Vapor retarder
A vapor retarder is a layer that slows the diffusion of water vapor through a wall, floor or roof assembly.
A vapor retarder is a layer that slows the diffusion of water vapor through a wall, floor or roof assembly.
Definition
A vapor retarder is a material that reduces how fast water vapor moves by diffusion through an assembly. Building codes group them into classes based on how strongly they resist vapor, and the right class depends on climate and assembly.
A plain example
In a cold climate, a designer may place a vapor retarder on the warm, interior side of insulation so that indoor moisture does not reach cold surfaces and condense. In another climate or assembly the answer can be different.
Why it matters
Moisture trapped in an assembly can lead to mold, rot and corrosion, so correct placement matters. Adding the wrong layer in the wrong place can trap moisture rather than prevent it, which makes this a design decision.
Common confusion
Vapor retarder, vapor barrier and air barrier are used loosely in the trade. Vapor moves by diffusion, air carries vapor by leakage, and they are controlled differently. Always ask what the product is rated for and where the design wants it.
On a project
Find the vapor retarder class stated in the wall assembly and check it against the code table for your climate zone. Keep the product data on file for the inspector.
Quick answers
- Is a vapor retarder the same as a vapor barrier? The terms overlap in speech. Codes use classes of retarders, which describe how strongly a layer resists vapor.
- Which side of the wall does it go on? It depends on climate and assembly. Follow the design and local code.
- Can a vapor retarder cause problems? If placed wrongly it can trap moisture. That is why placement is decided by the designer.
Related terms
Always check the project drawings and specification, and confirm requirements with the engineer of record and the local authority.
Questions people ask
Is a vapor retarder the same as a vapor barrier?
The terms overlap in speech. Codes use classes of retarders, which describe how strongly a layer resists vapor.
Which side of the wall does it go on?
It depends on climate and assembly. Follow the design and local code.
Can a vapor retarder cause problems?
If placed wrongly it can trap moisture. That is why placement is decided by the designer.
Sources and further reading
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