Building envelope · Control Layers
Vapour Diffusion Control System
This entry treats vapour control as a relationship between the resistances of successive layers and the drying paths left open, so a reader can see why an identical membrane can help one build-up and harm another.
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 vapour control system is
Vapour diffusion control is a property of an order, not of a sheet. What matters is the sequence of resistances through a build-up: which layer resists diffusion most, what sits either side of it, and whether moisture reaching a cold surface within the assembly has anywhere to go afterwards. Two assemblies built from identical materials in a different order are different systems.
Every construction becomes wet at some point, from weather during the works, from water used in building it, from a leak, or from vapour driven through it in use. The useful question is therefore not whether moisture arrives but whether the assembly can dry once it has, and in which direction. Where layers of high resistance sit on both sides of the insulation, the diffusive routes out are closed in both directions, and whether that matters in a given build-up is a question for the professional assessing it.
Whether a higher-resistance layer is wanted at all, and which side of the insulation it would belong on, depends on the climate, on the internal moisture conditions of the building and on the rest of the build-up. This entry states no position on that question, no resistance figure and no climate rule; assessing the risk of interstitial condensation is work for a qualified professional reading the whole assembly.
The system most often confused with this one is the air barrier, and the confusion has consequences. A sheet can be a competent vapour control layer while contributing nothing to resisting air movement, and a well-sealed enclosure can still have both of its drying routes closed. The test is what is being looked at: gaps, laps and joints belong to the air barrier, while the relative resistance of layers in sequence belongs here.
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.
- vapour control strategy
- vapor diffusion control
- vapour retarder system
- moisture diffusion control
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.
- Set the relative resistance of layers so that vapour movement through an assembly is a designed matter rather than an incidental one.
- Keep at least one drying route open so that moisture arriving inside the build-up has somewhere to go.
- Support an assessment of where within a build-up the risk of interstitial condensation is likely to sit.
- Give the internal moisture conditions of the building a defined relationship to the construction enclosing them.
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 6. Order is meaningful.Control layerHigher-resistance layer
The layer nominated to slow diffusion through the build-up, whether a membrane, a faced board, a foil surface or a dense applied coat. Where it sits in the sequence is the design decision; its mere presence tells a reader almost nothing about how the assembly behaves.
- Layer 2 of 6. Order is meaningful.Thermal layerInsulation and the temperature gradient
The insulating layer sets where within the build-up the temperature falls away, and therefore where vapour arriving from the warm side is likely to meet a cold surface. Moving the insulation moves that point without altering any other material in the assembly.
- Layer 3 of 6. Order is meaningful.SubstrateLayers either side and their resistances
The sheathing, lining, render or covering inboard and outboard of the nominated layer. Vapour behaviour is read from the whole ordered set of resistances, so these are components of the system rather than context sitting around it.
- Layer 4 of 6. Order is meaningful.Cavity or voidVentilated or drained void
A void connected to outside air that lets moisture leaving the assembly be carried away. Where a build-up relies on drying outward, that void and its openings are working parts of the vapour strategy rather than a convenience of construction.
- Layer 5 of 6. Order is meaningful.Control layerCapillary-active or buffering layer
Some build-ups include layers that absorb moisture, redistribute it and release it later rather than resisting its passage. Where such a layer is present the assembly behaves differently over a season, and that behaviour has to be recognised in any assessment.
- Layer 6 of 6. Order is meaningful.Jointing and sealingContinuity at laps and penetrations
Where the nominated layer is interrupted at a joist end, a lap or a service box, moisture can bypass it by travelling with air rather than by diffusing. Continuity therefore matters here even though this layer is not being asked to stop air on its own.
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 nominated layer and the insulation only make sense read together. The insulation decides where the build-up becomes cold; the nominated layer decides how much vapour reaches that point. Placed on the cold side of the insulation it becomes the surface where arriving moisture is most likely to condense; placed on the warm side, less arrives, but everything outboard of it must then be able to release whatever does get through.
Resistances act in sequence, so the relationship between them matters more than the value of any one. An assembly can be made to behave worse by adding a resistant layer outboard of the insulation even when nothing else has changed, because a drying route that used to be available has quietly been closed. That is precisely why a membrane cannot be judged from a page about the membrane.
A ventilated void is the release valve for outward drying. If the covering above it is closed and the void is unventilated or has been blocked, by insulation pushed into it, by debris at an opening, or by a later added layer, the assembly keeps the resistance of the outer covering with none of the drying it was designed around. Whether ventilation is wanted at all is an assembly-specific determination.
Diffusion and air movement interact rather than substitute for one another. Moisture carried by air through a gap can arrive far faster than moisture diffusing through material, so a nominated layer left open at joist ends or service boxes can be bypassed entirely. The two systems fail together, which is why one sheet is often asked to do both jobs and why that combination has to be a deliberate choice rather than an accident of specification.
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
This group spans both higher and lower resistance to diffusion, and members of it are commonly encountered on either side of an insulation layer. Which resistance is wanted, and where, is a determination for a qualified professional assessing the whole build-up.
Thermal layer
Insulating families are commonly encountered in this position and differ markedly in how they interact with moisture. Whether any of them fits a particular assembly is a matter for the manufacturer's documentation and for the designer.
Substrate
Sheathing boards are commonly encountered outboard of insulation, and their own resistance to diffusion forms part of the sequence being assessed. None of that resistance is stated here, and it belongs with the product documentation.
Finish surface
Internal finishes and coatings are commonly encountered on the warm face and carry resistance of their own, which is why redecoration can change an assembly. Whether a given finish belongs on a given build-up is a question for the designer.
Jointing and sealing
Sealing components of these families are commonly encountered where a nominated layer is lapped or interrupted. Compatibility with the sheet and with the surface behind it is established from the documentation for both, not from the role.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
The air barrier line
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 Air Barrier System →Ventilated roof void above insulation
Where an assembly dries upward and outward into a roof void, the ventilation of that void is part of this system. Insulation pushed into the ventilation path, or an opening that later becomes blocked, removes the drying the build-up was designed around.
Read about Roof Void Ventilation →Retrofit lining on an existing solid wall
Adding insulation to the inner face of an existing wall makes the original masonry colder and wetter than it was, and changes which directions remain open for drying. This is the case where a vapour assessment of the existing construction matters most.
Read about Internal Wall Insulation →Cold deck flat roof
In a cold deck the insulation sits between or below the joists and an air space is kept clear above it, beneath the deck, so what dries this assembly outward is a designed ventilated void rather than anything in the covering. What makes it consequential here is that the void is a feature which can be closed off, obstructed or left out, and the sequence of resistances then has to hold without it.
Read about Cold Flat Roof →Drained and ventilated outer leaf
A ventilated cavity outboard of the sheathing gives a wall a route to dry outward that a closed outer skin does not. Whether that route is being relied on changes what resistance is acceptable in the layers behind it.
Read about Rainscreen Facade →
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
- Whether interstitial condensation is a risk in a given build-up depends on the whole sequence of layers, the internal conditions and the climate, and is assessed by a qualified professional rather than inferred from any component.
- Thermal
- Because the insulation sets where the assembly becomes cold, changing the insulation position or the layers around it changes the vapour question as well, even where the thermal intent is unchanged.
- Durability
- Assemblies that hold moisture rather than releasing it place organic components, fixings and finishes under conditions they may not have been chosen for, and the effects appear gradually rather than as a single event.
- Interfaces
- The nominated layer stops at joist ends, party junctions, service boxes and openings, and what is done at each of those places determines whether the sequence assessed on paper exists in the built wall.
- Documentation
- Recording which layer was nominated, where it sits and what was assumed about internal conditions is what lets a later alteration be judged, since the next person to add a lining will otherwise be working blind.
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 layer in this build-up is intended to be the most resistant to diffusion, and where does it sit relative to the insulation?
- In which direction is this assembly expected to dry, and what would close that route if it were added later?
- Whether the internal moisture conditions assumed in the assessment match how the building will actually be used is worth settling early.
- Whether an existing construction has been surveyed before insulation is added to it changes what any assessment is worth.
- Whether one sheet is being asked to control both diffusion and air movement should be a stated decision rather than a coincidence.
- Whether any ventilated void the assembly relies on can be kept clear over time is a design question, not only a maintenance one.
- Whether a later change of internal finish could alter the sequence of resistances is worth recording for whoever redecorates.
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A vapour control layer is assumed to be beneficial everywhere, when the same sheet can close the only drying route a build-up had.
- The terms breathable and vapour open are used as though they described a single property, when they cover a wide range of behaviours.
- Vapour control is treated as interchangeable with air sealing, when the two are judged complete by entirely different tests.
- Condensation is imagined only as water forming on a visible surface, when the consequential case forms inside the build-up where nobody sees it.
- Adding insulation is treated as a change to one property, when it also moves where the construction is cold and how it dries.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Has an interstitial condensation assessment been carried out for this build-up, and what internal conditions did it assume?
- Which layer is nominated as the higher-resistance one here, and why is it on that side of the insulation?
- In which direction can this assembly dry, and what in the construction would prevent it?
- How is the nominated layer kept continuous at joist ends, party junctions and service penetrations?
- If the existing wall or roof already contains a resistant layer, how has that been taken into account?
- What would change in this assessment if the building were used more humidly than assumed?
- Does any part of this build-up rely on ventilation, and how is that ventilation path kept open?
What this page does not do
- No layer makes an assembly safe from condensation on its own, and no value or position is stated here because the answer depends on the whole build-up and its climate.
- Retrofitting insulation to an existing construction changes its moisture behaviour, and that change should be assessed before work rather than observed afterwards.
- Assemblies described as breathable still require assessment, since the term covers a wide range of behaviours and says nothing about the sequence of layers.
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.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
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 →