MEV, MVHR, PIV, natural ventilation: what is the best solution for a damp house?
A damp house does not always have a structural problem: in many cases, the observed humidity comes from insufficient ventilation, unable to evacuate the water vapor produced by normal occupation (cooking, showering, breathing, drying laundry).
The choice between Mechanical Extract Ventilation (MEV), Mechanical Ventilation with Heat Recovery (MVHR), Positive Input Ventilation (PIV), and natural ventilation depends on the building's airtightness, age, and the nature of the humidity. Ventilation, however, never treats rising damp or active seepage, which require dedicated structural treatment.
Why a Damp House Often Has a Ventilation Issue
An occupied home constantly produces water vapor: cooking, showering, breathing, and indoor laundry drying represent several liters of evaporated water per day in the ambient air. Without sufficient evacuation, this vapor condenses on the coldest surfaces — windows, wall corners, thermal bridges — creating condensation that promotes mold growth.
This condensation phenomenon is distinct from rising damp or active seepage, although all three can coexist. A home renovated with better insulation, without a corresponding improvement in ventilation, often sees its indoor humidity levels rise because the reinforced airtightness limits the natural leaks that previously evacuated part of the water vapor.
MEV (VMC Simple Flux): Principle, Cost, Use Case
Mechanical Extract Ventilation (MEV) extracts stale air from wet rooms (kitchen, bathroom, WC) via a network of ducts connected to an extraction unit, generally installed in attics. Fresh air is introduced passively into living rooms through air inlets provided in windows or walls.
This is the most common solution in the French housing market, particularly because it has been mandatory in new constructions for several decades. Its installation cost remains moderate (see our 2026 price guide), and its maintenance is relatively simple.
Its main limitation: in a poorly insulated home or with many uncontrolled cold air inlets, it can create a feeling of thermal discomfort in winter, as the incoming fresh air is not preheated.
MVHR (VMC Double Flux): Principle, Advantages, Limits
Mechanical Ventilation with Heat Recovery (MVHR) combines stale air extraction and filtered fresh air input, passing both flows through a heat exchanger that recovers part of the heat from the extracted air to preheat the incoming air. This system offers better thermal comfort than a single flux system, particularly in winter, while ensuring effective and controlled air renewal.
Its installation cost is significantly higher due to the complexity of the duct network (two separate circuits) and the unit with the heat exchanger. It also requires a sufficiently airtight building to be fully effective, making it particularly suitable for new constructions or global energy renovations.
PIV (VMI): Principle and When It Is Recommended
Positive Input Ventilation (PIV) works inversely to conventional extraction: it pushes filtered fresh air, slightly preheated in winter, into living areas, creating a slight overpressure in the home that forces stale air to evacuate naturally through existing gaps and leakage points rather than through a dedicated extraction network.
This system is particularly recommended in older houses that are not perfectly airtight, where installing a complete extraction duct network would be complex or costly. PIV has the advantage of limiting depressurization in the building, which can reduce the risk of Radon gas suction or cold air ingress through weak points in the envelope.
Its main limit: its effectiveness directly depends on the existence of sufficient and distributed air exit points throughout the home, making it less predictable than a classic mechanical extraction system in a very airtight building.
Natural Ventilation: When It Is Still Sufficient
Natural ventilation relies on air inlets through leaks in the building fabric (old windows, chimneys, gaps) combined with regular manual aeration. In very old, leaky constructions, it can be enough to ensure correct air renewal without mechanical devices, provided that occupants effectively ventilate regularly. This solution becomes insufficient as soon as the building is renovated with better insulation or window replacements — a frequent and underestimated cause of air quality degradation after energy renovation works.
Comparison Table
| Criteria | MEV (Simple flux) | MVHR (Double flux) | PIV (VMI) | Natural Ventilation |
|---|---|---|---|---|
| Installation cost | Moderate | High | Moderate to high | Low |
| Winter thermal comfort | Medium | High (preheated air) | Good (slightly preheated) | Variable |
| Old buildings | Yes, with adaptation | No, except global renovation | Yes, recommended | Yes, historically |
| High-perf. buildings | Yes | Highly recommended | Possible | Insufficient alone |
| Maintenance | Simple to moderate | Complex (filters, exchanger) | Simple | No device |
| Renewal control | Good | Very good | Average | Low |
Ventilation Alone vs Combined with Core Treatment
Essential point: improving ventilation never treats rising damp, active seepage, or basement problems subject to hydrostatic pressure (see our dedicated articles on rising damp and damp basements). These phenomena require specific structural treatment — resin injection, external drainage, tanking — regardless of the ventilation quality.
On the other hand, in a home where a structural treatment has already been carried out, adapted ventilation remains essential to accelerate the drying of the treated wall and prevent condensation from masking the progress of the core treatment.
Maintenance and Lifespan of Ventilation Systems
A ventilation system requires regular maintenance to remain effective: cleaning extraction and supply vents every 6 to 12 months, cleaning or replacing filters (especially important for MVHR), and periodic checking of the motor unit. A poorly maintained system sees its flow rate gradually decrease, reducing its efficiency without the occupant necessarily noticing before new signs of condensation appear. The lifespan of a ventilation motor unit is generally 10 to 15 years before replacement.
FAQ
Can ventilation treat rising damp?
No, a ventilation system only treats humidity related to water vapor produced in the home's air (condensation), not water migrating from the ground through capillary action in wall materials. These two phenomena require completely different treatments.
Should I install MVHR (double flux) in an old house?
This is generally not recommended without a prior global energy renovation, as its efficiency depends on the building's airtightness. PIV or a suitable MEV system is often more relevant in this context.
How much does annual ventilation maintenance cost?
Routine maintenance (cleaning vents, checking the motor) remains modest, while filter replacement on MVHR systems represents a significant recurring cost to be included in the overall budget.
Can PIV be sufficient alone in a very damp house?
It can significantly improve air quality and reduce condensation, but it never treats a structural problem (rising damp, seepage). A prior diagnosis determines if PIV alone is sufficient or if a deep treatment is also necessary.
How do I know if my damp problem comes from ventilation or a structural issue?
Condensation humidity mainly affects cold surfaces and varies with heating and aeration, while rising damp remains localized at the bottom of walls with slow, steady evolution. A detailed comparison table can be found in our article on rising damp.
Is regular manual aeration enough without a mechanical device?
In an old building that is not very airtight and occupied by people who effectively ventilate several times a day, it can be enough. But this solution depends entirely on the occupant's consistency, making it less reliable than an automated mechanical device over the long term.
ZeroHumi carries out a complete diagnosis to determine if your damp problem stems from ventilation, structural treatment, or a combination of both. Request a diagnosis →
Sources et références
Cet article s'appuie sur des sources officielles et vérifiées pour garantir la fiabilité des informations.
- [1]Ventilation in housing— CSTB
- [2]Indoor air quality— ANSES
- [3]Decree of March 24, 1982 - Housing aeration— Légifrance
Rédaction ZeroHumi
Équipe éditoriale ZeroHumi
Documentation technique vérifiée à partir des sources citées (ADEME, ANSES, OMS, CSTB, CEREMA)