The sub-floor void, that is the area below suspended floors, is perhaps the least inspected and therefore least understood part of a property.
High relative humidity
High sub-floor humidity, sustained above 85%RH tells us:
- there is a risk of rot, including Dry rot,
- there is a risk of moisture condensing on the damp proof course (DPC), which often looks like rising damp,
- there is (or was) a source of moisture, which may include vapour produced by growing rot,
- there is insufficient ventilation.
Why the sub-floor is rarely investigated
Until the 1989 when the British Wood Preserving Association (BWPA) merged with the British Chemical Dampcourse Association (BCDA) it was common to inspect the sub-floor void and report on high sub-floor humidity. Unfortunately, commercial pressure to recommend treating a property for rising damp with chemicals has meant that damp ness from high sub-floor humidity is rarely investigated.
Tips for solving high sub-floor humidity
- Inspect sub-floor looking for the centre of dampness, trace damp to its source, this may be;
- rainwater such as from a broken gulley,
- a vent open to rainwater, or slope directing into the sub-floor,
- a sub-floor air vent open to a source of internal humidity, such as kitchen, bathroom, drying clothes etc.,
- rotting timber (as a bi-product of respiration),
- a leaking mains water pipe, wastewater pipe or central heating system,
- Improve natural ventilation by clearing objects away from vents, uncovering the original vents or increasing the size of vents, there should be a flow from front to back of a property.
- Monitor sub-floor humidity with a datalogger or humidity probe. Sub-floor humidity will fluctuate. Ideally it should never exceed 90%RH and regularly be below 75%RH.
- Consider installing an inline extractor, such as £29.99 OOPPEN 4 inch Quiet Axial Flow Inline Ventilation Extractor Fan.
- If the a previously open vent is compromised, such as following an extension, consider using rigid ducting to draw air from far away from the vents, so as to create airflow.
- It is also possible to use multiple ducts, with different sized apertures, to vary the flow rates, to create balanced sub floor ventilation.
- Use a timer to vary the schedule.
- It is possible to use dehumidifiers to control sub-floor humidity. However, make sure the is a continuous flow safely out of the sub-floor.
- If water is collecting in the base of the sub-floor, consider installing a sump pump.