bricks are at a uniform level of water saturation. In other words if there is a dry brick next to the wet brick the water will seep from the wet brick to the dry one until both bricks have the same water and salt content.
Nature always follows the path of least resistance and water will rise in a wall until it arrives at a point where it evaporates. This is usually above your floor inside your house which is why the paint and render often look damaged above floor level.
Installing sub floor fans only lowers the evaporation point to below your floors and because there is a much higher level of evaporation caused by the fans, there is an increased volume of salt water being sucked from the soil below into your bricks and mortar, but you can’t see it because it is hidden below your floors.
• Rapid Deterioration.
As the salty water enters the bricks and mortar of your house it collects inside the pores of the bricks and mortar. The sub floor ventilation fans increase the evaporation from the sub-floor walls and piers causing the salts to crystalise and grow. This applies pressure to the internal bonds which hold your bricks and mortar together.
Speeding up the drying process will cause more salty water will rise to replace the water that has evaporated. This will deposit more salt crystals inside the pores of your bricks and mortar until a tipping point is reached and the growing force from the salt crystals break all the bonds within the bricks and mortar of the walls and piers that support your building.
In short. Sub floor fans give the appearance of curing rising damp, but they just lower the problem to a place where you can’t see it (below your floors).
If you have mould in your house then sub floor fans can work wonders, but you need to be sure that the foundational structures of your house are protected and are at no risk of accelerated deterioration before you install them.
Here’s another video of a house where there are no fans installed but the sub-floor has a high ventilation and the deterioration is critical. See Below.
Sub floor fans are great for minimising mould that can grow inside your house as a result of moisture being present under your house. The additional ventilation will dry the soil from below your building and keep it under control. .
2. When you have damp smells.
Mould grows in damp areas and as it is organic it releases a waste product. It is this waste product that causes the smell in your building. Sub floor fans will reduce the moisture in the environment under your building so it is not suitable for the mould to survive.
3. When you excessively damp earth below your building.
Having a lot of water below your house can leave everything in your house damp such as your clothes, books and furniture. Sub-floor ventilation fans can greatly help minimise these problems.
These are the ONLY 3 situations that you should use sub floor ventilation fans.
Sub floor fans do NOT fix rising damp. In many cases it will make the damage to your house worse.