Cellar Damp Calculator

Cellar damp has three main causes: rising damp (moisture wicking up through walls and floor from the ground), penetrating damp (water entering through cracks, failed mortar joints, or porous masonry from external ground water pressure), and condensation (warm moist air meeting cold cellar surfaces). Most cellars experience a combination of all three. A damp survey by a PCA-qualified surveyor can identify the primary cause.

A cavity drain membrane system (Type C per BS 8102) is generally preferred for existing cellars because it manages water rather than fighting it. The studded membrane creates an air gap allowing moisture to drain to a sump. Tanking (Type A) applies a barrier coating but can fail under hydrostatic pressure. Combining both (Type A + C) gives the highest protection for habitable conversions.

A dehumidifier is useful during the initial drying-out period after treatment (typically 3-6 months) and may be needed permanently if ventilation is poor. For habitable basement spaces, relative humidity should be maintained below 60% to prevent mould growth. A dehumidifier rated at 10-20 litres/day extraction capacity is suitable for most domestic cellars.

Options include: natural ventilation through existing airbricks (0.5-1 air changes per hour), mechanical extract ventilation through the wall or via ducting, or a positive input ventilation (PIV) system which pushes filtered dry air into the cellar. For habitable conversions, mechanical ventilation is usually needed per Part F. Ventilation alone will not cure penetrating damp — the water source must be addressed.

For storage, relative humidity (RH) below 70% is acceptable. For habitable rooms, target 40-60% RH for comfort and health. Above 70% RH, mould growth becomes likely within days. Above 80% RH, structural timbers are at risk of wet rot. Measure RH with a digital hygrometer — readings above 80% indicate moderate to severe damp requiring professional treatment.