Why is Airtightness Important?

The 2023 Indoor Air Quality Report by the UK Parliament outlined that 80-90% of people’s time is spent indoors. This highlights how important it is to ensure good air quality within our buildings. Airtightness is important in achieving this, for several reasons:

Energy Efficiency
Buildings that are not airtight lose heat through gaps and cracks, leading to higher energy use and bills. Improving airtightness helps maintain stable indoor temperatures and reduces the amount of energy needed to heat or cool a building, lowering both costs and carbon emissions. This supports climate goals and helps meet UK building regulations like Part L, which promote energy-efficient, environmentally friendly buildings.

Even with good insulation, a building will not perform well if air is leaking in or out. When air moves through insulation, it reduces its effectiveness. Airtightness protects the insulation and allows it to work properly by keeping the building warm in winter and cool in summer.

Comfort
Airtight buildings are more comfortable to live or work in because they reduce draughts and cold spots. They also help heating and cooling systems perform more effectively, ensuring a consistent and comfortable indoor environment year-round.

Moisture and Ventilation
Uncontrolled air leaks can allow dust, pollen, and other outdoor pollutants to enter a building. They can also carry moisture into walls and ceilings. Over time, all of this can lead to damp, mould, and irreparable damage to the building’s fabric. Airtightness reduces this risk by limiting where and how air enters or exits the building.

It also enables controlled ventilation systems like Mechanical Ventilation with Heat Recovery (MVHR) to work properly. This way fresh air is supplied, filtered, and distributed as intended rather than leaking in or out through cracks and gaps.

Noise Reduction
Improved airtightness also acts as a sound barrier, reducing the amount of external noise entering the building. Sealing up gaps in walls, floors, and windows can effectively block out external sounds, making indoor spaces quieter. This is especially beneficial in busy urban areas or buildings near roads, railways, or airports.

Future Homes
Airtightness also plays a key role in meeting future UK housing targets, like those set out in the Future Homes and Buildings Standards, which aim to have new homes be “zero-carbon ready” from 2025 onwards.

Standards like Passivhaus already demonstrate what this looks like in practice. They rely on very high levels of airtightness to meet their strict energy performance goals. Without airtight construction, even the most advanced insulation or heating systems can fall short.

Airtight construction is therefore essential not just for today’s standards, but also for building homes ready for the future.

Common Air Leakage Paths

By understanding how and where air leakage occurs, we can target weak spots in the building fabric.

Some common leakage paths include:
Floor to wall junctions
Ceiling to wall junctions
Partition walls
Windows
Doors
Loft hatches
Chimneys and flues
Lighting fixtures
Service penetrations

The following diagram illustrates common air leakage paths.

Methods for Achieving Airtightness

Airtightness focuses on reducing the air leakage in a building. It can be achieved by making sure there is a continuous airtight layer around the entire building envelope. The position of the layer, material or product used will vary according to construction method (e.g. timber frame, masonry, steel, SIPs). However, the key principle remains the same: the airtight layer or barrier must form a complete, sealed envelope without gaps or weak points.

The air barrier must be impermeable or nearly impermeable, meaning that air will not be able to pass through at 50 Pa. In most cases, it will be located on the warm side of the insulation, surrounding the entire heated volume of the building.

The airtightness zone can be achieved:

• with monolithic construction of airtight materials
• with an airtightness membrane over materials that are permeable
• by sealing all joints and junctions with airtight tapes and gaskets
• by lapping and taping all membranes at overlaps, penetrations and junctions
• with proprietary airtightness systems designed for specific build types

Air barriers can come in different forms including boards, membranes, tapes, paints, sealants, foams, and sprays.

Typical air barrier materials include:

• Vapour control layers (VCLs)
• Damp proof membranes (DPMs)
• Concrete cast in situ, precast concrete panels with proper seals at edges and joints
• OSB class 3 or higher (used for SIPS panels and sheathing in timber frame construction)
• Plywood sheathing
• Cross laminated timber (CLT)
• Parging coat – applied to masonry
• Gypsum boards

It is important to select materials that are robust, and suitable to form the air barrier. They must also be appropriate for the location, and compatible with adjacent construction elements to ensure continuity. Some may even require testing before specification.

Generally, in dwellings, the barrier will be within the external walls, the ground floor and line of the roof or the upper most ceiling depending on the roof construction. Its role is to separate the heated spaces from the unheated spaces. As outlined earlier, some of the most common and vulnerable areas for air leakage include joints between constructions, wall to floor, floor to ceiling or roof etc.

It is important that the line of the air barrier is clearly identified on drawings, and that it is continuous. Where possible, avoid complex detailing which will prove difficult to achieve on site. Also, make sure you pay close attention to the common air leakage paths as these will require careful detailing and sealing.

Achieving airtightness is not just about materials it also depends on good design, clear detailing, careful planning, and quality workmanship on site. Everyone involved in the project from the architects to construction workers need to understand the airtightness strategy and how to deliver it effectively during construction.

Application by Building Element

Now, we will go through some practical airtightness guidance for each building element.

Floors
With a ground bearing slab, a damp proof membrane provides suitable protection. It must be lapped, fully taped, and sealed at all edges. Any penetrations through the membrane must be sealed with compatible tapes, grommets, or sealants to ensure continuity. The membrane needs to be protected from damage during construction.

Where the screed forms the air barrier, an insulated perimeter upstand is normally included to accommodate expansion. A durable airtightness tape must also connect the screed to the wall to prevent tearing when skirting boards are installed.

A suspended beam and block floor would provide a good standard of airtightness if a membrane is laid over the block floor. The membrane must be lapped and sealed at joints. Any service penetrations through the barrier must also be boxed and sealed to the manufacturer’s instructions.

Masonry Walls
Air leakage has a tendency to occur in cavity walls; this can be remedied by sealing the internal blockwork wall by ensuring the mortar joints are suitably filled. Any penetrations into the blockwork must be sealed appropriately. A parging layer can be used to cover the blockwork (which can then be dry lined afterward).

Cavities must be closed at window and door openings using cavity closers and sealed appropriately.

Joists built into the wall must be sealed as these are considered to be problem areas. They can be sealed with flexible sealant, proprietary joist sealing boxes or sealed backer strips.

All service penetrations into the wall must be sealed, using either flexible sealant, or spray applied foams. Their application must be according to the instructions of the manufacturer.

Timber Frame Walls
The sole plate and floor slab junction must be sealed using gun applied flexible sealant, foam tape, or gasket systems to ensure air tightness.

A vapour control membrane is a suitable material to achieve a good level of airtightness. All joints must be lapped and well-sealed. Any gaps or penetrations into the membrane must be taped or sealed, with any damages being repaired.

Roof and Ceiling
Seal the junction between ceiling boards and the external wall so it forms a continuous air barrier layer. Any recessed lighting that penetrates the airtight layer can be treated with airtight backing caps to ensure a good seal.

For a dropped ceiling, the wall air barrier must be completed before the ceiling is installed.

If the airtight layer is positioned on top of joists, the membrane must be carefully sealed around each joist to maintain continuity with the wall air barrier. Where the airtight layer is below the rafters, pay special attention to rafter penetrations and insulation compression.

The attic is a common source of air leakage. Access points like loft hatches can suffer with air leakage, which can be remedied by using a suitable draught seal. Ensure continuity of the air barrier across ceilings and loft hatches, and that insulation is properly fitted without gaps.

Windows and Doors
Windows and doors often have small gaps in their frames, so weatherstripping is essential. Factory-applied strips are helpful but do not remove the need to seal the unit to the structural opening on site. They must be firmly connected to the air barrier at the head, jambs, and threshold to ensure continuity.

Ensure all windows and doors specified provide a suitable airtight seal when closed. Use tape where the frame meets the structure, and fit draught excluders around external doors to minimise leakage.

Other considerations
Although careful design and detailing are of utmost importance in creating a good airtightness level, much of the work is to be carried out on site. It is recommended that an ‘airtightness champion’ is appointed for the project who can ensure all site operatives are aware of the importance of the airtight barrier throughout the build. The ‘airtightness champion’ will often have a checklist of duties that will assist in achieving this goal, including communications, supervising, site testing, taking readings, checking materials, preparing for testing, and many other duties. This person would be site based to ensure good communication with all sub-contractors and trades visiting the site.