Key takeaways
- Four control layers: water, air, vapour, thermal. All four have to be somewhere in the build-up.
- Continuity beats thickness — the weakest point governs the assembly.
- Vapour layer position depends on climate, and getting it wrong traps moisture inside the wall.
- Draw the layers on a section and trace each one around the whole building without lifting the pen.
Continue exploring all sustainable and climate-responsive design guides.
Four things to control
Every exterior assembly — wall, roof, floor, junction — has to manage four flows. They are separate problems and each needs a layer, though one material can sometimes serve two.
Liquid water. Rain, run-off, splash. Controlled by shedding surfaces, a drainage plane and flashings. This is the largest and fastest-acting risk.
Air. Uncontrolled air movement carries far more heat and moisture through an assembly than diffusion does. Controlled by a continuous air barrier.
Water vapour. Moisture moving by diffusion through materials. Controlled by a vapour retarder, positioned for the climate.
Heat. Controlled by insulation.
The order they appear, and which side of the structure they sit on, follows from the climate. What does not vary is that all four have to be present and continuous.
Continuity beats thickness
This is the single most useful idea in envelope design, and the one most often lost during value engineering.
An assembly performs at its weakest point, not its average. Thick insulation interrupted by steel fixings, a slab edge and an uninsulated window reveal delivers substantially less than its calculated value. A continuous air barrier at a modest specification outperforms an excellent one with gaps at every floor line.
The practical test is the pen test: on a drawn section, trace each control layer around the whole building without lifting the pen. Every place you have to lift it is a discontinuity that needs a designed detail — a slab edge, a window perimeter, a roof-to-wall junction, a service penetration, a balcony.
Where each layer usually sits
| Layer | Common position | Failure symptom |
|---|---|---|
| Water control | Outboard, as a drainage plane behind the cladding | Staining, damp interiors, decay in the assembly |
| Air control | Continuous, often combined with the water layer | Draughts, heat loss far above calculation, moisture in the wall |
| Vapour control | Climate-dependent side of the insulation | Condensation within the assembly, mould, corrosion |
| Thermal | Outboard of the structure where possible | Cold surfaces, condensation at bridges, high running cost |
Placing insulation outboard of the structure keeps the structure warm and reduces bridging, which is why continuous external insulation performs so much better than the same amount between studs.
The layers interact
They are not independent, and most real failures come from the interaction:
- Air leaks carry vapour. A gap in the air barrier moves far more moisture into a wall than diffusion through the vapour layer ever will. Sealing the air barrier is usually the higher-leverage moisture measure.
- Insulation changes where condensation forms. Adding insulation moves the dew point within the assembly. Adding it on the wrong side can create a condensing surface where there was not one.
- A vapour-tight layer on both sides traps moisture. An assembly needs to dry in at least one direction. Sealing both faces is a common and damaging mistake, particularly in renovation.
- Water control depends on drying. Everything eventually admits some water; whether that matters depends on whether the assembly can dry. See ventilated facade vs direct-applied cladding.
Climate decides the arrangement
There is no universal correct build-up. Cold climates drive vapour outward for most of the year; hot humid climates drive it inward; mixed climates reverse seasonally and need assemblies that tolerate both directions.
This is why importing a detail from another region is risky, and why the climate data has to come first. See climate-responsive facade strategies.
What to check on a project
- Are all four layers identified on the section, by name?
- Does each pass the pen test around the whole building?
- Is every discontinuity — slab edge, window, roof junction, penetration, balcony — drawn as a detail?
- Is the vapour layer positioned for the actual climate, in both seasons?
- Can the assembly dry in at least one direction?
- Is the insulation continuous, and where are the remaining bridges?
For where envelope work sits relative to the other decisions, see passive design basics and the sustainable and climate-responsive design guide.
Frequently asked questions
What is the difference between an air barrier and a vapour barrier?
An air barrier stops bulk air movement carrying heat and moisture through the assembly. A vapour retarder slows moisture diffusing through materials. One material can do both, but they are different functions and confusing them causes real failures.
Which side should the vapour layer go on?
It depends on climate and on which direction vapour predominantly moves. In cold climates it usually sits toward the warm interior; in hot humid climates the drive can reverse. Mixed climates need an assembly that tolerates both, which is why details do not transfer between regions.
What is a thermal bridge?
A path where heat bypasses the insulation — a steel fixing, a concrete slab edge, a stud, a balcony. Bridges reduce the assembly's real performance below its calculated value and create cold surfaces where condensation forms.
How do I know if my envelope is continuous?
The pen test. On a section, trace each control layer around the entire building without lifting the pen. Wherever you have to lift it — at a slab edge, a window, a roof junction — is a discontinuity that needs a detail.
Sources
- Building Envelope Design Guide — Whole Building Design Guide, National Institute of Building Sciences Accessed August 28, 2026.
- Opaque Envelope — Building Technologies Office, U.S. Department of Energy Accessed August 28, 2026.