Envelope and protection · Sealants & Membranes
Liquid-Applied Air and Water Control Coating
Describes the fluid-applied wall control layer as a different failure mode from every sheet and tape in this category, and as the wall relative of the liquid roof coating already published.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What liquid-applied air and water barrier is
A liquid-applied air and water control coating is applied wet, by roller, brush or spray, onto the outer face of a sheathing board, a masonry background or a concrete wall, and cures into a continuous elastomeric film bonded to what it was applied to. It does the job that a breather membrane and its tapes do between them: it resists liquid water arriving from outside while remaining relatively open to vapour passing outwards, and it forms the plane at which air movement through the wall is intended to stop.
The difference from a sheet is that there is nothing to lap. A taped sheet is only as continuous as its laps, its tapes, its fixings and its adhesion to whatever it was stretched over; a fluid-applied barrier follows every irregularity and closes around every fixing as it cures. That is what makes it attractive where the substrate is uneven, where the fixing pattern is dense, or where the geometry has more corners than flat areas.
In exchange, the risk moves rather than disappearing. Everything now depends on what cannot be seen once the cladding is on: whether the film went on wet enough, whether the substrate was dry and clean enough, whether the temperature and humidity were within what the manufacturer's documentation describes, and whether board joints and fixings were pre-treated as the system requires. A lap can be lifted and looked at. A cured film can only be believed, on the strength of the record made at the time.
It is also applied as a system rather than out of a single pail: a primer where the substrate calls for one, a joint and fixing treatment, transition materials at openings, penetrations and terminations, and then the field coat. The published liquid roof coating is its close relative — the same idea of a seamless cured film whose result rests on preparation and continuity — put to work on a wall instead of a roof.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Fluid-applied air barrier
- Liquid-applied air barrier
- Spray-applied air barrier
- Fluid-applied water-resistive barrier
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Continuous air and water control over sheathing boards in framed external wall construction, behind a rainscreen or a cladding.
- Control layers over masonry and concrete backgrounds too irregular for a taped sheet to follow.
- Transitions at openings, parapets and penetrations, where the layer has to continue across a change of material.
- The junction between an above-ground wall control layer and a below-ground waterproofing layer at the base of a wall.
- Localised application where a densely fixed or heavily penetrated substrate would defeat a sheet and tape approach.
- Continuations and repairs where an existing control layer has been opened up by later work.
Consideration
Where it is commonly considered
- Where the substrate is irregular, densely fixed or geometrically complicated and a taped sheet would be hard to keep continuous.
- Where the design treats the control layer as a plane that is drawn, walked and inspected before the cladding goes on.
- Where the programme allows the coating to be applied, cured, inspected and recorded before it is covered.
- Where the applicator and the conditions on site match what the manufacturer's documentation describes.
- Where the system's own transition materials are being used at openings rather than improvised on site.
- Where the wall as a whole has been assessed for vapour behaviour with the coating in the position proposed.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where the substrate is damp, frozen, dusty or newly placed and its condition has not been assessed.
- Where the weather on site would not allow the film to be applied and cured as the manufacturer's documentation describes.
- Where it would go over a substrate the documentation does not describe, or over an existing coating of unknown type.
- Where the vapour behaviour of the whole wall has not been looked at, since a coating in the wrong plane changes where moisture accumulates.
- Where nobody is inspecting and recording the application before the cladding covers it permanently.
- Where the same product is being asked to serve as a below-ground waterproofing layer, which is a different discipline entirely.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- It is intended to resist liquid water from outside while remaining relatively open to vapour outwards. Where that plane sits within the build-up is a design question about the whole wall rather than about the coating.
- Substrate dependency
- Bond decides everything: board type, joint condition, dust, moisture content, release agents on concrete, and any existing coating all change whether the film adheres or not.
- Installation dependency
- Wet film thickness, temperature, humidity, curing time and the pre-treatment of joints and fixings are the whole of the workmanship, and none of it can be checked once the cladding is fitted.
- Weathering and exposure
- How long the cured film may stand exposed before it is covered, and what sunlight does to it in the meantime, is set by the manufacturer's documentation rather than by site convenience.
- Repairability
- A damaged area is simply recoated, which is easy while the wall is open and effectively impossible afterwards. The window for repair closes with the cladding.
- Documentation
- The application record — conditions, coverage, dates, who applied it — is the only later evidence that the layer exists in the form it was designed in.
- Appearance
- It is usually a strong colour so that thin areas and missed patches show while they can still be corrected. It is a buried layer, not a finish.
Exposure
Exposure and environmental conditions
- What weather is expected during application, and what does the manufacturer's documentation say about temperature, damp and curing?
- How long will the coating stand exposed before the cladding goes on?
- What is the exposure of this elevation to driving rain once the building is complete?
- What is the drainage and drying path behind the cladding, and does the coating form the surface water runs down?
- Is there a risk of condensation within the build-up, and who has assessed the wall as a whole?
- Are there elevations where the substrate will be wetter, colder or in shade for longer than the rest?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The sheathing board type, its joints and its fixing pattern determine what pre-treatment the system calls for before the field coat.
- Board joints, fixings, internal corners and changes of plane are treated as separate operations rather than absorbed by the field coat.
- Transitions at openings, at the roof junction and at the base of the wall are detailed with the system's own materials.
- Penetrations through the wall are sealed into the coating rather than over the top of it.
- Cladding support brackets and rails pass through the plane, and how they are treated where they do is part of the design.
- Where the coating meets a sheet membrane, a tape or a below-ground layer, the products have to be compatible and the junction drawn.
Appearance
Appearance and finish considerations
- It is a buried layer and not a finish; the colour exists so that coverage and missed areas are visible while the wall is still open.
- Brush, roller and spray leave different textures, and a sprayed film that is back-rolled is a different operation from one that is not.
- Thin areas, holidays and shadowing around fixings are obvious on the open wall and invisible a day after the cladding goes on.
- Overspray onto frames, glazing and adjacent finishes is a real site consideration and a common source of damage claims.
- A photographic record taken before cladding is usually the only image of this layer that will ever exist.
- Where a coating runs out at a termination, the neatness of that edge is what a later inspection will judge the whole application by.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What does the manufacturer's documentation describe for this substrate and for this period of exposure?
- How is film thickness being controlled and checked as the work proceeds?
- What happens where the coating is damaged by trades following on?
- How are brackets, fixings and penetrations made after the coating is applied treated?
- Is the coating compatible with every sealant, tape and membrane it meets?
- Who is on site when conditions change, and who decides whether work stops?
Upkeep
Maintenance and repair considerations
- Is there a record of what was applied, where, in what conditions and by whom?
- If water appears behind the cladding, what can be reached without taking cladding off?
- Who would be called back, and what does any workmanship warranty actually cover?
- Is there a drawing showing the control layer as a continuous plane around the whole building?
- How are future penetrations — a new flue, a bracket, a service — to be made without breaking the plane?
- Does anyone hold the product information for what was used, years after the job?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- It is applied as a system: primer where required, joint and fixing treatment, transition materials, then the field coat.
- Conditions govern the work — substrate moisture, air and surface temperature, humidity, and rain falling within the curing period.
- Coverage is controlled by the applicator, because there is no lap to prove anything after the event.
- Sequencing with the openings, the cladding support and the roof decides whether the plane can be made continuous at all.
- Inspection happens before the wall is covered, because there is no later opportunity of any kind.
- Recoating windows, overcoating and the compatibility of anything applied over it are set by the manufacturer.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's system documentation, naming which primer, joint treatment and transition materials belong with the field coat.
- The applicator's record of conditions, coverage and dates for each elevation.
- A drawing of the control layer plane showing how continuity is achieved at every junction.
- Photographic records of each elevation taken before cladding.
- Compatibility information for every sealant, tape and membrane the coating meets.
- The terms of any warranty offered and the conditions attached to it.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which plane in this wall is the air control layer, and which is the water control layer?
- Why a fluid-applied product here rather than a sheet, and what does that change about how it is inspected?
- What pre-treatment do the board joints and the fixings require?
- How does the coating continue at openings, at the roof junction and at the base of the wall?
- How are cladding brackets and services treated where they pass through the plane?
- Who inspects this layer, at what point, and what is recorded?
- Has the wall's vapour behaviour been assessed with this layer in this position?
- What conditions would stop work on site, and who has the authority to stop it?
Blind spots
Commonly overlooked points
- It removes laps and replaces them with a film thickness nobody can verify later; the risk moves rather than disappearing.
- The product is a system, and the pail of field coat is the part of it that matters least.
- Air control and water control are two different jobs, and a wall can be watertight and still leak air.
- The layer only exists as a plane if it is continuous at openings, at the roof and at the base — and those junctions are where it is usually broken.
- Once the cladding is on, a photograph taken during construction is the only evidence anyone will ever have.
- Every trade that follows can damage it, and the last one to leave the scaffold is rarely the one who applied it.
What this page does not do
- Where a control layer sits within a wall build-up, and how that wall handles vapour and condensation, is design work for the specific building.
- Nothing here states that a coating makes a wall airtight or watertight. A control layer exists only where it is continuous, and continuity is a construction outcome rather than a property of a product.
- Substrate, conditions, curing and compatibility are governed by the manufacturer's documentation for the actual system supplied, and by no general rule.
- Below-ground waterproofing is a separate discipline; a wall air and water control coating is not a tanking product and must not be used as one.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Related entries
Often used alongside
Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Sealants & Membranes
Reference entries for the thin control layers — joint sealants, damp-proof courses and membranes, vapour control layers, breather membranes, tanking systems, tapes and foams.
Browse all sealants & membranes entries →