Ventilation

For a long-term mold-resistant tiny house, an appropriate ventilation concept is necessary. In this article, you will find information on what such a concept can look like and why window ventilation is usually not sufficient.

The necessity of a ventilation concept

In Germany, for conventional new buildings, a ventilation concept according to DIN 1946 must be created. This is often not done for tiny houses, even though they are structurally susceptible to mold growth due to the often poor insulation and other factors. In a tiny space (when used as a residential building), a similar amount of moisture is regularly released as in much larger apartments or houses, whether through cooking, showering, breathing, or laundry.

As a general guideline for residential buildings, at least one air exchange every two hours is recommended. The guideline is 30 m³/h per person (minimum: 20 m³/h).

My tiny house has a volume of about 35 m³. Therefore, if unoccupied, a full air exchange would be required every two hours, and if occupied by only one person, every hour (even at night!). Wall structures are generally as airtight as possible for energy reasons; nevertheless, there is a natural air exchange through leaks (e.g., window seals). In practice, the necessary additional air exchange through ventilation/other measures is slightly lower. Nevertheless, it is clear that window ventilation alone is often not sufficient.

If tiny house manufacturers do not provide a ventilation concept and instead refer to, for example, a “breathable wall“, it’s a red flag.

The goal of ventilation is to achieve good indoor air quality with a comfortable humidity level between 40% and 60%. In winter, an air humidity of approx. 40% is advantageous from the point of view of mold prevention.

Ventilation systems

In my opinion, a humidity and temperature-controlled extractor fan above the shower and an extractor hood are always necessary for on-demand ventilation. Additionally, a permanent ventilation system is required, which can be with or without heat recovery.

Without heat recovery

The minimum solution is window rebate ventilation or external wall openings, i.e. controlled leaks in the outer shell. The exhaust fan in the bathroom can also run permanently and thus (assuming enough supply air) ensure the necessary air exchange.

However, for a new build, I would always recommend a system with heat recovery.

With heat recovery

There are two basic concepts: centralized and decentralized residential ventilation.

Central Residential Ventilation

In this system, a central ventilation unit is connected to all rooms through pipes. The heat of the exhaust air is transferred to the supply air using a heat exchanger. Enthalpy heat exchangers release part of the humidity from the exhaust air back to the supply air.

The space requirements are comparatively large, and the duct routing is challenging; therefore, these systems are rarely used in tiny houses. However, from a technical perspective, they are by far the best solution.

Decentralized living space ventilation systems

Decentralized living space ventilation systems consist of several units.

They usually consist of pairs of fans inserted into wall penetrations with as much distance as possible. The fans always run in opposite directions (one blows out, one draws in) and change their direction every few minutes. This way, the heat exchanger in the wall penetration is alternately warmed by the exhaust air and releases the heat to the supply air after the direction change.

Due to the system design, condensed humidity from the exhaust air may be reintroduced to the supply air. The actual dehumidification is unclear to me. In forums, there is sometimes criticism of the changing volume during direction changes and musty smells in the heat exchangers.

An alternative is decentralized systems with a cross-flow heat exchanger (e.g., from Bayernluft, not recommended). The airflow is low, so several devices must be installed.

Solar air collectors

With solar air collectors, air is heated in/on black plates and fed into the tiny house. I am somewhat skeptical about how significant the heat gain is in everyday life. It usually contributes little to dehumidification.

Dehumidifier

Dehumidifiers do not provide any air exchange, so they are only a supplement to other ventilation systems.

Presumably, the air is already rather dry in winter with a good ventilation concept. However, a dehumidifier can still be useful when needed (e.g., after doing laundry) or during transitional periods. Electric dehumidifiers consume a lot of power. Generally, devices with a compressor are often recommended at temperatures around and above 13°C (efficiency), and at lower temperatures, adsorption dryers.

Common mineral dehumidifiers are often insufficient as they may not absorb enough moisture.