Golden Rule: Never Insulate a Wet Wall
A wet wall loses 30% of its insulating power (CSTB source). Insulating would trap water in the wall, accelerating its degradation and promoting invisible mold growth between the wall and the insulation.
- • Hidden mold (health risk)
- • Saturated insulation (zero effectiveness)
- • Wall structure degradation
- • Redoing work within 2-5 years
- • Diagnose the cause
- • Treat the humidity
- • Allow to dry (3-6 months)
- • Insulate with suitable materials
📊 Case Study: 1950s House, Normandy
❌ Initial mistake (2019)
- • Polystyrene insulation on wet walls
- • Cost of work: 8,500€
- • Result after 18 months: mold, smell, degraded insulation
- • Demolition + reprocessing cost: 12,000€
✓ Correct solution (2021)
- • Rising damp treatment: 4,200€
- • Drying time: 4 months
- • Wood fiber insulation: 6,800€
- • Total: 11,000€
Cost of mistake: +9,500€ |Result after 3 years: no problem
The 3-Step Method
Diagnose and treat humidity
Identify the origin of the humidity (infiltration, rising damp, condensation) and treat it with the appropriate solution. Diagnosis cost: 150-400€.
Resin injection: 80-150€/ml
VMC: 400-3,000€
Choose a breathable insulation
For walls that have experienced humidity problems, prioritize breathable insulation (which allows water vapor to pass through). The μ (mu) coefficient indicates vapor resistance: the lower it is, the more the insulation "breathes".
Install insulation correctly
Avoid airtight vapor barriers on the interior side of old walls. Use hygrovariable membranes (Sd variable from 0.2 to 10 m) that regulate the passage of vapor according to ambient humidity. Membrane cost: 3-8€/m².
Insulation Comparison: Which to Choose?
| Insulation | λ (W/m.K) | μ (vapor) | Price/m² | Verdict |
|---|---|---|---|---|
| Wood fiber | 0.038-0.042 | 3-5 | 15-25€/m² | Recommended |
| Hemp wool | 0.039-0.042 | 1-2 | 20-30€/m² | Recommended |
| Expanded cork | 0.038-0.043 | 5-30 | 25-50€/m² | Recommended |
| Polystyrene (PSE) | 0.030-0.038 | 20-100 | 8-15€/m² | Avoid |
| Glass wool | 0.032-0.040 | 1 | 5-15€/m² | Avoid |
λ = thermal conductivity (lower = better insulation) | μ = vapor resistance (lower = more breathable)
2026 Insulation Financial Aid
Possible accumulation up to 90% of the cost
MaPrimeRénov' 2026
- • EWI: 75€/m² (very modest) to 15€/m² (intermediate)
- • IWI: 25€/m² (very modest) to 7€/m² (intermediate)
- • Sieve exit bonus: +10%
Other aids
- • CEE: 5-15€/m² depending on zone
- • VAT 5.5% (vs 20%)
- • Eco-PTZ: up to 50,000€ at 0%
- • Variable local aids
EWI or IWI: Which Solution for Wet Walls?
EWI (external) - Recommended
- • The wall stays warm (reduces condensation)
- • Residual moisture escapes inward
- • Thermal bridges removed
- • No loss of living space
- Cost: 100-200€/m²
IWI (internal) - With precautions
- • Cheaper but less effective
- • Breathable insulation mandatory
- • Hygrovariable membrane necessary
- • Loss of 5-10 cm per wall
- Cost: 40-80€/m²
In Summary
- 1. Always treat humidity BEFORE insulating (savings: up to 50% of total cost)
- 2. Choose breathable insulation (μ < 10): wood fiber, hemp, cork
- 3. Avoid polystyrene and glass wool on old walls
- 4. Prioritize EWI when possible (best thermal solution)
- 5. Take advantage of aids: up to 90% of the cost covered in 2026
Sources et références
Cet article s'appuie sur des sources officielles et vérifiées pour garantir la fiabilité des informations.
- [1]Thermal insulation guide— ADEME
- [2]
- [3]RE2020 - Environmental regulation— Ministère de la Transition Écologique
- [4]Pathologies of old buildings— Maisons Paysannes de France
Rédaction ZeroHumi
Équipe éditoriale ZeroHumi
Documentation technique vérifiée à partir des sources citées (ADEME, ANSES, OMS, CSTB, CEREMA)