CalcKit
Permeability Converter
Table of Contents
What is permeability?
Water vapour permeability describes how easily water vapour passes through a material, such as a wall board, a roofing membrane, a paint film or a plastic sheet. It is one of the key figures in building science, where it decides whether moisture can dry out of a wall or gets trapped inside it and causes condensation, mould and rot.
Two closely related figures are used:
- Permeance is the rate at which vapour passes through a given layer of material, whatever its thickness. Its unit is the perm: one grain of water vapour per hour, through one square foot, for each inch of mercury of vapour pressure difference.
- Permeability is a property of the material itself, independent of thickness: the permeance of a layer of standard thickness. Its unit is the perm-inch, the permeance of a layer one inch thick.
A material with a low perm rating holds vapour back. Building codes call a material with a rating of 0.1 perm or less a vapour barrier (Class I), from 0.1 to 1 perm a vapour retarder (Class II), and from 1 to 10 perms a semi-permeable layer (Class III).
This tool converts between the perm and perm-inch units, each at the two reference temperatures used in testing, 0 °C and 23 °C.
Permeability units
- Permeability (0°C): the perm rating based on the inch of mercury measured at 0 °C (32 °F), the original definition of the unit.
Conversion Factor: 1 Permeability (0°C) (Base Unit) - Permeability (23°C): the perm rating based on the inch of mercury measured at 23 °C (73.4 °F), the temperature of the standard ASTM E96 test.
Conversion Factor: 1 Permeability (23°C) = 1.00418 Permeability (0°C) - Permeability inches (0°C): the perm-inch, the rating for a layer one inch thick, based on the inch of mercury at 0 °C.
Conversion Factor: 1 Permeability inches (0°C) = 0.0254 Permeability (0°C) - Permeability inches (23°C): the perm-inch based on the inch of mercury at 23 °C.
Conversion Factor: 1 Permeability inches (23°C) = 0.025506 Permeability (0°C)
The factor between the two kinds of unit, 39.37, is the number of inches in a metre: in this tool, a value in the Permeability units is taken per metre of material, and a value in the Permeability inches units per inch.
Why the temperature matters
The perm is defined through the inch of mercury, the pressure of a column of mercury one inch tall. Mercury expands as it warms, so a one-inch column is slightly lighter at 23 °C than at 0 °C, and the inch of mercury is about 0.42% smaller.
Because the perm divides by that pressure, a perm at 23 °C is about 0.42% larger than a perm at 0 °C, and the same material has a slightly lower rating in 23 °C perms. The difference is small, but test reports and product data sheets state which one they use, so values should be converted before they are compared.
Frequently asked questions
What is a perm?
A perm is a unit of water vapour permeance: one grain of water vapour passing through one square foot of material per hour, for each inch of mercury of vapour pressure difference across it. In SI units, 1 perm (0 °C) is about 57.2 nanograms per second, per square metre, per pascal.
What is the difference between a perm and a perm-inch?
A perm measures permeance, the vapour flow through a layer as it is, whatever its thickness. A perm-inch measures permeability, a property of the material: the permeance of a layer exactly one inch thick. A material's permeance is its permeability divided by its thickness in inches.
How many perms is a vapour barrier?
A Class I vapour barrier, such as polyethylene sheet or aluminium foil, has a rating of 0.1 perm or less. A Class II vapour retarder, such as kraft-faced insulation, is between 0.1 and 1 perm, and a Class III retarder, such as latex paint, between 1 and 10 perms.
How do you convert perms at 0 °C to perms at 23 °C?
Multiply by 0.99584. To go the other way, from 23 °C to 0 °C, multiply by 1.00418.