The sub-floor void, that is the area below suspended floors, is perhaps the least inspected and therefore least understood part of a property.

Yet misdiagnosis can cause Dry rot and useless chemical damp proofing treatment.

High relative humidity

High sub-floor humidity, sustained above 85%RH tells us:

  1. there is a risk of rot, including Dry rot,
  2. there is a risk of moisture condensing on the damp proof course (DPC), which often looks like rising damp,
  3. there is (or was) a source of moisture, which may include vapour produced by growing rot,
  4. 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

  1. 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,
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. If water is collecting in the base of the sub-floor, consider installing a sump pump.

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