Natural ventilation is a simple, maintenance-free, and energy-efficient way to exchange indoor air, but its effectiveness may decrease over time or due to changing usage conditions. In older buildings, problems often arise from insufficient replacement air supply, clogged exhaust ducts, and structural changes that can disrupt the natural airflow. With the right measures, ventilation can be significantly improved without major structural modifications.
6 ways to improve natural ventilation efficiency
1. Ensure sufficient replacement air
Keep replacement air vents clean and open.
Add adjustable vents if there's not enough incoming air.
Don’t block structural air pathways like door bottom gaps.
2. Maintain and optimize exhaust ducts
Clean extract air vents and ensure air flows out without obstruction.
Order duct cleaning if the draft is weak or the vents are dirty.
Check that the chimney outlets are unobstructed and properly protected.
3. Leverage natural forces
Install wind-assisted chimney caps to enhance exhaust airflow.
Use window ventilation, especially on cool summer nights.
Avoid unnecessary airflow obstacles, such as thick curtains in front of vents.
4. Balance indoor air circulation
Keep interior doors open or add transfer air openings to allow free air movement.
Ensure the building has no uncontrolled air leaks that could disrupt ventilation.
5. Adjust ventilation seasonally
In winter: Ensure a sufficient temperature difference between outdoor and indoor air to maintain the draft.
In the summer: Open windows in the evening and at night when the outdoor air is cooler and ventilation works better.
6. Consider a hybrid system or mechanical ventilation
Install mechanical exhaust if you want more consistent and controlled air circulation.
If you’re considering upgrading from natural to mechanical ventilation, test IVAeris Oy's energy consumption calculator to see how the change affects energy consumption and indoor air quality.
Adequate and properly routed replacement air
Natural (gravitational) ventilation only works when enough fresh air can enter the building from outside. In many older buildings, problems arise when window renovations or additional insulation make the building too airtight, causing ventilation imbalance. This can lead to excessive underpressure, which in the worst case draws impurities into indoor air from structures or soil. A common solution is to add replacement air vents into windows or walls. Adjustable vents help avoid a drafty feeling in winter, and filters can improve the quality of incoming air.
In this context, it is worth reading IVAeris Oy's article on installing a replacement air valve in a window , which discusses the selection and placement of valves in detail.
Duct functionality and improving draft
Extract air ducts are a key component of gravitational (natural) ventilation. They create negative pressure that allows fresh outdoor air to enter the building. If the ducts become clogged with dust or debris, the effectiveness of the draft may decrease, so regular inspection and cleaning when needed is recommended. If exhaust air is not flowing properly, it's important to ensure there are no blockages at the duct outlets and to check whether changes in the building’s pressure conditions—such as sealing renovations—have affected performance.
Wind naturally enhances chimney draft, but especially wind-assisted chimney caps—such as rotating draft boosters—can significantly improve airflow. A typical design value for living spaces is that supply airflow should be at least ~0.35 l/s per m² of floor area (equivalent to an air change rate of about 0.5 ACH) to ensure good ventilation. Wind-assisted chimney caps help achieve or exceed this target level, especially in windy conditions.
Studies show that a rotating chimney cap can improve the draft efficiency enough to raise the air change rate to around 0.40 ACH compared to a standard chimney. Depending on the situation, this can mean a several dozen percent increase in ventilation efficiency. In addition, wind caps prevent backdrafts, helping air flow more consistently and in the correct direction.
To improve ventilation performance and boost draft, duct cleaning and the installation of wind-assisted chimney caps are recommended when needed. IVAeris Oy's article on the importance of cleaning ventilation ducts provides more information on when duct cleaning is necessary.
Impact of Energy Consumption and Savings through Optimization
Gravitational (natural) ventilation operates using natural air circulation and does not consume electrical energy, as it has no fans. However, warm indoor air escaping through ventilation leads to heat energy loss, which can make up a significant portion of a building’s heating demand. By optimizing ventilation, unnecessary heat loss can be reduced while still maintaining adequate air exchange.
One key method to save energy is adjusting replacement air vents based on demand. This helps avoid excessive airflow and the related heat loss. Another option is to add wind-assisted draft boosters to the natural system, which improve airflow without additional energy consumption.
The greatest savings, however, come from heat recovery systems, even though this means partially switching to a mechanical ventilation system. Calculations show that in an average home, ventilation-related heating energy consumption can be around 5790 kWh per year with only gravitational ventilation. With an efficient heat recovery unit (around 75% efficiency), up to 4340 kWh can be saved, reducing heating energy demand to around 1450 kWh per year. If the ventilation unit consumes, for example, 1000 kWh of electricity annually, the net benefit is still significant: about 7500 kWh less heating energy per year compared to ventilation without heat recovery.
On the other hand, a well-optimized gravitational ventilation system can be competitive in terms of total energy consumption compared to mechanical systems. This is because users often reduce ventilation to the minimum required level during winter, which decreases excess heat loss. Optimal natural ventilation ensures adequate air exchange without unnecessary energy use. Key methods include duct maintenance, correct vent placement, installing draft boosters on chimneys, and educating residents on effective window ventilation.
The end result is improved indoor air – better control of humidity and pollutants – while saving heating energy. Typically, optimization can improve ventilation energy efficiency by tens of percent, translating into annual energy savings worth hundreds of euros.
If upgrading from natural to mechanical supply and exhaust ventilation is being considered, it’s important to understand the change in energy consumption. A mechanical system increases electricity use but improves indoor air quality and reduces heating costs. IVAeris Oy's ventilation energy consumption calculator helps you estimate how energy costs will change and what benefits can be achieved.
Indoor Air Quality in Natural Ventilation – Managing Humidity and Pollutants
Optimizing airflow has a direct impact on indoor air quality. The purpose of gravitational (natural) ventilation is to remove moisture and pollutants from indoor spaces to keep the environment healthy and comfortable.
Humidity balance and its importance
Indoor spaces constantly generate moisture from structures, furniture, and human activity. Inadequate ventilation can lead to moisture accumulation, causing window condensation, damp structures, and eventually mold problems. Studies show that with good ventilation, indoor relative humidity stays within recommended levels (20–40% in winter and 50–70% in summer). Optimized natural ventilation ensures proper humidity control, especially in bathrooms and kitchens where moisture loads are highest.
Pollutant control and ventilation’s impact on air quality
Ventilation’s role is not only to manage moisture but also to remove indoor air pollutants such as carbon dioxide (CO₂), volatile organic compounds (VOCs, odors), and fine particles. To maintain healthy air, CO₂ levels should stay below 1000 ppm. Comparative studies show that in homes with natural ventilation, the average winter CO₂ concentration is around 795 ppm, while in mechanically ventilated homes it's about 667 ppm. Although mechanical systems keep ventilation consistent, optimized natural ventilation can still ensure healthy air quality.
Active measures by residents—like short cross-ventilation or boosting exhaust airflow—can quickly reduce pollutant levels. Improved ventilation can also lower radon concentrations, as it reduces underpressure in the house and allows replacement air to be drawn in more controllably through filters.
Boosting and potentially transitioning to a mechanical system
Optimizing natural ventilation can significantly improve indoor air quality without major investments. In practice, this involves maintaining ventilation ducts and vents, installing draft boosters on chimneys, and using the system correctly. If indoor air problems persist despite enhancements, switching to mechanical exhaust ventilation can be considered. This increases energy consumption but improves control over air quality, especially in areas with high moisture loads.
Airflow optimization is a key factor in achieving good indoor air quality. A well-functioning gravitational ventilation system can effectively manage moisture and pollutants, but when necessary, mechanical exhaust ventilation can offer an even better and more stable indoor air solution in the long term.
What to Avoid When Improving Natural Ventilation
Optimizing natural (gravitational) ventilation can significantly improve indoor air quality and energy efficiency, but certain mistakes can weaken system performance and cause problems.
1. Over-sealing the building
Sealing a building can reduce uncontrolled air leakage and improve energy efficiency, but overdoing it can block the flow of replacement air. This can lead to indoor moisture buildup, causing window condensation, damp structures, and eventually mold. For gravitational ventilation to work properly, adequate intake of replacement air is essential.
2. Incorrect integration of mechanical and natural ventilation
If mechanical solutions like roof fans are added to a gravitational ventilation system, it's essential to ensure no uncontrolled underpressure is created. Excessive negative pressure can reduce ventilation efficiency, increase air leaks through structures, and even impair fireplace operation. Adding mechanical exhaust requires careful planning to ensure the system remains balanced.
3. Unnecessary duct cleaning
Duct cleaning is only beneficial if there is significant contamination in the ducts—such as dust, dirt, or mold. Excessive cleaning does not improve air quality unless airflow regulation is also addressed. Optimization should focus on ensuring proper airflow routes and sufficient ventilation.
4. Balancing energy efficiency and ventilation
Gravitational ventilation uses natural forces (temperature difference and wind) and doesn't consume electricity for fans. However, in winter, warm air escaping through ventilation causes heat loss, since there's no heat recovery (HRV), which is common in mechanical systems. HRV can recover 50–80% of exhaust air heat, significantly reducing heating demand.
In gravitational ventilation, all incoming air must be heated from scratch, which can increase heating demand, even though no electricity is consumed. Mechanical fans consume 300–1000 kWh/year depending on airflow levels, but the recovered heat energy can outweigh that many times. On the other hand, if airflow is naturally lower in gravitational systems, heat losses may be smaller compared to continuous mechanical ventilation. Therefore, energy efficiency comparison is not entirely straightforward.
5. Air quality control and impact of weather conditions
Both natural and mechanical ventilation can provide healthy indoor air, but their mechanisms differ. Mechanical ventilation maintains consistent airflow year-round, whereas gravitational ventilation is dependent on weather. During cold and windy conditions, it may be too strong, causing drafts and heat loss; during calm weather, ventilation may weaken, leading to increased CO₂ and moisture levels.
Mechanical supply air systems also include filters that remove outdoor pollutants like pollen and fine particles before the air enters. In natural systems, intake air usually comes directly through vents without such filtration.
Measurements show that mechanical ventilation typically results in lower CO₂ levels and pollutant concentrations. For example, in winter, the average CO₂ level in homes with natural ventilation was about 795 ppm, while in mechanically ventilated homes it was around 667 ppm. The difference is especially evident when natural ventilation is insufficient.
Dreaming of boosting your natural ventilation?
When optimizing gravitational (natural) ventilation, it’s important to avoid over-sealing, poorly implemented combinations of mechanical and natural systems, and unnecessary duct cleanings. From an energy efficiency perspective, ventilation performance directly impacts heating demand, and each system has its own strengths. In natural ventilation, air quality control depends on weather conditions, whereas mechanical ventilation ensures consistent airflow throughout the year.
Optimized natural ventilation can provide good indoor air quality efficiently, but it requires careful planning and active participation from the user in managing ventilation.
If you have questions about your natural ventilation system or need cleaning, inspection, or airflow adjustment, IVAeris Oy's professionals are here to help. If you're considering upgrading from natural to mechanical ventilation, we also provide expert consultation and solutions to ensure optimal indoor air quality and energy performance. Contact us today and let's find the best solution for your building together!
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