Building envelope · Control Layers
Air Barrier System
This entry describes the air barrier as a single line traced around a whole enclosure, so that a reader can see why its behaviour is set by connections and changeovers rather than by the resistance of any component sheet.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What air barrier system is
An air barrier is not a product but a line. It is the connected set of surfaces a designer nominates to resist bulk air movement between the occupied interior and outside, together with everything that joins those surfaces to one another. On a section it should be possible to trace that line around the enclosure without lifting the pen, and the same should hold on plan.
The nominated plane changes as it travels. It may be a taped sheathing board across the walls, a wet plaster coat behind a lining, a membrane in the roof and the structural slab at the base. Because each of those planes belongs to a different trade and often to a different work package, the barrier is defined less by what carries it than by whether the changeovers between them were designed at all.
The confusion this entry exists to remove is with the vapour diffusion control system. An air barrier resists air moving through gaps; a vapour control layer resists moisture diffusing through material. A fully taped breather membrane is an air barrier while remaining open to vapour, and a vapour control layer left loose-lapped and unsealed at its edges is not an air barrier at all, however high its resistance to diffusion.
Because performance belongs to the connections rather than to the components, the line is normally verified by pressurisation testing of the finished enclosure rather than by inspecting materials. Anything that will be buried behind finishes therefore has to be complete, checked and recorded before it is covered over.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- airtightness layer
- air control layer
- air-sealing system
- airtight line
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Resist uncontrolled air movement between the occupied interior and outside across the whole of the enclosure.
- Give the ventilation strategy a defined enclosure to act within, rather than leaving air exchange to chance.
- Reduce the quantity of moisture that moving air can deposit in the cold parts of a construction.
- Provide one traceable line that drawings, work packages and testing can all refer to without ambiguity.
- Allow the completed enclosure to be tested and, where a fault appears, to be located and repaired.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 7. Order is meaningful.Control layerNominated barrier plane
The surface chosen to carry the line through each part of the enclosure, whether a sheathing board, a membrane, an applied coat or the structure itself. Its identity matters less than whether it is named and drawn continuously from one element into the next.
- Layer 2 of 7. Order is meaningful.Jointing and sealingLap and joint treatment
Tapes, sealants, bonded strips and compressed gaskets that close the seams between adjacent pieces of the barrier plane. These are the parts most often left to site judgement, and they are where the line is most often broken.
- Layer 3 of 7. Order is meaningful.Edge and terminationChangeover to the adjoining plane
The detail that carries the line from one nominated plane into the next, such as wall sheathing into roof membrane or wall lining down to floor slab. Each changeover is a designed junction rather than a place where two trades happen to meet. The general mechanism of a changeover - the bond chain, peel against shear, differential movement and the window in which it can still be verified - belongs to the control layer transition system; what this entry owns is that a changeover is nominated at all, and that the plane either side of it is named.
- Layer 4 of 7. Order is meaningful.Service zonePenetration and service seals
Collars, grommets, sealed enclosures and sleeves where cables, pipes, ducts and flues cross the line. How many exist is set by the services layout, so a barrier drawn without reference to that layout will be perforated after the fact.
- Layer 5 of 7. Order is meaningful.AttachmentBonding and mechanical restraint
Primers, adhesives, battens and clamped fixings that hold the barrier against the wind pressure acting on it. A seam stuck to dust, frost or a damp surface behaves as an unbonded seam whatever the tape wrapped over it.
- Layer 6 of 7. Order is meaningful.SubstrateSupporting background
The board, blockwork, plaster or structure behind the barrier that gives it a stable, clean and continuous surface to bond against. Where that background is missing, across a joist zone or over a soft insulation face, the barrier has nothing to bear on.
- Layer 7 of 7. Order is meaningful.ProtectionProtection and recorded access
Sequencing and coverings that keep the finished line from being cut, punctured or abraded by following trades, together with the photographic record that lets a later fault be found without opening the whole build-up.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The barrier plane and its joint treatment are not two components but one, since neither has meaning without the other. A board of high resistance to air movement contributes nothing across an open seam, and the most capable tape contributes nothing where it bridges a gap wide enough to leave it unsupported. What the pair achieves at any point is set by the weaker of the two there.
Changeovers govern the whole. Where the plane passes from a sheathing board into a roof membrane, the two materials move differently, present different surfaces and usually belong to different installers. If the detail does not say which laps onto which and what bonds them, the line simply stops at the changeover, and the enclosure behaves as though the barrier ended there even though both planes are individually complete.
Services and the barrier compete for the same plane. Every socket enclosure, waste pipe, extract duct and cable route that crosses the line turns a designed surface into a set of holes, and if the barrier is drawn only in section then the services layout decides where those holes fall. Putting a service void inboard of the line changes the interaction, because most terminations then stop in the void and only services that must pass outward reach the barrier.
Background and bonding decide whether any of this survives. Tapes and sealants transfer wind pressure into whatever they are stuck to, so the bond available at a seam is set by the condition of the background beneath it, and a friable, dusty or wet surface gives that seam little to develop against. The dependency runs the other way at movement joints, where the barrier has to be free to move with the structure instead of being stretched across a joint that will open.
Protection is a continuity matter rather than a housekeeping one. Once the line is buried, a puncture made by a following trade is indistinguishable from a design fault when a test finds leakage, and the search then runs over the entire enclosure rather than over the one area that was disturbed.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Control layer
Membrane, board and wet-applied families are all commonly encountered carrying the barrier plane itself. Whether a particular product can serve that role in a given build-up is a matter for the manufacturer's documentation and for the designer.
Jointing and sealing
Sealing components of these families are commonly encountered at laps, perimeters and penetrations. Compatibility between a tape, a sealant and the surface each is asked to bond to is established from the documentation for both products rather than assumed from the role.
Substrate
Backgrounds of this kind are commonly encountered behind a nominated barrier plane. Whether a given surface is sound, dry and clean enough for a particular adhesive or tape is set by the product documentation and by the designer, not by the family name.
Edge and termination
Termination components of these kinds are commonly encountered where the line meets a frame, a threshold or an opening. Their behaviour depends on joint geometry and on expected movement, both of which are project-specific and belong with the designer.
Components
Building components that fill these roles
The discrete elements commonly occupying each role in this assembly. This is a different statement from the material families above: those name what a role is commonly made of, these name the positioned elements that occupy it. Commonly encountered, never recommended.
Control layer
- Inner Leaf →Commonly the plane on which the wall's air barrier is formed, where a continuous parge or membrane is applied.
- Window Frame →Every frame is a place the air barrier must be continued onto rather than stopped at.
- Sash →Every opening light is a deliberate break in a continuous air barrier, closed by compression seals rather than by the barrier itself.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
Changeover between nominated planes
Wherever the barrier moves from one plane to another it becomes a transition detail with two substrates, two installers and usually two work packages. The transition system owns the geometry of that handover; this entry owns the requirement that the handover exists at all.
Read about Control Layer Transition →Services crossing the line
Ducts, flues, pipes and cables that pass outward turn the barrier into a set of designed holes. The sealing detail at each has to accommodate the movement, temperature and vibration of the service passing through it, which is why a generic seal repeated everywhere rarely suits every crossing.
Read about Penetration Sealing →Window and door perimeters
The gap between a frame and the structural opening is a continuous slot around every opening, and the barrier has to be carried from the wall plane onto the frame across it. Because the frame arrives later than the wall, the connection depends on what was left ready for it.
Read about Window Opening Interface →Intermediate floor zone in a framed wall
Where a floor is built into an external wall, the barrier plane on the inner face is interrupted by the floor zone and has to be carried around or through it. The detail has to be resolved before the floor is decked, because afterwards the zone is unreachable.
Read about Timber Joisted Upper Floor →Wall head into roof
At the eaves the wall barrier, the roof barrier, the insulation line and any ventilation path all converge in a confined space that is usually built by roofing trades. Which layer runs through and which stops has to be decided in the detail rather than on site.
Read about Eaves System →Void inboard of the line
A void formed between the barrier and the internal finish moves most first-fix work off the barrier plane. It changes what has to be coordinated, the depth available for services and the point at which the line can be inspected before it is closed.
Read about Service Void →Timber Platform Frame
The band of horizontal timber between storeys is where the barrier has to cross a junction that will move, be penetrated by joist ends and be buried early in the programme. It is the position where continuity is most often lost and least easily inspected afterwards.
Read about Timber Platform Frame →Post and Beam Frame
Any member passing from inside to outside carries the barrier line across a permanently moving junction. An exposed frame therefore multiplies the number of these crossings, and each one is a detail rather than a repetition of the same detail.
Read about Post and Beam Frame →Mass Timber Structure
The joints between plates are numerous, repetitive and covered early, which makes them the natural line for the barrier and the easiest place to lose it. Sealing is applied as the structure goes up rather than revisited once the building is enclosed.
Read about Mass Timber Structure →Structural Insulated Panels
Panel joints, the base plate line and every penetration are where continuity of the enclosure is either achieved or lost. The field of a panel contributes almost nothing to the difficulty, and the perimeter contributes nearly all of it.
Read about Structural Insulated Panels →Suspended Timber Ground Floor
Sealing at deck level is a length of the whole building's air barrier, so it has to join the wall's line rather than stop at the skirting. Where the two are not connected, the perimeter becomes the leak the rest of the work was meant to close.
Read about Suspended Timber Ground Floor →Vapour Control System
Where one sheet is asked to control both diffusion and air movement, it has to satisfy the stricter definition of complete. A lap that is adequate for diffusion control is not necessarily adequate for the air line, and the drawing has to say which convention applies.
Read about Vapour Control System →Structural Thermal Break
Has to be carried around the projecting element, since the structure passing through cannot close the line itself.
Read about Structural Thermal Break →Masonry Veneer Wall
The membrane is frequently asked to carry air control as well as water shedding, which changes the requirement from lapping to sealing. Where that dual role is intended, every penetration made by an anchor, bracket or service becomes part of the air line.
Read about Masonry Veneer Wall →Timber Frame Wall
The wall's control layer is one length of a line that also runs through floors, the roof and the ground floor construction. Where that line changes plane, the wall's layer has to be joined to something else, and those changeovers are outside the wall's own scope.
Read about Timber Frame Wall →Precast Facade Panels
A facade of separate panels cannot be continuous by itself, so continuity is carried by the backing wall and closures behind. Where those are absent at a floor edge, the joint between panels becomes the only line between inside and outside.
Read about Precast Facade Panels →Curtain Walling
The frame carries the air control line across the facade, but at every abutment it has to be handed onto a wall, a slab or a roof. That handover involves two trades and two materials, and it is where an otherwise continuous line is most often left open.
Read about Curtain Walling →Cold Flat Roof
The ceiling plane has to join the air barrier of the walls somewhere, usually at the wall head where the ceiling, the wall lining and the roof structure all meet. A ceiling barrier that stops at the plaster line leaves the void connected to the wall construction.
Read about Cold Flat Roof →Cold Pitched Roof
The ceiling barrier has to meet the wall barrier somewhere around the perimeter, usually at the wall head where the ceiling, the lining and the roof structure converge. Where it simply stops at the plaster, the void is connected to the wall construction.
Read about Cold Pitched Roof →Roof Void Ventilation
The envelope-wide air barrier decides how much internal air can be delivered into the void. A circuit designed against a continuous barrier is being asked to do something quite different from the same circuit under a leaky ceiling.
Read about Roof Void Ventilation →Framed Partition
An internal partition running into an external wall interrupts whatever continuous layers the external wall carries. The abutment has to let those layers run past or be joined to them, and a partition cavity opening into an external wall cavity is a route neither designer intended to create.
Read about Framed Partition →Cantilevered Balcony
The air barrier of the wall and floor has to be closed around the projecting element. The detail is small, is built where structure, insulation and finishes converge, and cannot be judged by looking at the completed elevation from outside.
Read about Cantilevered Balcony →Mass Timber Wall Build-Up
The barrier has to continue off the wall panel onto the floor, roof and ground construction. Each changeover is from a timber face onto something else, which is where a barrier made of the structure itself is most vulnerable and where the taping sequence has to be planned before the panels go up.
Read about Mass Timber Wall Build-Up →Sheeted Metal Wall
The liner is the wall's contribution to a barrier that has to continue onto the roof, down to the floor and around every opening. Every one of those changeovers is from a profiled sheet onto something with a different shape, which is the recurring difficulty of making a profiled liner airtight.
Read about Sheeted Metal Wall →Masonry Partition
An internal partition running into an external wall interrupts whatever continuous layers that wall carries. A masonry partition bonded into the inner leaf makes that interruption a permanent one, so whether the layers run past the partition or are joined to it is settled by the envelope designer rather than at the abutment.
Read about Masonry Partition →
Internal junctions
Junction conditions inside this system
Junctions between the components within this assembly, rather than at its boundary. An entry exists only where the condition between two elements governs how the assembly behaves — two parts touching is not on its own a junction worth naming.
Inner Leaf to Wall Plate
Where a bedded wall plate meets the top of the inner leaf, spreading roof and floor bearings and tying the wall head to the structure above.
Structural transfer · Thermal continuity · Air leakage · Moisture · Buildability
Insulation Layer to Parapet
Where the thermal layer of the roof and the thermal layer of the wall arrive at a wall that continues past the roof, and have to meet round it.
Thermal continuity · Moisture · Air leakage · Buildability
Mullion to Transom
Where the vertical and horizontal members of a glazed facade cross, and where the facade's internal drainage has to change direction.
Drainage · Air leakage · Structural transfer · Movement
Slab Edge to Spandrel Panel
Where the edge of a floor meets the opaque panel concealing it, forming a continuous slot at every storey that several separate planes have to cross.
Fire continuity · Thermal continuity · Acoustic continuity · Air leakage · Movement
Window Frame to Window Sash
Where the moving light is carried by the fixed frame and closes against it, which is the line at which a window's air and water tightness is decided.
Air leakage · Movement · Weathering · Moisture · Buildability
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- Air moving through a gap carries moisture into cold construction far more readily than diffusion through the material around it, which is why an air barrier is a moisture measure as much as a comfort or energy one.
- Buildability
- Most of the line is completed long before finishes, by trades working from different drawings. A joint that cannot be reached once the next layer is up has to be sequenced deliberately rather than promised and then chased.
- Interfaces
- The boundaries between work packages, frame to roof, wall to floor, structure to opening, are where the line is most often left unowned. Naming an owner for each is a project management act as much as a technical one.
- Fire
- Where the line crosses a compartment or separating element, the sealing components used there are governed by fire design carried out separately. This entry states no rating or class, and the tested detail and the relevant authority govern.
- Documentation
- A drawn and photographed record of the completed line, made before it is covered, is what makes a later fault findable. Without it, the search for leakage runs over the whole enclosure instead of over the part that was disturbed.
- Maintenance and access
- Later work crosses the line: a new socket, an extract fan, a loft hatch, a rooflight, a rewire. Whether the household and its trades know the line is there determines whether it is reinstated after each of them.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Which plane carries the barrier through each element, and where the plane deliberately changes, belongs on the drawings before any work starts.
- Whether the line can be traced continuously in section and on plan, including around stairs, thresholds and party junctions, is worth testing on paper first.
- Whether a service void is formed inboard of the line changes how many penetrations the barrier itself has to accept.
- Who owns each stretch of the line across the work packages determines who is accountable where two packages meet.
- When the line will be inspected and recorded, relative to the point at which it is covered, is a programme decision as much as a technical one.
- Whether the ventilation strategy has been designed alongside the barrier rather than after it determines how the enclosure behaves in use.
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- An air barrier and a vapour control layer are treated as the same thing, when a taped breather membrane can be one without being the other.
- Sealing is read as a finishing activity, when most of the line is buried well before any finishing trade arrives on site.
- Leakage in a finished enclosure is assumed to be a materials problem, when the fault is almost always a junction, a penetration or a changeover between planes.
- Air movement is treated as a comfort issue alone, when the moisture that moving air deposits in cold construction is often the more consequential effect.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which surface carries the air barrier in each part of this enclosure, and where exactly does the plane change?
- How is the line detailed where it passes from the walls into the roof and down into the ground floor construction?
- Has the services layout been coordinated with the barrier, and is a service void being formed inboard of it?
- What testing or inspection is proposed, at which stage of the programme, and who witnesses and records the result?
- How would a fault found after completion be located and repaired without opening up finished construction?
- Which sealing components are being relied on at movement joints, and what movement have they been chosen to accommodate?
What this page does not do
- No single component makes an enclosure resistant to air movement; the behaviour belongs to the completed and correctly connected assembly.
- Reducing uncontrolled air movement without a designed ventilation strategy changes how moisture and indoor air behave, and the two belong together.
- Where the line crosses a compartment or separating element, the provisions there are determined by fire and acoustic design carried out separately by qualified professionals.
- Older buildings often rely on incidental air movement, and altering that in a retrofit is a decision for a professional who has assessed the whole construction.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Commonly built alongside
Systems routinely present in the same building without a junction recorded between them and this one. Where two systems do meet, the junction is set out above instead.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Building Envelope · Envelope
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Envelope Control Layers and Continuity
The continuous planes running around an enclosure — air, vapour, water and heat — and the interruptions that decide whether that continuity actually exists.
Browse all control layers entries →