Every winter, millions of people discover fog on their windows when they wake up. While this phenomenon may seem trivial, it often reveals a hygrometric imbalance which, if untreated, can have consequences for health and the building.
Why Does Condensation Appear?
Condensation forms when warm, moist air comes into contact with a cold surface. According to the CSTB, the phenomenon starts when the gap between the indoor air temperature and that of the glazing exceeds 8°C.
Concrete example: in a room at 20°C with 70% relative humidity, condensation appears as soon as the glass drops below 14.5°C – which is common with single glazing in winter.
📊 Case study: Martin Family, Rennes
Initial situation
- • 1975 house, single glazing
- • Daily condensation (3-4h/day)
- • Mold around the windows
- • Heating bill: 1 850€/year
After work
- • Argon double glazing installed
- • Humidity-sensitive VMC
- • Condensation eliminated by 95%
- • Heating bill: 1 350€/year (-27%)
Investment: 6 200€ (8 windows + VMC) | Grants obtained: 2 100€ (MaPrimeRénov' + CEE) | Return on investment: 8 years
The 3 Types of Condensation: Diagnosis and Costs
| Type | Frequency | Cause | Solution | Cost |
|---|---|---|---|---|
| Interior (inner face) | 85% | Air humidity + cold surface | Ventilation + heating | 0 - 3 000€ |
| Between the panes | 10% | Failing seal | Glazing replacement | 150 - 400€/window |
| Exterior | 5% | Good insulation (normal) | No action | 0€ |
Source: ADEME, indicative prices 2025-2026
Effective Solutions to Eliminate Condensation
1. Improve ventilation
This is the most effective solution. A humidity-sensitive VMCevacuates excess humidity while limiting heat loss.
2. Reduce humidity sources
Cost: 0€. Avoid drying laundry indoors (releases 1.5L of water/load), use an extractor hood in the kitchen, ventilate for 10-15 min/day even in winter.
3. Replace old windows
Double or triple glazing significantly reduces condensation. The inner surface remains warmer (17-18°C vs 8-10°C in single glazing).
4. Use a dehumidifier
As a supplement, keeps the humidity level below 60%. Ideal for rooms without VMC or while waiting for larger works. Cost: 150-400€ (electric).
Financial Aid and Regulation 2026
Aid available for window replacement
MaPrimeRénov' 2026
- • Very low-income households: 100€/window
- • Low-income households: 80€/window
- • Intermediate: 40€/window
Other stackable aids
- • CEE (Energy Savings Certificates)
- • Reduced VAT at 5.5%
- • Eco-PTZ (up to 50 000€ at 0% rate)
RE2020 and air quality
Since 2022, the RE2020 imposes reinforced requirements on indoor air quality. New constructions must integrate efficient ventilation and joinery with high thermal performance (Uw ≤ 1.3 W/m².K). For renovation, the DPE now takes into account the performance of joinery in the energy classification.
When to Consult a Professional?
Slight morning fog on windows is normal. However, consult a humidity expert if:
- • Water runs down the panes and damages the frame (risk of wood/PVC degradation)
- • Mold appears around the windows (health risk)
- • Condensation persists all day (probable structural problem)
- • You observe fog between the panes (failing seal = mandatory replacement)
- • Your heating bills are abnormally high (+20% vs average)
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]
- [4]RE2020 - Environmental regulation— Ministère de la Transition Éclogique
Rédaction ZeroHumi
Équipe éditoriale ZeroHumi
Documentation technique vérifiée à partir des sources citées (ADEME, ANSES, OMS, CSTB, CEREMA)