The Role of Humidity Recovery in Modern Ventilation Systems

In modern buildings, ventilation is essential for maintaining healthy indoor air quality. Fresh outdoor air must continuously replace stale indoor air in order to remove excess carbon dioxide, odors, and pollutants. During colder seasons, however, ventilation also removes moisture from indoor air and replaces it with colder and significantly drier outdoor air. This often causes indoor humidity levels to fall to uncomfortable levels during winter.

Humidity recovery systems were developed to reduce this problem. Unlike traditional heat recovery ventilation systems that transfer only temperature, humidity recovery systems — often called enthalpy recovery systems — also transfer part of the moisture contained in the exhaust air back into the incoming fresh air. This helps slow down the drying of indoor air and improves comfort during the heating season.

It is important to understand, however, what humidity recovery systems can and cannot do. A common misconception is that humidity recovery “creates” humidity indoors. In reality, these systems cannot generate moisture. They can only recover and reuse part of the humidity that already exists inside the building. This distinction becomes especially important in colder climates where outdoor air contains very little absolute humidity.

How Humidity Recovery Works

When warm indoor air is exhausted from a building, it contains both heat and moisture. In a humidity recovery system, part of this moisture is transferred through a membrane or rotary exchanger into the incoming outdoor air stream. As a result, the supply air entering the rooms contains more moisture than it otherwise would in a standard ventilation system.

This process reduces the rate at which indoor humidity is lost. The indoor environment therefore remains more stable and comfortable, particularly during cold winter periods when outdoor air is extremely dry after being heated indoors.

The system does not add new moisture into the building. Instead, it reduces how quickly existing indoor moisture escapes through ventilation. If moisture is continuously produced indoors — for example by occupants breathing, cooking, showering, drying laundry, or indoor plants — humidity recovery can noticeably improve indoor humidity levels by retaining a larger portion of this generated moisture.

Without indoor moisture generation, however, the indoor air will eventually approach the same absolute humidity level as the outdoor air, regardless of humidity recovery efficiency.

Short-Term and Long-Term Effects

The short-term effects of humidity recovery are often very noticeable. During cold weather, buildings ventilated without humidity recovery can become dry very quickly. Humidity recovery slows this process considerably, which improves comfort and reduces symptoms associated with dry air such as irritated eyes, dry skin, or throat discomfort.

In the longer term, humidity recovery also improves energy efficiency because latent heat contained in water vapor is partially recovered together with sensible heat. This slightly reduces the overall heating demand of the building. In addition, more stable indoor humidity levels reduce stress on humidification systems and may improve the durability of certain building materials and furnishings.

However, humidity recovery should not be viewed as a complete solution to low indoor humidity in cold climates. Its main benefit is moisture retention, not moisture creation. If a building has very low internal moisture production and high ventilation rates, indoor humidity can still become very low during winter.

The Importance of Air Exchange Rate

The air exchange rate plays a major role in determining indoor humidity levels. Air exchange rate, commonly expressed as ACH (Air Changes per Hour), describes how many times the indoor air volume is replaced with outdoor air every hour.

Higher ventilation rates remove indoor moisture faster and therefore dry the building more rapidly. Humidity recovery systems reduce this drying effect because some of the exhausted moisture is returned back into the supply air. Nevertheless, the final equilibrium condition is still governed by the moisture content of the outdoor air if no new moisture is added indoors.

This means humidity recovery changes the speed of drying rather than the final physical equilibrium.

Example Calculation

To better understand the effect, let us examine a practical winter example.

Assume the outdoor temperature is 0°C with an outdoor relative humidity of 90%. Although the relative humidity sounds high, cold air can contain only a small amount of water vapor. Under these conditions, the outdoor absolute humidity is approximately 4.3 g/m³.

Now assume the indoor temperature is 20°C. At this temperature:

• 40% relative humidity corresponds to approximately 6.9 g/m³ absolute humidity,
• while 25% relative humidity corresponds to approximately 4.3 g/m³ absolute humidity.

This means that outdoor air at 0°C and 90% RH, when heated indoors to 20°C without adding moisture, results in indoor air close to 25% RH.

Next, assume the ventilation system has:

• a humidity recovery efficiency of 70%,
• and an air exchange rate of 5 air changes per hour.

Under these conditions, the humidity recovery system significantly slows down the loss of indoor moisture. If the indoor humidity initially starts at 40% RH, the indoor air will dry much more slowly than with a conventional ventilation system. However, if there are no continuous indoor moisture sources, the indoor humidity will still gradually move toward approximately 25% RH over time, because this reflects the moisture content of the outdoor air.

If the indoor humidity already starts at 25% RH, humidity recovery alone cannot raise it further because no additional moisture exists within the system to recover.

Mathematically, the indoor absolute humidity over time can be approximated as:

where:

• ACH is the air exchange rate,
• and 𝜂 is the humidity recovery efficiency.

This equation illustrates that humidity recovery reduces the drying rate by reducing the effective moisture loss through ventilation.

Practical Implications

In real buildings, indoor moisture is usually generated continuously by occupants and daily activities. In such cases, humidity recovery can provide substantial benefits because the system continuously reuses part of the generated moisture instead of exhausting it outdoors.

For many homes, this means:

• improved comfort during winter,
• reduced need for humidification,
• lower heating energy consumption,
• and more stable indoor conditions.

At the same time, humidity recovery systems also introduce additional maintenance requirements compared to standard heat recovery systems. The long-term value therefore depends on climate conditions, occupancy patterns, ventilation rates, and overall building design.

Conclusion

Humidity recovery systems are highly effective at slowing down wintertime indoor drying caused by ventilation. They improve comfort, help retain indoor moisture longer, and recover part of the latent heat that would otherwise be lost.

Their limitations must nevertheless be understood correctly. Humidity recovery systems do not create humidity and cannot maintain high indoor humidity indefinitely without continuous indoor moisture generation. In buildings with little internal moisture production, indoor humidity will eventually approach the outdoor absolute humidity level even when humidity recovery is used.

For this reason, achieving optimal indoor humidity in colder climates may still require a combination of:

• demand-controlled ventilation,
• moderate ventilation rates,
• humidity recovery,
• indoor moisture generation,
• or dedicated humidification systems.

When properly applied, humidity recovery remains an important and valuable component of modern energy-efficient ventilation design.