This is a Passivhaus Basics blog post that gives an overview of a specific aspect of the Passivhaus Standard.
The Passivhaus Standard requires airtight construction. What does this mean?
Essentially it means a draught-free building envelope.
A clear airtightness strategy is required to achieve this. The airtight line needs to be continuous even when formed of different materials. And it needs to be joined up, even where there are penetrations.
Sometimes airtight construction gets confused with how a building is ventilated or with ‘breathing construction.’ This post clears up these particular confusions.
And why does the Passivhaus Standard require airtight construction?
Airtight construction is draught-free construction. It is an essential part of the Passivhaus Standard to protect the building envelope, to ensure radical energy efficiency and to provide exceptional comfort.
Airtight Construction is a Draught-Free Building Envelope
Airtight construction, or airtightness, quite simply means that there are no unintended gaps in the building envelope that allow air to leak in or out of the building. This means there are no cold draughts coming in through the building envelope. It also means there are no unintended gaps that allow air from inside the building to leak into the materials of the building envelope.
Unintended gaps can be a result of poor design and detailing that doesn’t allow junctions to be constructed well and fully sealed. It can also be the result of poor quality work on site or mistakes during the construction.
To achieve airtight construction there needs to be an identified air barrier (or airtightness line) that is continuous and joined up to form a complete loop. It might not be the same material in all locations, but it must be continuous and joined up. The airtightness line should be possible to draw as a continuous joined up line on any plan, section or detail drawing of the building. (See also: What is the Passivhaus Building Envelope?)
Penetrations through the air barrier are possible. However, penetrations should be planned and suitably detailed to ensure that the penetrations aren’t a cause of unintended air leakage. Unplanned penetrations can be difficult to resolve on the construction site. Every effort should be made to identify, locate and detail an airtight solution for all penetrations during the design stage.
Airtight construction can be formed from many different materials and components. These materials and components should be clearly identified on drawings as forming the airtight line. Some typical materials used for airtight construction include; wet plaster on masonry construction, reinforced concrete, Oriented Strand Board (OSB) of a suitable thickness, and specifically designed airtight membranes. In addition to the materials that make up the building fabric, windows, doors, curtain wall systems and roof lights need to be airtight components.
In most cases, the insulation material is not airtight and does not form the air barrier.
To achieve airtight construction there needs to be a clear strategy. It should identify the location of the air barrier, what materials and components it is made of and how junctions and penetrations are resolved.
What Airtight Construction is Not
Is airtight construction a problem for ventilation?
No. Every building, airtight or not, needs a properly designed, installed and functioning ventilation system. Plentiful fresh clean air at comfortable temperatures needs to be provided to the people inside the building, all year round. Infiltration air – where it has leaked in through gaps in the building envelope – cannot and will not provide this.
Ventilation air needs to enter the building where it is designed to do so – whether this is through opening windows or vents, or through a mechanical ventilation system. With airtight construction, this is possible to achieve as air movement in and out of the building is controlled. With a building envelope that is not airtight, the ventilation is less reliable as it is not possible to control air movement in and out of the building.
You can read other posts I have written about ventilation here.
Is airtight construction a problem for ‘breathing construction?
No. ‘Breathing Construction’ or ‘Breathing Wall’ type construction is the commonly used name for vapour permeable construction. It is entirely possible to have a building envelope that is both vapour permeable (allows the movement of water vapour through the construction) and airtight (draught-free). There are many examples of Passivhaus Standard buildings with a building envelope that is both ‘breathing’ and airtight.
For a more detail look at this subject read: Is a Passivhaus Building a “Hermetically Sealed” box?
Airtight construction ensures ventilation air comes into the building where it is meant to. And, yes, airtight construction can also be ‘breathable’.
Why is Airtight Construction Required?
1. Airtight construction is required to protect the building envelope.
Airtight construction prevents the moisture and humidity inside the building from being carried by unintended air leaks into the building envelope. The moisture and humidity is mainly created by daily activities such as heating, cooking, bathing, washing, breathing etc.. There is considerable risk that moisture in the building envelope will come into contact with a colder surface and condense. Condensation within the building envelope (‘Interstitial condensation’) increases the risk of damage to the building envelope. A gap only 1mm wide will allow enough unintended airflow to carry 360g of water per day into the building envelope.
2. Airtight construction is required for energy efficiency.
Airtight construction prevents heat energy from escaping out through unintended gaps in the building envelope. The main reason why Passivhaus buildings require so little heating is because of the high-performance building envelope that reduces heat movement in and out of the building. Insulation plays a key role preventing unwanted heat-loss and so does airtightness. The air barrier preserves the performance of the insulation which would otherwise be reduced by draughts through it. The air barrier also keeps the warm air inside the building, preventing it being lost through gaps in the building envelope. There is more about this particular aspect on the post: What is the Passivhaus Building Envelope?
3. Airtight construction is required for comfort.
Airtight construction is a draught-free building envelope. This means that the building envelope prevents cold draughts from entering the building and causing discomfort. This may seem trivial since we are all used to experiencing cold draughts now and then. However, cold draughts are a significant source of discomfort. To counter the uncomfortable effect of cold draughts, we turn the temperature up a few degrees. This results in more energy being used!
4. Airtight construction is required for efficient heat recovery ventilation.
As already mentioned above, ventilation air needs to enter and leave the building where it is designed to do so. With airtight construction, air movement in and out of the building is controlled and this can be achieved. Without airtight construction, the heat energy in warm indoor air leaks out through the building envelope and cannot be recovered, it is lost. The heat recovery ventilation system cannot be very efficient in this situation.
Airtight construction protects the building fabric, ensures energy efficiency, provides draught-free comfort and is required for efficient heat recovery ventilation.
What is Airtight Contruction?
Airtight construction is draught-free construction.
It takes a good clear strategy at the design stage to enable airtight construction to be achieved successfully. The air barrier needs to be continuous and joined up. There is no single material that needs to be used for air barrier though. The right materials will be those that integrate with the type of construction system being used. Penetrations need to be thought through carefully and early, so airtight details can be resolved.
Airtight construction doesn’t need to be confusing to understand. It really is about preventing unwanted draughts through the building envelope. It’s still entirely fine to open windows and doors! And it’s also entirely fine to use ‘breathable’ materials to form the air barrier – ‘breathable construction’ can and should also be draught-free.
Airtight construction is not just about energy efficiency, as it is sometimes emphasised. It is vital for protecting the building envelope from moisture damage. It is also important for indoor comfort and for the efficiency of the heat recovery ventilation system.
All of these aspects make airtight construction essential for any high-performance building, and indispensible for the Passivhaus Standard.
Some additional resources:
- Listen to episode 105 “What is a Blower Door?“of the Home Style Green podcast to understand how the airtight construction of a building is tested.
- Watch this free 30 minute documentary: “The Future of Housing – And How Airtightness Can Help” from House Planning Help (disclaimer: I had a minor role in the production) for a non-technical and visual understanding of airtight construction.
- Download (for free) Airtightness Report – A Practical Guide from bere:architects.
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