Humidity in farmhouse basement and floor:
diagnosis and technical solutions
Capillary humidity affects 73% of farmhouses with earthen or stone floors in the Basque Country. It is not just an aesthetic problem: chronic humidity degrades oak beams from the inside, promotes xylophage growth, and reduces the thermal efficiency of the entire structure. This guide explains how to diagnose the real origin and what solutions are compatible with heritage protection.
Contents
- 1Why does the farmhouse basement get humid?
- 2Types of humidity and how to distinguish them
- 3Technical diagnosis: thermography and measurements
- 4Solution 1: perimeter drainage barrier
- 5Solution 2: cross ventilation of the basement
- 6Solution 3: floor treatment
- 7Compatibility with BIC/KOI heritage protection
- 8Indicative budgets and ROI
Why does the farmhouse basement get humid?
Basque farmhouses were built on land with a high water table. The floor was compacted earth or irregular stone slabs, without any drainage barrier or waterproofing. Groundwater rises by capillarity through the dry stone walls and floor, condensing inside the basement.
Average rainfall
1,200–1,500 mm/year
Affected farmhouses
~73% with earthen floor
Basement relative humidity
85–98% all year
Types of humidity and how to distinguish them
| Type | Origin | Distinctive sign |
|---|---|---|
| Capillarity | Rising water from subsoil | Stains on lower walls, efflorescence |
| Condensation | Air/wall temperature difference | Droplets on cold surface, no stains |
| Infiltration | Crack or defective joint | Localised stain, worsens with rain |
| Pipe leak | Leak in buried installation | Growing circular stain, odour |
| Deficient drainage | Stagnant exterior water | Generalised humidity after rain |
Technical diagnosis: thermography and measurements
Infrared thermography of walls and floor
The thermal camera detects cold (humid) zones vs dry zones with 0.05°C precision. It allows mapping the exact extent of humidity without demolishing anything.
Contact hygrometers on walls
Direct measurement of relative humidity in stone walls. Values above 18% moisture content in the wall indicate an active problem.
Efflorescence analysis
White efflorescence on walls indicates evaporation of water with mineral salts. We analyse the chemical composition to identify the origin of the water (subterranean vs superficial).
Solution 1: perimeter drainage barrier
- 1Excavation of a perimeter trench 40-60 cm deep around the farmhouse
- 2Application of bituminous or bentonite waterproof membrane on the exterior wall
- 3Installation of perforated drainage pipe at trench bottom with slope to the exterior
- 4Filling with filtering gravel layer (20 cm) for capillary drainage
- 5Restoration of exterior finish compatible with heritage (lime, sand)
Indicative cost: 180-250 €/m linear of perimeter. For a typical 12×10 m farmhouse (perimeter 44 m): between 7,900 € and 11,000 €. Estimated durability: 25-40 years.
Solution 2: cross ventilation of the basement
Opposite ventilation grilles
Install ventilation grilles on opposite façades at basement level. Minimum section: 1/400 of floor surface. For 80 m² basement: 0.20 m² total section.
Intermittent mechanical extractor
Extractor fan with hygrostat (activates when RH > 75%). Consumption: 15-25 W. Annual electricity cost: ~35 €. Reduces RH from 90% to 65% in 4-6 hours.
Solar light tube
Solar tube with integrated extractor harnessing solar energy to force extraction of humid air. No electricity consumption. Works even on cloudy winter days.
Pollen and rodent mesh
All ventilation openings must have 2 mm stainless steel mesh to prevent entry of rodents, insects and pollen without obstructing airflow.
Solution 3: floor treatment
Concrete slab with vapour barrier
Inhabited useFor basements in use (storage, wine cellar). Compact the earth, place geotextile + 300 micron polyethylene sheet, and pour 8 cm concrete slab. Reduces evaporation by 95%.
Traditional paving with lime joints
Compatible BICFor farmhouses with heritage protection. On compacted earth place 5 cm sand layer and 4-6 cm stone slabs with hydraulic lime mortar joints. Allows compatible transpiration.
Expanded clay layer (Leca)
EcologicalEco alternative: 15-20 cm of expanded clay on geotextile acts as capillary drainage and thermal insulation. Heritage-compatible. Reduces evaporation by 70%.
¿Manchas de humedad en vigas o paredes?
La humedad crónica destruye la madera estructural desde dentro. Termografía infrarroja + resistógrafo: detectamos el origen exacto y el daño real sin demoler nada. Informe técnico en 3 semanas.
Compatibility with BIC/KOI heritage protection
- The perimeter excavation must be documented with pre-intervention photos and backfilling must use the same traditional materials (lime, local sand)
- The ground level grade cannot be modified nor the visual relationship of the farmhouse with its surroundings altered
- Ventilation grilles must be wrought iron or corten steel with traditional design, no PVC or anodised aluminium
- The concrete slab on the floor is only permitted in interior zones not visible from the exterior, and must be accompanied by a justifying technical report
Indicative budgets and ROI
| Intervention | Price range | Estimated annual saving |
|---|---|---|
| Perimeter drainage barrier | 7,900 – 11,000 € | 1,200 – 2,500 €/year |
| Cross ventilation + extractor | 890 – 1,800 € | 600 – 1,100 €/year |
| Floor treatment (lime slabs) | 2,500 – 4,200 € | 400 – 800 €/year |
| Complete package (3 solutions) | 12,000 – 18,000 € | 2,500 – 4,500 €/year |
Author: Steel&Stone KET Technical Team — Ondarezaintza · Ea, Bizkaia
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