A unit used by petroleum engineers to express the permeability of a porous
subsurface reservoir. (Sometimes cubic feet are used instead of barrels.)
At 0°C, 1 perm is approximately 5.72135 × 10⁻¹¹
kilograms per newton-second; at 23°C, approximately 5.74525 × 10⁻¹¹
kilograms per newton-second.
Frederick J. Ticknell, O. E. Mechem and R. C. McCurdy.
Some studies on the porosity and permeability of rocks.
Transactions of the AIME, vol. 103, no. 1, December 1933.
A unit used in the construction industry to express permeance: how easily water vapor
can flow through a barrier. The smaller the number, the harder it is for water vapor to get through the
barrier. Architects, building officials and contractors use it as a figure of merit, and it is often encountered in regulations and building codes. To be meaningful, a measurement in perms must be accompanied by a statement of the thickness of the barrier.
Several, differing perms exist.
The US perm is defined as
and the mass is the loss of mass of the container of water.
The metric perm (an unfortunate name, since it is not an
SI unit) is defined as
To convert metric perms to US perms, multiply by 1.51735.
Getting rid of the non-SI units, such as days and millimeters of mercury,
gives us the so-called SI perm:
The actual SI measure of permeance is the kilogram per second per square meter per pascal.
A list of perm values for common building materials is provided at
An interesting PowerPoint presentation at a 2013 ABAA conference:
ASTM E96/E96M - 16. Standard Test Methods for Water Vapor
Transmisson of Materials.
Vapour permeability is a material property, expressed independently of material
thickness, in units of ng/Pa s m, and given the symbol, μ. Vapour permeance
is a measure of the ease of vapour flow through a material layer, in units of
perms (equal to 1 ng/Pa s m2 or 1 grain/(hr·in Hg· ft2)) and given the symbol
M. Permeability and permeance are analogous to thermal conductivity and
thermal conductance respectively. Imperial US perms can be converted to
metric perms by multiplying by 57.1.
Many codes define a vapor barrier as any material or system that has a
permeance of less than 1 US perm. This is an arbitrary value based on a limited
and questionable study conducted in the 1940’s. Vapor diffusion flow through
a wall may need to be controlled with vapor resistant layer in some special
cases, but plastered strawbale walls usually don’t need them, and often appear to
perform much better without them.
Moisture Properties of Plaster and Stucco for Strawbale Buildings.
Available on the internet at the Canada Mortgage and Housing Corporation
Page 5. Reprinted here with the author's permission.
A unit of electric permeance suggested by Vladimar Karapetoff in
1911, but not adopted.
When permittance is measured in farads, elastance is measured in
darafs (see the chapter on the Electrostatic Circuit in the author's
Analogously, when two or more magnetic paths are in parallel
it is convenient to use the reciprocals of the reluctances. The
reciprocal of the reluctance of a magnetic path is called its permeance;
eq. (1) becomes then
Φ = ℘M, (2)
℘ = 1/ℛ. (3)
A script ℘ is used for permeance in order to avoid confusing it with power. For the unit of permeance corresponding to the rel, the author proposes the name perm. A magnetic path has a permeance of one perm when one maxwell of flux is produced for each ampere-turn of magnetomotive force applied along the path.
The unit “perm” has been in use among electrical designers
for some time, although no name has been given to it. Notably Mr. H. M. Hobart has used it extensively in his writings, in the calculation of the inductance of windings. He speaks of “magnetic lines per ampere-turn per unit length” (of the embedded part of a coil). This is equivalent to perms per unit length.
The Magnetic Circuit.
New York: McGraw-Hill 1911.
Printed sources refer to a unit called the “AFS perm”, which apparently measures a property of the sand from which molds are made in foundries (“AFS” is from American Foundrymen's Society, later renamed the American Foundry Society). So far we have not been able to find a definition of this unit in the Society's publications. If you have any knowledge of this topic, please
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Last revised: 26 April 2017.