General construction: how buildings go together
Building envelope layers explained: wall and roof assemblies
A wall or roof is a stack of layers, each with a job: structure, water control, air control, vapor control, insulation and finish. Knowing the jobs makes quotes and drawings easier to read.
A wall or roof is a stack of layers, each with a job: structure, water control, air control, vapor control, insulation and finish. Knowing the jobs makes quotes and drawings easier to read.
The layers and their jobs
| Layer | Job |
|---|---|
| Cladding or roof covering | Sheds most rain, takes sun and wear |
| Drainage gap or underlayment | Lets water that gets in run out |
| Water-resistive barrier | Last line of defense against liquid water |
| Sheathing | Bracing and a base for layers |
| Insulation | Slows heat flow |
| Air barrier | Stops air movement |
| Vapor control layer | Limits vapor diffusion, placed by climate |
| Interior board | Finish, fire protection, sometimes the air barrier |
One material can do more than one job, but each job needs a named layer.
Order matters
Layers must let moisture escape toward the side where it dries. See moisture in walls.
Reading drawings
A good wall section names every layer, its thickness and its job, and details the joints. If a drawing cannot be read this way, ask for a section detail before pricing.
Buying the layers
See buying guides for cladding, sheathing, barriers and steel-frame envelope coordination.
Roofs follow the same logic
Covering, underlayment, deck, insulation, air and vapor control and the ceiling, with ventilation or an unvented design.
Quick answers
- What is the difference between an air barrier and a vapor barrier? The air barrier stops air flow; the vapor layer slows vapor diffusion. They are different jobs.
- Can one layer do everything? Rarely. Some products combine jobs, but check each job is covered.
- What is a wall section? A drawing cut through the wall showing all layers.
Keep reading
- Moisture and condensation in walls
- Air barrier materials buying guide
- Exterior cladding buying guide
- Insulation types and where they go
Always check the project drawings and specification, and confirm requirements with the engineer of record and the local authority.
Questions people ask
What is the difference between an air barrier and a vapor barrier?
The air barrier stops air flow; the vapor layer slows vapor diffusion. They are different jobs.
Can one layer do everything?
Rarely. Some products combine jobs, but check each job is covered.
What is a wall section?
A drawing cut through the wall showing all layers.
Sources and further reading
Have a project outline or drawings?
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