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Building enclosure control layers · Lesson 1 of 4

Building enclosure control layers: principles and terminology

The four control layers of a building enclosure (water, air, vapor and thermal), the order BSC gives them, where they can sit, why continuity is the rule at the roof-to-wall junction, and where AFS sheet metal fits beside them.

You will be able to: Name the four control layers and their order of importance, say where they sit and why each must be continuous from roof to wall, and state who designs, supplies and verifies them.

10 min · 11 slides · 8 questions

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Slide 1 of 11

Four control layers, one order

Building Science Corporation (BSC) presents four control layers in order of importance: water, air, vapor, thermal.

Its 2010 insight calls the first one the rain control layer; the layer is the same.

Each layer controls one flow across the enclosure: rain, air, vapor or heat.

Four layers, in order of importanceFour stacked cards in order: water control, which stops rain and liquid water; air control, which stops air flow in and out; vapor control, which slows water vapor diffusion; thermal control, which slows heat flow. Each card carries its line style, solid blue, dashed green, dotted teal and amber hatch. A navy band says keep the rain out first and that moving air can carry far more moisture than diffusion does.Four layers, in order of importanceWater controlStops rain and liquid waterFirstAir controlStops air flow in and outSecondVapor controlSlows water vapor diffusionThirdThermal controlSlows heat flowFourthKeep the rain out first.Moving air can carry far moremoisture than diffusion does.Order of importance from Building Science Corporation.

Four stacked cards in order: water control stops rain and liquid water; air control stops air flow in and out; vapor control slows water vapor diffusion; thermal control slows heat flow. Each card shows its line style. A navy band says keep the rain out first and that moving air can carry far more moisture than diffusion does.

The order BSC gives the four control layers, and the moisture point from WBDG: air leakage can move several orders of magnitude more water than diffusion.

Sources

Videos for this lesson

What this video shows: Building Science Corporation on the four layers of the enclosure in order of importance (water, air, vapor, thermal) and why they are easiest to connect when all are on the outside. Checked against the video's description.

Video: The Perfect Wall. Finally. by BuildingScience-com Corporation, on YouTube. Chosen from the video's published description.

Glossary

Control layer
One or more materials acting together to control one flow across a building enclosure. BSC and Oak Ridge National Laboratory name four: water (rain), air, vapor and thermal.
Air barrier system
BSC: a system of materials designed and constructed to control airflow between conditioned and unconditioned space.
Vapor barrier
BSC: a Class I vapor retarder, meaning 0.1 perm or less. A vapor retarder retards the movement of water by vapor diffusion; it is not typically meant to stop air.
Thermal bridge
The BC Hydro bridging guide: part of the building envelope where otherwise uniform thermal resistance is changed. Parapets and slab edges are examples.