Vapour Resistance

Reference data and engineering information about vapour resistance for electrical applications.

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Overview

Engineering reference data for Vapour Resistance in electrical engineering.

Key Formulas

Power

$P = VI = I^2R = V^2/R$

Electrical power.

Variables

Symbol	Description	Unit
$V$	Voltage	V
$I$	Current	A
$R$	Resistance	Ω
$P$	Power	W

Vapor resistivity values for common building materials.
Material	Vapor Resistivity (GNs/kg·m)
Brickwork	45 - 70
Concrete blocks, light weight	15 - 150
Render	100
Plaster, cement	75 - 205
Wood, pine	45 - 1850
Plywood	150 - 2000
Fiberboard	15 - 375
Hardboard	230 - 1000
Plasterboard	30 - 60
Straw-board, compressed	45 - 70
Wood-wool, slab	15 - 40
Polystyrene, expanded	100 - 750
Glass wool	5 - 7
Phenolic, closed cells	150 - 750
Gloss paint, average	40 - 200
Polyethylene sheet	110 - 120
Aluminum foil	4000

Definitions

Vapor Resistance (Vr): The total resistance a material offers to the diffusion of water vapor, measured in N·s/kg. It is a property of the material layer as a whole.

Vapor Resistivity (νr): An intrinsic property of a material that describes its resistance to vapor diffusion per unit thickness, measured in N·s/(kg·m). It is a key parameter for calculating the total resistance of a layer.

Practical Insights

The vapor resistance of a building assembly is critical for managing condensation risk within walls, roofs, and floors. Materials with very high vapor resistivity (like aluminum foil) act as vapor barriers, while materials with low resistivity (like glass wool) are vapor-permeable.

The formula $V_r = t \cdot \nu_r$ shows that total resistance increases linearly with material thickness. When analyzing moisture risk, the vapor resistance values of individual layers are summed to assess the overall assembly's resistance profile.

References

Original Source

Source: engineeringtoolbox.com