As homes become more energy-efficient and airtight, a new challenge emerges—how to maintain fresh, healthy indoor air without sacrificing the benefits of insulation and thermal performance. While airtight construction reduces heat loss and lowers energy bills, it also traps moisture, carbon dioxide, and pollutants inside. This can lead to condensation, mould growth, and poor air quality, impacting both the home’s integrity and the occupants’ well-being.
So, how do you ensure fresh air circulation without compromising efficiency? The answer lies in Mechanical Ventilation with Heat Recovery (MVHR). In this blog, we’ll explore why airtight homes—especially those built to Passivhaus standards—need MVHR more than ever and why other ventilation methods simply aren’t enough.
The Rise of Airtight Homes and the Passivhaus Standard
What is Passivhaus?
The Passivhaus standard is a rigorous building certification that focuses on ultra-low energy consumption while maintaining high levels of comfort and indoor air quality. Developed in Germany, this standard is now widely adopted across Europe and beyond. Passivhaus homes achieve their efficiency through:
- Exceptional Airtightness – Limiting uncontrolled airflow to prevent heat loss.
- Super-Insulated Walls, Floors, and Roofs – Reducing heat transfer through the building envelope.
- High-Performance Windows and Doors – Featuring triple glazing and advanced sealing to prevent draughts.
- Thermal Bridge-Free Construction – Eliminating weak points where heat can escape.
- Mechanical Ventilation with Heat Recovery (MVHR) – Ensuring a continuous supply of fresh air without losing heat.
While these features dramatically reduce the need for active heating and cooling, they also mean that fresh air cannot enter naturally, and indoor pollutants cannot escape without a dedicated ventilation system.
The Problems Airtight Homes Face Without Proper Ventilation
1. Poor Indoor Air Quality (IAQ)
Airtight homes trap pollutants indoors, leading to degraded indoor air quality. Common indoor air contaminants include:
- Volatile Organic Compounds (VOCs) – Emitted from paints, cleaning products, furniture, and building materials.
- Carbon Dioxide (CO₂) – Produced by human respiration; excessive levels can cause drowsiness and headaches.
- Dust and Allergens – Accumulate due to lack of proper airflow.
- Cooking Fumes and Odours – Become more persistent in an airtight home without effective ventilation.
Without MVHR, these pollutants build up, leading to discomfort, health issues, and long-term damage to the building. High CO₂ levels, in particular, are a common issue in bedrooms, where fresh air intake is minimal overnight, leading to restless sleep and morning fatigue.
2. Moisture Buildup and Mould Growth
Airtight homes are particularly vulnerable to excess humidity, as moisture from daily activities (cooking, showering, drying clothes, and even breathing) has nowhere to escape. This can lead to:
- Condensation on Windows and Walls – Especially in colder months, when warm indoor air meets cooler surfaces.
- Damp and Mould Growth – Thrives in areas with persistent moisture, such as corners, behind furniture, and around window frames.
- Damage to Building Materials – Moisture buildup can cause structural issues, such as rotting wood and degrading insulation.
Mould is not just an aesthetic issue—it poses serious health risks, particularly for individuals with respiratory conditions such as asthma. MVHR systems actively remove humid air from bathrooms, kitchens, and utility spaces, replacing it with dry, filtered fresh air to maintain optimal humidity levels.
3. Lack of Oxygen and High CO₂ Levels
Many homeowners assume that an airtight home will have enough oxygen simply because it is well-sealed. However, this is a misconception. When people breathe in an enclosed space, they consume oxygen and exhale CO₂. If there is no fresh air supply, CO₂ concentrations increase, leading to:
- Drowsiness and Fatigue – High CO₂ levels can cause a lack of focus and energy.
- Headaches and Dizziness – Common symptoms of prolonged exposure to stale air.
- Poor Sleep Quality – Bedrooms often experience the worst air quality due to prolonged occupancy at night.
Opening windows is often impractical, especially in winter when doing so results in significant heat loss. MVHR systems continuously introduce fresh oxygen-rich air while expelling CO₂-heavy stale air, ensuring a healthier indoor environment.
4. Odour Accumulation and Stale Air
Without sufficient ventilation, odours from cooking, pets, and daily activities linger longer in an airtight home. While air fresheners and scented candles may mask these smells temporarily, they do not remove the underlying cause. Over time, stale air can make a home feel unpleasant and even contribute to sick building syndrome.
An MVHR system continuously extracts stale air while bringing in fresh, filtered air, preventing odour buildup and maintaining a fresh indoor atmosphere.
Why MVHR is the Only Effective Solution for Airtight Homes
Since airtight homes prevent natural airflow, the only effective way to maintain indoor air quality is through a controlled mechanical ventilation system. Here’s why MVHR is the best solution:
1. Continuous Fresh Air Without Energy Loss
MVHR systems work 24/7 to remove stale air and supply fresh air. Unlike opening windows, which results in heat loss, MVHR recovers up to 90% of heat from outgoing air and transfers it to the incoming fresh air, maintaining indoor temperature and reducing heating costs.
2. Humidity Control to Prevent Mould
By continuously removing moisture-laden air, MVHR systems keep humidity levels balanced, reducing condensation and preventing mould growth. This is particularly beneficial in airtight homes where moisture has no other escape route.
3. Filtration of Outdoor Pollutants
For homes in urban areas, opening windows can introduce traffic fumes, pollen, and fine dust particles. MVHR systems use high-grade filters (such as F7 or HEPA filters) to ensure that the air entering the home is clean and free from harmful pollutants.
4. Healthier Indoor Environment
By supplying oxygen-rich, filtered air, MVHR systems reduce CO₂ buildup, improving concentration, sleep quality, and overall wellbeing. This is especially important in airtight homes, where natural air exchange is minimal.
5. Even Air Distribution and Comfort
Unlike extractor fans, which create negative pressure, MVHR systems ensure a balanced airflow, eliminating draughts, maintaining consistent temperatures, and preventing cold spots.
Final Thoughts
Airtight homes, especially those built to Passivhaus standards, are designed for maximum energy efficiency. However, without proper ventilation, they can quickly become uncomfortable, unhealthy, and prone to moisture-related issues.
MVHR is not just a luxury—it is a necessity for any airtight home. It ensures continuous fresh air, prevents mould and humidity issues, and maintains energy efficiency. Unlike traditional ventilation methods, MVHR provides a long-term, sustainable solution for maintaining indoor air quality in modern homes.
If you’re building or living in an airtight home, investing in an MVHR system is the best way to protect both your health and your home while maintaining energy efficiency.