Ventilation for Passive Houses That Works

A Passive House should feel calm, fresh and consistently comfortable, even when it is cold and wet outside. That outcome depends heavily on ventilation for passive houses being designed as part of the building, not added as an afterthought. In a highly airtight home, stale air, moisture and indoor pollutants have nowhere to go unless a correctly sized system moves them out and brings filtered fresh air in.

For most Passive House projects, mechanical ventilation with heat recovery, commonly called MVHR, is the right solution. It recovers heat from extracted air while supplying fresh outdoor air to bedrooms, living areas and other habitable rooms. The result is cleaner air without the draughts and heat loss associated with relying on open windows alone.

Why passive houses need mechanical ventilation

A Passive House is built to minimise unwanted heat loss. Excellent insulation, high-performance windows and careful airtightness detailing reduce the energy needed to maintain a comfortable temperature. The familiar Passive House airtightness benchmark is very demanding, so natural leakage through gaps around floors, windows and service penetrations is deliberately kept to a minimum.

That is good news for energy use, but it changes the way a home needs to breathe. Cooking, showering, drying clothes and simply living in a house all create water vapour. People also produce carbon dioxide, while furniture, finishes and cleaning products can release pollutants into the air. Without planned ventilation, humidity can rise and air quality can fall, even in a new and well-built property.

Opening windows is still useful when residents want a breeze or extra cooling in summer. It is not, however, a dependable whole-house ventilation strategy. It relies on occupant behaviour, weather conditions and wind direction, and it can quickly remove valuable heat during winter. A balanced MVHR system provides controlled background ventilation every day, whether the property is occupied or empty.

Ventilation for passive houses starts with the design

The best systems are considered before walls and ceilings are closed. Early planning gives the installer room to route ducts efficiently, place the unit accessibly and avoid unnecessary bends that restrict airflow. It also prevents a common problem in retrofit or late-stage projects: trying to fit a capable ventilation system into spaces that were never allocated for it.

An MVHR system usually extracts air from wet and polluted rooms such as kitchens, bathrooms, utility rooms and WCs. It supplies filtered fresh air to bedrooms, sitting rooms, home offices and other living spaces. Air then moves through the home from supply areas to extract areas, normally through carefully sized gaps below internal doors or dedicated transfer paths.

The system itself is only one part of the result. Duct layout, terminal positions, external grilles, insulation, condensate drainage and electrical controls all matter. A premium unit connected to poorly designed ductwork may be noisier, less efficient and harder to balance than a modest unit installed properly.

Get airflow rates right

Airflow should be calculated for the property rather than guessed from its floor area alone. The number of bedrooms, expected occupancy, room use, kitchen arrangement and building layout all affect the requirement. A large open-plan kitchen and living area, for example, may need a different approach from a compact home with separate rooms.

The design must provide adequate whole-dwelling ventilation while also meeting extract needs in moisture-producing rooms. Boost settings should deal effectively with showering and cooking, but constant high-speed operation is rarely the answer. Oversizing can increase noise and electricity use, while undersizing risks lingering humidity and poor air quality.

For projects in Ireland, the system should be designed and commissioned with the relevant Irish Building Regulations and TGD F-2022 requirements in mind. Developers and builders should also consider Passive House certification criteria and the project specification from the outset, as these may require particular calculations, testing and documentation.

Keep ductwork short, smooth and accessible

Ductwork is where many ventilation systems win or lose performance. Long flexible runs, sharp bends and undersized ducts make fans work harder. They can also create turbulence and noise at valves. Rigid or semi-rigid duct systems, sized for the intended airflow, generally give better control and a more predictable commissioning result.

Ducts passing through colder spaces need suitable insulation and vapour control to reduce the risk of condensation. External intake and exhaust terminals should be positioned to prevent the system drawing discharged air back into the property. They should also be protected from obvious local pollutants, such as vehicle exhausts or kitchen extracts.

Access matters after handover too. Filters need changing, condensate drains need checking and the unit may need servicing. Placing an MVHR unit in a difficult-to-reach attic corner can turn simple maintenance into an avoided task.

Choosing the right MVHR system

When comparing units, heat recovery efficiency is relevant, but it should not be the only figure considered. A system also needs sufficient capacity at its normal operating speed, low sound levels, effective filtration and sensible controls. The published performance should be assessed at the airflow the home actually requires, not only at an attractive best-case test condition.

For a family home, look for quiet operation in bedrooms and living spaces, as constant background noise can lead occupants to switch the system off. Good filters are particularly valuable where occupants suffer from allergies or where outdoor air quality is affected by traffic, smoke or seasonal pollen. Filters protect the home and help protect the unit’s internal components too.

Summer bypass is another useful feature. When outdoor temperatures are lower than indoor temperatures, a bypass can allow cooler incoming air to avoid the heat exchanger. It does not replace shading, sensible window use or a proper overheating strategy, but it can support comfort during warmer periods.

Single-room heat recovery units can be a practical option where fitting central ductwork is not feasible, particularly in targeted retrofit rooms. For a full Passive House, though, a well-designed centralised MVHR system is usually the more consistent route to balanced, whole-house airflow. The right choice depends on the building fabric, available routes, budget and whether the work is new build or retrofit.

Installation and commissioning are not optional extras

A ventilation system is not finished when the unit turns on. Commissioning is the process of measuring and adjusting airflow at each supply and extract point so the completed system performs as designed. This is essential for comfort, compliance and efficient operation.

During commissioning, an experienced technician checks fan settings, balances individual valves and confirms the system achieves the specified airflow rates. They can identify issues such as a crushed duct, excessive resistance, a wrongly fitted valve or an imbalance between supply and extract air. Occupants should receive clear information on controls, filter changes and boost functions, along with commissioning records and certification where required.

Airtightness work and ventilation work should also be coordinated. Sealants, tapes and service penetrations need to be completed carefully around ducts and external terminals. If changes are made on site, they should be reviewed rather than assumed to be harmless. Small alterations can affect both the airtightness layer and the airflow design.

PureAir Solution supports projects from product selection and installation through to airflow testing, balancing and certification, helping homeowners, builders and property managers avoid the gap between a good specification and real-world performance.

Living with an MVHR system

MVHR is designed to run continuously at a low background rate. Switching it off to save electricity is usually false economy, particularly in an airtight property where moisture and pollutants can build up quickly. Use boost mode for showers, heavy cooking or periods of higher occupancy, then allow the system to return to its normal setting.

Filters should be checked regularly and replaced according to the manufacturer guidance and local conditions. A home near a busy road may need more frequent changes than a rural property. Dirty filters reduce airflow, increase fan effort and can undermine the air-quality benefits the system is meant to provide.

Residents should avoid blocking supply or extract valves with furniture, boxes or decorations. Valves should not be adjusted casually after commissioning, as changing one room can disturb the balance elsewhere. If a system becomes noticeably louder, leaves persistent condensation on windows or seems unable to clear bathroom humidity, it is worth arranging a professional inspection rather than simply turning up the fan speed.

A Passive House is built around careful decisions that work together. Treat ventilation with the same care as insulation and airtightness, and the reward is a home that feels fresher every morning, stays comfortable through the seasons and gives its occupants a healthier place to live.