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False.
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.

004 Airtight-Passivhaus-MVHR

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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11 thoughts on “Airtight (Passivhaus) buildings need Mechanical Ventilation.
True or False?”

  1. Hi Elrond,

    EXCELLENT article, very clear indeed!

    It raises lots of related questions but here are two to start:
    1) How should we judge when the balance between natural ventilation and MVHR occurs? We wouldn’t, for example, simply say “OK, Mrs Client, please switch this system ON on October 1st and Off on April 1st next year” in all cases;

    2) What effect on indoor air quality, comfort and health do you think natural, moisture-buffering (hygroscopic) materials have, especially in winter when a MVHR system is more likely to be used?

    Look forward to more blogs!

    Sam

  2. Sam and all,

    The key line in Elrond’s article for designers, builders and clients alike is “Mechanical ventilation is needed for comfort, not because the building is airtight.”

    If used in the UK outside the context of this article I would modify the above as a one-liner to “”Mechanical ventilation is needed for comfort in cold weather, not because the building is airtight.”

    1) If the occupant likes to have the windows open then they should keep them open and the MVHR off. When they feel uncomfortable they should close the windows and switch on the MVHR. It’s that simple, and applies in summer too if the MVHR has a simple to operate summer bypass.

    This assumes the designer of the house or flat has paid sufficient attention to the passivhaus guidance on natural/manual ventilation strategies, where applicable.

    After 5 years living in Y Foel, including 2 record winters, we have never used the MVHR for more than 7 months in any one year. As I’m writing this I have a lovely 18°C cool breeze on my face from the window, while sitting in a sumptious bath of 24°C radiant warmth. This is by choice. We have purged at night on such days to get 21-22°C but I confess I now prefer 23-24°C during summer days, as long as the RH is around 50% or less. The bedroom is kept between 21-23°C simply by leaving more windows open for longer.

    I have compared IAQ levels over several years, using CO2 as an indicator, between mechanical ventilation and a number of combinations of open windows. This has also helped me understand the less obvious differences, as well as the limits of both.

    Suffice it to say that the range of options one has in a passivhaus with MVHR compared to a building-regs-minimum build are wonderfully liberating. In the UK climate at least.

    One thing that concerns me is that new occupants of current UK passivhaus developments are likely being advised by their designers/builders with the best of intentions, but with a lack of first-hand passivhaus experience. What I would like to see happen is passivhaus designers feeding much, much more off the experience and knowledge of multi-year occupants. For UK domestic new builds I would like to see the emphasis moved away from an all year round reliance on MVHR.

    2) I’ve monitored temperature and relative humidity in 5 points in our house for 5 years now: http://passivebuild.co.uk/bronhaul/ . We have roughly 15 tonnes of Ibstock Ecoterre earth blocks in the ground floor internal walls, with lime render. The main thermal envelope fabric is: Plasterboard->cavity-> OSB->I-beam filled with Warmcell->Unger diffutherm (wood fibre). It was designed with the hope that the fabric would buffer moisture levels well. We are in a 1,500-2,500mm rainfall zone where outdoor RH can max-out for weeks at a time.

    Sadly I have no way of directly comparing our house with another passivhaus using less hygroscopic materials in the same climate conditions. But, our indoor humidity levels are remarkably constant, very, very pleasant, and effortless to regulate. They are 10-30% lower than outdoor RH levels.

    My subjective view is that the timber-based, vapour permeable construction and the earth blocks have a very large positive contribution to the indoor climate of our house. If I ever built another house for myself I would certainly follow similar principles as we did with Y Foel.

    Mark

  3. Hi Elrond, nice website! What you haven’t explained is why someone would want natural ventilation over HRV. Maybe that’s obvious to architects but I don’t know. Is it to reduce energy consumed or just because it’s nice to have a breeze going through on a warm summer day or people just want a window open?
    Potentially either scenario could be at least partly automated. Electric windows could open when appropriate for natural ventilation. For reduced energy use, at least in fairly windy places like Palmerston North, you could use one of those rotating wind powered extraction fans and control the airflow with a shutter that can change the size of the opening to restrict airflow as appropriate.

    • Hi Benjamin,

      Thanks for your comment and positive feedback. Maybe I didn’t quite hit the “Plain English” fully in this case though! Kara already gave one answer to the question and here’s my other thoughts on why people might want ‘natural ventilation’ over mechanical ventilation with heat recovery:
      – Psychology. Many people are most familiar and comfortable with the sense of control that manually opening a window gives them. People often don’t realise that you can still open a window in a passivhaus building. This applies to enjoying a breeze also.
      – A misplaced belief in energy efficiency. People often assume that opening windows must be more energy efficient than mechanical ventilation because the later requires electrical energy to run it. (I used to think that this was the case myself.) As explained in the blog, this is simply not true with a passivhaus building. The ventilation fans use less energy than you would otherwise lose (as heat) through open windows.

      In my experience, automating windows is only a good idea when there is no other practical option, for example high level windows in a school. The controls and mechanisms involved are expensive and it is often difficult to ensure that they work as desired in the long term. If electrical controls are necessary, manual operation is still often best.

      Unfortunately the wind is never reliable enough for any kind of consistent ventilation, even in a place where it seems very windy. Shutters on such a fan are unlikely to be airtight or insulated so the fan will pretty much be a hole in your roof (or wall) letting heat out and cold in! So poor for energy consumption and uncomfortable too. It may seem ok in a very poorly performing house but it wouldn’t be suitable in a passivhaus or other low energy building.

      Sorry that doesn’t sound very positive, ventilation does need to be done right for energy efficiency, comfort and indoor air quality though.

  4. The reason why someone would prefer “natural” ventilation? Semantics. Natural must be good, must it not? The opposite is of course unnatural, aberrant, bizarre – and who wants that in their home?
    Can we please change the terms here, as there is nothing “natural” about “natural” ventilation of a home. While there is a reliance on wind for it to work (hardly ever satisfactorily), it is even debatable whether wind in the built environment is “natural”. In addition to changing wind patterns through site modifications, according to the dictionary, one possible definition of “natural” is: not caused by humankind. We all know that humankind is responsible for changing climate patterns, ergo the wind. Furthermore: unless your building envelope is a sieve, the concept requires human intervention, and should thus rather be called “manual” ventilation. The dichotomy between natural and mechanical ventilation is unreasonable (I was tempted to say: unnatural). It’s funny that no one demands natural ventilation of cars or natural anaesthetisation, though. But for some reason with houses, many people like to go au nature in every way, and believe that nature is always good-natured, whereas technology is poisoning us, making us dependent, addicted even.
    Finally, we should make up our minds as to whether humans are part of nature – rendering everything we do natural, or outside of it. I am convinced of the former, in which case mechanical ventilation is just as natural as opening a window, and any differentiation becomes as pointless as it seems to me. Ventilation has a purpose, and mechanical forces are much better suited to meeting this purpose most of the year. An airtight building envelope is a precondition to doing any type of purposeful ventilation effectively. End of story.

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