Interior dampness in a buried building comes from two sources that look alike: leaks through the structure, and condensation when warm, humid air meets cool earth-coupled surfaces. Tell them apart by watching where and when the wet appears, fix leaks at the source, and control condensation by ventilating when the outside air is drier than the inside and by letting walls dry through breathable finishes.
Interior moisture management is the control of humidity and surface condensation inside a space whose walls stay at ground temperature.
A Cool Wall Is A Condensing Surface
There are two sources of dampness, leaks and condensation, and they look alike. The ground holds the interior surfaces near a steady cool temperature. In summer, outside air is warmer and often holds more water vapour. When it enters and meets a surface cooler than its dew point, water condenses on the surface, even though nothing is leaking. The wall looks wet; the cause is air, not groundwater.
| Observation | Likely cause |
|---|---|
| Wet patch in one place after rain, dry between storms | Leak through the membrane or roof |
| Wet patch that grows at the foot of the wall | Rising water, drain failure |
| Wet all over in warm, humid weather, dry in winter | Condensation |
| Wet streaks below a shaft or window | Condensation or leak at the penetration |
What You Need
- Ventilation openings high and low; see Ventilation Shafts and Chimneys.
- Breathable interior finishes, such as earth or lime plaster, which absorb and release moisture.
- A way to judge humidity, such as a hygrometer if salvageable; see Humidity Control.
- Heat or sun to dry the space occasionally.
Method: Diagnose, Then Control
Step 1: Map the wet. Note where water appears and when: after rain, in summer, in winter.
Step 2: Stop leaks first. Repair the membrane or drain; see Waterproofing and Drainage.
Step 3: Ventilate when the outside air is the drier air. In warm humid weather, closing the openings can keep the space drier than opening them, because the warm outside air is the source of the moisture. In cold weather the reverse applies, because cold outside air holds little water.
Step 4: Avoid adding moisture inside. Drying laundry, boiling pots and many occupants all add water vapour.
Step 5: Keep surfaces breathable. Do not seal a wall with an impermeable paint that traps water behind it.
Common Mistakes
- Opening everything up on a hot humid day. It floods the space with moist air.
- Sealing interior surfaces. Moisture behind them cannot leave.
- Ignoring mould. Persistent damp grows mould, which harms breathing.
Safety
Mould, radon and stale air
Damp, closed spaces grow mould whose spores harm breathing; clean visible growth with a dust mask on, and fix the cause. Radon can accumulate underground in some geological areas, which is another reason ventilation is mandatory. Never burn fuel indoors to dry a buried room without a flue and fresh air; see Carbon Monoxide Risks.
What’s Next
- Learn next: Ventilation and Light
- Combine with: Root Cellars — where humidity is wanted