What causes damp in cellars?

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.

Is tanking or membrane better for a damp cellar?

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.

Do I need a dehumidifier in a cellar?

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.

How do I ventilate a damp cellar?

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.

What relative humidity is acceptable in a cellar?

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.

TradeCalculator

Cellar Damp Calculator

Calculate damp treatment areas for cellar walls and floors, compare tanking vs membrane solutions, and size dehumidification and ventilation systems.

Wall Area

Total cellar wall area to treat

Floor Area

Cellar floor area to treat

Treatment Type

Tanking (barrier) vs membrane (managed drainage)

Current Relative Humidity

Measure with a digital hygrometer

How We Calculate This

This calculator assesses cellar damp severity, calculates treatment material quantities, and sizes dehumidification and ventilation systems based on the cellar dimensions and current moisture levels.

Damp assessment

Relative humidity (RH) is the primary indicator: below 70% is low risk, 70-80% is mild (condensation risk), 80-90% is moderate (rising damp likely), and above 90% indicates severe damp with active water ingress.

Treatment comparison

Tanking: Cementitious slurry applied in 3 coats at 1.5 kg/m² per coat to walls and floor. Creates a barrier against water penetration.

Membrane: Studded HDPE cavity drain membrane fixed to walls and floor. Water drains behind the membrane to a perimeter channel and sump pump. Area includes 10% overlap allowance.

Dehumidification

Dehumidifier capacity is estimated from the cellar volume and the difference between current and target relative humidity. A 0.5 L/day per cubic metre rate is used for initial drying.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.