Permeability Converter
Print pageAll Equivalents Reference Table
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Water Vapor Permeability (also known as Moisture Transmittance Coefficient, Vapor Permeance per Unit Thickness, or Porous Diffusion Rating, symbolized by δ or Wp) measures the rate of water vapor mass transmission through a material of unit thickness per unit area under a unit vapor pressure differential (˙m = δ · A · (ΔP ÷ L) ⇒ Permeance M = δ ÷ L = ˙m ÷ (A · ΔP), where ˙m is moisture mass flow rate, ΔP is vapor pressure difference across the barrier, A is surface area, and L is material thickness). Across architectural building envelope design, basement wall waterproofing, roof underlayment moisture control, HVAC insulation jacketing, pharmaceutical blister packaging, and civil hydrogeology, water vapor permeability is categorized across three major engineering unit families: International System of Units (SI metric fundamental: Kilogram per Pascal Second Sq. Meter / kg/(Pa·s·m2), Nanogram per Pascal Second Sq. Meter / ng/(Pa·s·m2)), US Customary Building Science standards (US Perm (23°C / 73.4°F), US Perm (0°C / 32°F)), and Thickness-Normalized Material Permeability metrics (Perm-Inch (23°C), Perm-Inch (0°C)).
Our free online Permeability Converter provides instant, high-precision conversions across all SI metric, US Perm (ASTM E96 standard), perm-inch, and building envelope moisture transmission units:
- US Perm (23°C / 73.4°F) to kg/(Pa·s·m2) [ASTM E96 Standard]:
1 perm (23°C) = 5.74525 × 10-11 kg/(Pa·s·m2) = 57.4525 ng/(Pa·s·m2) = 0.0574525 mg/(Pa·s·m2). - US Perm (0°C / 32°F) to kg/(Pa·s·m2):
1 perm (0°C) = 5.72135 × 10-11 kg/(Pa·s·m2) = 57.2135 ng/(Pa·s·m2). - Perm-Inch (23°C) to kg/(Pa·s·m2) [Material Permeability Standard]:
1 perm-in (23°C) = 1.45929 × 10-12 kg/(Pa·s·m2) = 1.45929 ng/(Pa·s·m2) = 0.0254 perm (23°C). - Perm-Inch (0°C) to kg/(Pa·s·m2):
1 perm-in (0°C) = 1.45322 × 10-12 kg/(Pa·s·m2) = 1.45322 ng/(Pa·s·m2). - Kilogram per Pascal Second Sq. Meter to US Perms:
1 kg/(Pa·s·m2) = 17,405,683,000.0 perms (23°C) = 17.4057 × 109 perms. - Nanogram per Pascal Second Sq. Meter to US Perms: Multiply ng/(Pa·s·m2) by
0.01740568(100 ng/(Pa·s·m2) = 1.74057 perms (23°C)).
Master Water Vapor Permeability & Permeance Conversion Table
The table below displays exact mathematical conversion relationships, SI kg/(Pa·s·m2) multipliers, and US Perm (23°C) equivalents relative to 1 US Perm at 23°C (1 perm = 57.4525 ng/(Pa·s·m2)):
| Permeability / Permeance Unit Name | Symbol | Exact Value in kg/(Pa·s·m2) | US Perm (23°C) & ng/(Pa·s·m2) Equivalent | Domain & Technical Application Standard |
|---|---|---|---|---|
| 1 US Perm (23°C / 73.4°F) | perm (23°C) |
5.74525 × 10-11 kg/(Pa·s·m2) |
1.0 perm (57.4525 ng/(Pa·s·m2) / 1.00419 perm (0°C)) |
US ASTM E96 Standard Water Vapor Permeance Metric |
| 1 US Perm (0°C / 32°F) | perm (0°C) |
5.72135 × 10-11 kg/(Pa·s·m2) |
0.99584 perm (23°C) (57.2135 ng/(Pa·s·m2)) |
Freezing Temperature Water Vapor Transmission Metric |
| 1 Perm-Inch (23°C / 73.4°F) | perm-in (23°C) |
1.45929 × 10-12 kg/(Pa·s·m2) |
0.02540 perm (23°C) (1.45929 ng/(Pa·s·m2)) |
1-Inch Thickness Normalized Material Vapor Permeability |
| 1 Perm-Inch (0°C / 32°F) | perm-in (0°C) |
1.45322 × 10-12 kg/(Pa·s·m2) |
0.02529 perm (23°C) (1.45322 ng/(Pa·s·m2)) |
Freezing Temperature Material Permeability Standard |
| 1 Kilogram per Pascal Second Sq. Meter | kg/(Pa·s·m2) |
1.0 kg/(Pa·s·m2) (Base SI Unit) |
17.4057 × 109 perms (1.0 × 1012 ng/(Pa·s·m2)) |
SI Fundamental Physical Permeability & Diffusion Metric |
| 1 Nanogram per Pascal Second Sq. Meter | ng/(Pa·s·m2) |
1.0 × 10-12 kg/(Pa·s·m2) |
0.017406 perm (23°C) (0.68530 perm-in (23°C)) | European Metric Building Physics Permeance Unit |
Step-by-Step Wall Assembly Vapor Transmission Calculation Example
To convert an unfaced Extruded Polystyrene (XPS) rigid foam insulation board with a material permeability rating of 1.10 Perm-Inches (1.10 perm-in at 23°C) into permeance (perms) for a 2-inch thick board (L = 2.0 inches) and SI ng/(Pa·s·m2):
Step 1 (2-Inch Board Permeance Calculation): Permeance M = Perm-Inches ÷ Thickness = 1.10 perm-in ÷ 2.0 in = 0.55 US Perms (0.55 perms)
Step 2 (SI Permeability Conversion): δ = 1.10 perm-in × 1.45929 = 1.6052 ng/(Pa·s·m2) (1.605 × 10-12 kg/(Pa·s·m2))
Step 3 (SI Permeance Conversion): M = 0.55 perms × 57.4525 = 31.599 ng/(Pa·s·m2)
Thus, the 2-inch XPS insulation board has a permeance of 0.55 perms (Class II Semi-Impermeable Vapor Retarder) and a material permeability of 1.605 ng/(Pa·s·m2).
Real-World Building Envelope Vapor Barrier Permeability Classifications (ASTM E96)
Below is a comparative reference chart showing vapor permeance ratings across building materials under ASTM E96 classifications:
| Building Membrane / Insulation Material | Vapor Permeance in US Perms (23°C) | SI Metric Equivalent (ng/(Pa·s·m2)) | International Building Code (IBC) Permeability Class |
|---|---|---|---|
| 6-Mil Polyethylene Vapor Sheeting / Foil Facing | ≤ 0.06 perms | ≤ 3.447 ng/(Pa·s·m2) | Class I Vapor Impermeable (Vapor Barrier) |
| Asphalt-Coated Kraft Paper Facing / 2-Inch XPS Foam | 0.10 – 1.0 perms | 5.745 – 57.453 ng/(Pa·s·m2) | Class II Vapor Semi-Impermeable Retarder |
| 1/2-Inch Exterior Plywood / Latex-Painted Drywall | 1.0 – 10.0 perms | 57.453 – 574.525 ng/(Pa·s·m2) | Class III Vapor Semi-Permeable Retarder |
| Commercial Tyvek Housewrap Weather Barrier | 50.0 perms | 2,872.6 ng/(Pa·s·m2) | Vapor Permeable Air & Water Weather Barrier |
| 3.5-Inch Unfaced Fiberglass Insulation Batt | > 100.0 perms | > 5,745.25 ng/(Pa·s·m2) | Highly Breathable Open Insulation Cavity |
History & Physics: 1856 Henry Darcy Law vs ASTM E96 US Perm Standard
1856 Henry Darcy & Porous Media Flow (q = -k ÷ μ · ∇P)
In 1856, French hydraulic engineer Henry Darcy published Les Fontaines Publiques de la Ville de Dijon, formulating Darcy’s Law for fluid filtration through sand beds: q = -k ÷ μ · ∇P. Darcy introduced intrinsic material permeability k (measured in Darcy or m2), forming the physics foundation for vapor diffusion through porous building envelopes.
ASTM E96 Standard Test Method for Water Vapor Transmission
In the mid-20th century, the American Society for Testing and Materials established ASTM E96 (“Dry Cup” desiccant and “Wet Cup” water methods), defining the US Perm (1 grain of water vapor per hour per square foot per inch of mercury vapor pressure difference). At 23°C (73.4°F), 1 US Perm = 57.4525 ng/(Pa·s·m2).
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Frequently Asked Questions (FAQ)
How do you convert US Perms (23°C) to ng/(Pa·s·m2)?
To convert US Perms to metric ng/(Pa·s·m2), multiply perms by 57.4525. For example, a 0.50 perm Kraft paper facing × 57.4525 = 28.73 ng/(Pa·s·m2).
What is the difference between Permeability and Permeance?
Permeance (M in Perms or ng/(Pa·s·m2)) measures moisture flow through a specific finished wall layer of given thickness. Permeability (δ in Perm-Inches or ng/(Pa·s·m)) is an intrinsic material property normalized for 1 unit of thickness: Permeance = Permeability ÷ Thickness.
What is a Class I Vapor Barrier?
Under the International Building Code (IBC), a Class I Vapor Barrier is defined as a material having a permeance of 0.10 perms or less (≤ 5.745 ng/(Pa·s·m2)), such as 6-mil polyethylene plastic sheeting or foil facings.
Why does temperature affect perm ratings (0°C vs. 23°C)?
The US Perm definition relies on saturation vapor pressure of water, which varies non-linearly with temperature. At 23°C (73.4°F), 1 Perm = 57.4525 ng/(Pa·s·m2); at 0°C (32°F), 1 Perm = 57.2135 ng/(Pa·s·m2).