Meet the Mechanical Ventilation System

With this article, we’d like to cover a topic that is very important for both comfort and energy efficiency of buildings: the ventilation of the house.

Unfortunately, this matter is as important as it is unknown to most people. When it come to business professionals, at least in Italy, we see an overall lack of any knowledge on this topic, which is combined with the widespread resistance to change. This situation is currently fostering a building practice, for both renovations and new constructions, where the indoor air quality of houses and buildings in general is worse than some decades ago.

We’ll try and cover the topic one step at the time.

3D model of a passive house
3D model of a passive house


In order to live in comfortable conditions, an adult human being needs about 30 m3 (1060 ft3) of fresh air every hour. A child requires less, starting at 10 m3 (350 ft3) per hour, and increasing with age. Of course, this amount changes depending on the physical activity: in case of very active sports, the amount required can be a lot more.

A classic example is provided by the bedroom: in Italy, this room has an average net treatable area of about 15 m2 (160 ft2), and a height of 2,7 m (8,8 ft), so that the volume of air available inside the room is about 40 m3 (1400 ft3). If two adults sleep in the bedroom, the air available in the room allows them to breathe comfortably for less than one hour. They are going to spend the rest of the night with less oxygen than they would need, and with an excess of carbon dioxide and water vapour. Sleeping with the bedroom door open is not going to help much.

This is one of the main causes of mold and condensation appearing on the internal surface of walls and ceilings.

Overall, we are used to an extremely low quality of air inside our buildings.

Besides breathing, other daily activities in a house produce water vapour, including cooking, showering and personal hygiene, washing clothes and so on. In average, a family of four people produces about 10 litres (2,6 gal) of water vapour every single day.

To avoid the risk of mold and condensation, you need to eliminate this vapour somehow.


Contrary to popular belief, transpirability of the structures (including walls, roof and so on) that build up the thermal envelope can expel only a minimum part of the water vapour produced inside the house. Even in the case of the “greenest” structure, with straw bale and timber walls and clay or lime plasters, the amount of vapour that actually goes through those structures is only about 5% of the total.

A straw bale house under construction.
A straw bale house under construction.

For this reason, stating that an external insulation layer “chokes” your house, does not make sense: there’s really not much to choke to begin with.

The only way to guarantee indoor air quality in a building, and to eliminate the water vapour produced, is to provide ventilation.

According to Italian standard UNI10339, the ventilation required in a house is 0,5 1/h: for example, if the net volume of your apartment is 200 m3 (7060 ft3), you need to provide 100 m3 (3530 ft3) of fresh air every hour, to maintain it livable. If you live outside of Italy, you have different codes and therefore different numeric requirements, but the base concept is most likely the same.


The very first way to manage this air exchange is by opening the windows. It corresponds to the image of the housewife that opens up the house every morning, to air it out.


However, in the 21st Century, this scenario raises the following doubts:

1- In today’s day and age, adults have jobs and kids go to school, so that the house remains closed up and empty throughout the day;

2 – The guarantee the required air exchange described above, you’d need to open all windows of your house for 10-15 minutes at regular intervals, 6 or 7 times per day, including at night;

3 – air moves naturally only due to temperature difference (including wind), therefore natural ventilation is effective only in winter. An ineffective manual ventilation fosters the risk of mold and condensation in mid season;

4 – exchanging 100 m3 (3530 ft3) of air every hour causes an extraordinary waste of energy for heating and cooling.


This type of ventilation is represented by uncontrolled air leaks through cracks and gaps of the thermal envelope.

Although these leaks contribute to getting rid of the water vapour you produce inside the house, these air leaks only happen in winter, when the temperature difference between inside and outside is high enough.

For comfort and energy efficiency, air leaks are bad news.

Minnesota wall rot
Damages to a wooden house caused by leakages of warm and humid air from the inside of the house to the outside, through a non-airtight thermal envelope.

As far as the durability of the house, this type of ventilation can cause some serious damages. If warm and humid air leaks through the structures of the house towards the outside, it may bring to heavy water condensation within those structures. In winter, a one meter long (3,3 ft) thin crack can bring up to 800 g (28 ounces) of condensation water into the wall/roof every single day.

In case of a masonry or concrete structure, this water diminishes the insulative properties of the structure. In case of a wooden house, this phenomenon brings the structure to rot in a relatively short time (5-10 years).

For both renovations an new constructions, eliminating the uncontrolled natural ventilation (air leaks) needs to be one of the top priorities.


3D model of a passive house, with the ventilation system highlighted.
3D model of a passive house, with the ventilation system highlighted.

Structures are not transpirant enough, manual ventilation is difficult and air leaks dangerous. For these reasons, the market offers a variety of systems to mechanically ventilate buildings. These systems allow to bring filtered fresh air to the living areas, and to extract humid air from kitchen, bathrooms, laundry rooms. The same systems come with an integrated heat recover, to warm up the incoming air using the heat contained in the exhaust air, without mixing the two flows.

In Italy, these systems are still mostly considered as an additional “gadget”, because most professionals – unaware of the indoor air quality requirements – only see them under the energy efficiency point of view. Only 1% of Italian houses is provided with mechanical ventilation.

You can divide these systems into two groups: centralised systems (one machine for the whole house, with ductwork to distribute the air), and decentralised systems (one small machine per room, no ductwork, less efficient heat recovery).

All of these systems are made by:

– a set of fans to move the air;

– a set of filters (that remove dust and pollen from the air);

– a heat recovery system;

– centralised systems include ductwork to distribute the air to different rooms.

At the time of this article, these systems are not yet compulsory in Italy. However, we include a mechanical ventilation system in all of our projects, whether it is a new construction or a renovation.


Just like any other innovation, mechanical ventilation is facing some resistance from the most conservative part of the building industry. It is perceived as an “unnatural” solution. However, these professionals don’t provide real technical motivations, and are unable to provide with valid alternatives to fulfil the requirements described in this article, as well as to avoid mold and condensation problems.


In order to live in a healthy way, the human body requires certain environmental conditions. Our natural metabolism needs to receive a set amount of fresh air every hour, and to expel water vapour and carbon dioxide.

Since we spend most of our time inside a constructed environment, the buildings we live in need to match our metabolism.

In the last few decades, we changed the way we use buildings. Until the end of World War II, housewives were in charge of indoor air quality of the house, which they took care of by opening up the windows for a considerable amount of time. In the 21st Century, this model is outdated. Homes are inhabited mainly at night, while they remain closed and unoccupied during the daytime. This changed way we use buildings requires a different way we manage them, in order to guarantee comfort and hygiene.

Even though local legislation may be late on the subject of ventilation, the market already offers a variety of solutions to integrate the manual ventilation with a mechanical one.

Theses systems allow to guarantee the indoor air quality, keeping proper levels of oxiden, water vapour and carbon dioxide.

With the way we use houses today, the alternative to these systems is a moldy house.


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