I often read and hear people, including passivhaus experts, state that once a building is airtight it requires mechanical ventilation. This is not strictly true. And there are many examples of passivhaus buildings that operate using ‘natural ventilation’ during the summer months. An airtight building significantly reduces air movement through the building fabric: walls, floor, roof, window frames and junctions etc.. And we should never rely on this kind of air movement to provide the air that we breath inside a building, for ventilation, that is. (Fancy drinking the water that seeps through a leaking roof? It’s kind of similar.) A ventilation strategy and system that genuinely does work, and is used properly all year round, is needed in any building, airtight or not. Once you add comfort and energy efficiency into the equation, this almost always means mechanical ventilation with heat recovery will be required for some of the year. The comfort and energy efficiency benchmarks of the Passivhaus Standard certainly mean this is the case.
And this is where airtightness and mechanical ventilation converge – both are needed for comfort and energy efficiency.
Passivhaus with ‘natural ventilation’ – is it possible?
It certainly is possible in relatively mild climates, like the UK, for a passivhaus building to operate using ‘natural ventilation’ during the summer months. This is the time of the year when outdoor air temperatures are suitable for the indoor comfort benchmarks to be met without ‘natural ventilation’ causing cold draughts. (Have you read my “Jab Jab Jab, Right Hook” blog post on comfort?) There are many passivhaus buildings in the UK that have been designed for and operate with mixed-mode ventilation. That is to say, they rely on ‘natural ventilation’ during the summer and mechanical ventilation with heat recovery for the rest of the year.
Windows, doors and dedicated vent openings that are operated manually can provide summer ventilation. Or electronic controls can be used to automatically operate windows and vents according to certain conditions. Whatever approach is taken, there absolutely needs to be a designed comprehensive ventilation system to ensure it does work, does provide adequate fresh air and is used properly. Too much indoor air movement due to the wind and people will simple close the windows or vents and there won’t be enough ventilation to provide adequate fresh air. This is often the case where ‘natural ventilation’ is the only ventilation system provided, especially during the colder times of the year.
And there need to be secure, bird and insect proof, windows or vents that can be left open overnight to help cool and refresh the rooms in the building in the summer months.
An airtight building could rely on ‘natural ventilation’ all year round, just as many buildings that aren’t airtight do. The trouble is, it wouldn’t be comfortable and people would close the windows or vents. Consequently there wouldn’t be adequate fresh air provided indoors. It’s worth bearing in mind that this is exactly what does happen in many buildings that aren’t airtight!
Air leakage is not ventilation.
Mechanical ventilation is needed for comfort, not because the building is airtight.
One of the key reasons to make a building airtight is to ensure good draught-free indoor comfort. This is also a reason why mechanical ventilation, with heat recovery, is needed for some of the year. How much of the year it is needed for depends on the local climate conditions.
Mechanical ventilation can provide a gentle consistent supply of fresh air so that there are no obvious draughts in the rooms. Since it operates constantly it can blow the air very gently at low speed. No gusts of wind, no torrents of cold air racing through tiny vent holes. And you can choose the best location for the fresh air to be provided in the room, unlike ‘natural ventilation’, it doesn’t need to be on an external wall.
And with heat recovery, which is critical for passivhaus, fresh air from the outside is pre-warmed before it enters the room. Again, this eliminates draughts that would otherwise be caused by the temperature difference between the warm indoor air and cold fresh air coming in. The pre-warming process doesn’t mix the stale air being extracted from the room and the fresh outdoor air though. Only the heat is transferred from the stale extract air. The incoming air remains fresh, but at a pleasant temperature.
Mechanical ventilation is needed for energy efficiency, not because the building is airtight.
The other key reason to make a building airtight is energy efficiency. Air leaks in the building fabric let heat out. This is also a reason to have mechanical ventilation with heat recovery: it saves heat energy that is otherwise lost through ventilation. And a mechanical ventilation system with heat recovery in an airtight building uses less energy to run than the heating energy it saves.
Firstly, once the building is airtight and fresh air is gently consistently provided by a mechanical ventilation system with heat recovery, there are no longer cold draughts. So you don’t need to turn the heating up and consume more energy to combat cold draughts. You can stay comfortable using less energy.
And secondly, the heat recovery process keeps heat inside the building instead of throwing it out the window (or some other vent). This is a crucial aspect of the integrated design approach required by the Passivhaus Standard. Once a building is super insulated and airtight all the heat you put into a building stays there, except when it gets thrown out of the building by the ventilation system! Using a super efficient mechanical ventilation system with heat recovery means at least 75% of the heat and sometimes over 90% of the heat doesn’t get thrown away.
However, this efficiency is only possible with an airtight building. Uncontrolled draughts and air leaks interfere with the balance of air being supplied and extracted by the ventilation system. When the system gets unbalanced the heat exchange process can’t operate efficiently. And of course draughts and air leaks let heat out of the building and let uncontrolled cold air in, again reducing the amount of heat keep inside the building.
So, do airtight buildings need mechanical ventilation?
Strictly speaking, no. However, without mechanical ventilation with heat recovery, an airtight building will be uncomfortable and is likely to need much more energy to combat the discomfort, if it is ventilated adequately.
The Passivhaus Standard doesn’t require mechanical ventilation; it requires that strict comfort and energy efficiency benchmarks be met. And the only way in most climates to meet the benchmarks is to include mechanical ventilation with heat recovery.
Buildings need to be airtight to be comfortable and to be energy efficient. Mechanical ventilation with heat recovery provides adequate fresh air at a comfortable temperature and is highly energy efficient in an airtight building.
Airtightness and Mechanical Ventilation with Heat Recovery: two essential elements of comfortable, radically energy efficient buildings. Buildings certified to the Passive Standard.
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