TOP 10 Airtight Products
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.
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Airtight Construction is a Draught-Free 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 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.
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, specifically designed airtight boards, and specifically designed airtight membranes and the new emergence of liquid applied 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.
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.
Is airtight construction a problem for ‘breathing construction?
No. Breathable Construction or Vapour Open Construction are the commonly used names 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 – water vapour is a gas) and airtight (draught-free). There are many examples of Passivhaus Standard buildings with a building envelope that is both ‘breathing’ and airtight.
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 that of the high-performance building envelope that reduces heat movement in and out of the building. Insulation plays a key role in 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.
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 Construction?
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.
Common air leakage paths
- Suspended floors (timber and concrete beam and block): Gaps between floorboards or concrete blocks around the perimeter of the dwelling/junction between floor and walls. Large gaps left around services that penetrate through the floor (e.g. soil vent pipes).
- Gaps left between floorboards or blocks and also gaps around services (e.g. pipes and cables).
- Window/door components: Windows and doors that do not close tightly resulting in large air leakage paths.
- Joists that penetrate into wall construction: Masonry walls: Gaps left around joists that penetrate into the inner leaf of external walls. Air leakage from the cavity into the upper floor void leaking into the dwelling through gaps between flooring and through any penetrations in the ceilings, eg recessed lights and ceiling light roses. Timber frame construction: Gaps left around joists, where they penetrate through the air barrier, allowing air leakage into the dwelling through penetrations in the walls and ceilings
- Window sills and reveals: Air can leak directly to the outside or into the cavity through gaps between the window frame and wall reveals. Gaps around window casements (component air leakage). Gaps between doors and frames. Gaps at bottom of the door across the threshold.
- Gaps between dry lining and ceilings: Gaps and insufficient sealing at the wall to ceiling junction allowing air to leak into, and out of, the unheated loft void.
- Internal partition walls: Air leakage can occur through internal partitions if the detailing or location of the air barrier leaves a pathway between indoors and outdoors. Gaps in the air barrier allowing air into the partition which then leaks through penetrations such as light switches and power sockets.
- Loft hatches: Loft hatches that do not fit properly (prefabricated loft hatches can become twisted as they are installed). Inadequate seals between the hatch and the frame. (Note: condensation can be an issue if the loft hatch does not fit – warm moist air from the dwelling rises into the loft and condenses on cold surfaces, such as roof timbers and roof underlay.
- Ceiling roses and recessed ceiling lights: Holes made through the upper ceiling for lights creating air leakage paths into the loft space.
- Gaps around soil and vent pipes and flue stacks: Gaps in ceilings around soil vent pipes and passive flue stacks allowing air leakage paths.
- Gaps around extractor fans and cooker hoods: Poorly fitted extractor fans and cooker hoods allowing air leakage through gaps left between the wall and the ventilation duct.
- Gaps around service pipes (these gaps can often provide the largest air leakage paths in dwellings) Gaps left around service pipes, cables and ducts that pass through the dwelling’s external fabric can be a major contributor to poor airtightness. Large holes often created for much smaller diameter pipes to pass through. Gaps and holes around service penetrations often hidden from view behind baths, vanity units and kitchen units. Cuts and holes in vapour control membranes (used as an air barrier for framed construction) made to accommodate pipes, cables and ducts as they penetrate through the dwelling’s external walls resulting in large air leakage rates.
- General air leakage through walls Gaps in mortar joints (or in some cases missing mortar joints) between concrete blocks on the inner leaf allowing significant air leakage from the cavity, the illustration below shows cold external air being drawn in through gaps and missing mortar joints in the blockwork wall behind the dry lining. Draughts will be felt at the base of the wall (under skirting boards), through electric sockets, around light switches and light roses/recessed light fittings. Gaps between walls and solid ground floors:
- Gaps left between the sole plate of a frame and the ground slab due to undulations in the concrete surface.
Note on point 13. The illustration left shows how air can move through gaps in mortar creating a direct path from the outside to the inside. This is highlighted in the real-life example below, the photo on the left shows the building wall in daylight using a normal camera, the thermal image to the right shows the same section but reveals leakage through the bricks and blockwork behind the render.
You can read more about airtightness in our news Blog
For a view on how airtightness and construction methods can drastically affect the performance, running costs and comfort of your home watch this documentary The Future of Housing, which clearly illustrates the poor quality often delivered currently in the UK.
Editor's Note: Our thanks to Elrond Burrel and Greenspec for their contribution to this article.