What is Water Vapor Transmission Rate and Why Should You Care?
Water Vapor Transmission Rate (WVTR) measures how much water vapor passes through a material over a specific time and area. This is critical for packaging, construction, and textile industries where moisture control is essential.
Understanding WVTR helps you select appropriate materials for moisture-sensitive applications.
How to Calculate WVTR
The formula for Water Vapor Transmission Rate:
[\text{WVTR} = \frac{\text{Weight Change (g)}}{\text{Area} \times \text{Time (days)}}]
Where:
- Weight Change is the mass of water vapor transmitted (grams)
- Area is the exposed test area (m² or 100 in²)
- Time is the test duration in days
Calculation Example
A packaging film sample has these test results:
- Weight change: 0.5 g
- Test area: 0.01 m²
- Duration: 1 day
[\text{WVTR} = \frac{0.5}{0.01 \times 1} = 50 \text{ g/m}^2\text{/day}]
WVTR Values for Common Materials
| Material | WVTR (g/m²/day) |
|---|---|
| Paper/Cardboard | 200-500 |
| LDPE (Low-Density Polyethylene) | 10-20 |
| HDPE (High-Density Polyethylene) | 3-8 |
| PET | 10-15 |
| Aluminum Foil | < 0.1 |
| Metalized Film | 0.5-2 |
Values at 38°C and 90% RH
Applications
- Food Packaging: Prevent moisture absorption or loss
- Pharmaceutical: Protect hygroscopic drugs and tablets
- Electronics: Shield sensitive components from humidity
- Construction: Select appropriate vapor barriers
- Textiles: Design breathable but weather-resistant fabrics
Test Conditions
Standard WVTR testing is typically performed at:
- Tropical: 38°C (100°F), 90% RH
- Temperate: 23°C (73°F), 75% RH
Always specify test conditions when comparing WVTR values.