Water Vapor Permeability and Perm Ratings
Permeability is the property of a substance that permits passage of water vapor. Permeance is the measure of the amount of water vapor (moisture) that can pass through a specified material in a certain amount of time. The measure and degree of permeability is expressed in units referred to as Perm-Inches, or sometimes simply Perms. Materials with high Perm levels will allow more moisture or water vapor to pass through than those with lower Perm values.
While no specific test procedure has been officially recognized, or sanctioned, by the construction industry to date, the following methods are being used by many companies to measure the permeability of mastics.
1) ASTM E-96 [Standard Test for Water Vapor Transmission of Materials]
- Procedure A is Desiccant Method at 73.4°F (23°C)
- Procedure B is Water Method at 73.4°F (23°C)
- Procedure BW is Inverted Water Method at 73.4°F (23°C)
- Procedure C is Desiccant Method at 90°F (32.2°C)
- Procedure D is Water Method at 90°F (32.2°C)
- Procedure E is Desiccant Method at 100°F (37.8°C
2) ASTM E398 [Standard Test Method for Water Vapor Transmission Rate of Sheet Materials Using Dynamic Relative Humidity Measurementis], and
3) ASTM F1249 [Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor]
There are two variations that are commonly used in the mastic: ASTM E96 Procedure A and Procedure E.) As the tests are run, the samples are placed in a constant temperature and humidity controlled area and the sample dishes are weighed to determine if the dish is gaining (desiccant method) or losing moisture (water method). The weight changes are documented over a time period and then calculations are made to determine the water vapor transmission (WVT) and permeability of the material tested.
It is important to keep in mind that, depending on the test method used, reported permeability values can vary substantiality. Also, testing results can also vary widely unless a strictly controlled and consistently applied test protocol is followed.
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Tech Info & Conversion tables
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Vapor Retarders: Assemblies require a vapour barrier to minimize moisture transfer by diffusion to cause condensation. A Canadian spec (CAN/CGSB-51.33-M89) defines Type "1 VB" as Low Permeance (i.e. not greater than 15 ng/Pa.s.m²) to be used where high resistance is required. In other cases, use a VB Type 2 with initial permeance no greater than 45 ng/Pa.s.m² or (60 ng/Pa.s.m² after aging).
Any material with a perm rating higher than 60 ng/Pa.s.m² is not a Vapor Barrier
VAPOUR PERMEANCE:
Water vapour permeance is the rate of water vapour diffusion through a sheet of any thickness of material (or assembly between parallel surfaces). It is the ratio of water vapour flow to the differences of the vapour pressures on the opposite surfaces. Permeance is measured in perms (ng/Pa.s.m2). In metric units (SI), a perm is equal to the transfer of 1 nanogram of water per square metre of material per second under a pressure difference of 1 pascal. In imperial units, a perm is equal to the transfer of 1 grain (0.002285 oz.) of water per square foot of material per hour under a pressure difference of 1 inch of mercury (1.134 ft. of water). Materials which have a sufficient resistance to the flow of water vapour can be used as vapour diffusion retarders (VDR's). VDR's are rated in terms of their permeance. The lower the rating of a material, the more effectively it will retard diffusion.
PERMEABILITY is the measure of the ease with which water vapour passes through a unit thickness of a material.
| VAPOUR PERMEANCE |
| Material |
Metric
(ng/Pa.m²s)
|
Imperial
(grain/ft²hr (in.Hg))
|
| Common Vapour Diffusion Retarders |
| 0.03 mm (1 mil) aluminum foil |
0.0 |
0.00 |
| 0.01 mm (0.35 mil) aluminum foil |
2.9 |
0.05 |
| 0.15 mm (6 mil) polyethylene |
3.4 |
0.06 |
| 0.10 mm (4 mil) polyethylene |
4.6 |
0.08 |
| 0.05 mm (2 mil) polyethylene |
9.1 |
0.16 |
| Asphalt kraft paper facing |
17.0 |
0.30 |
| 0.05 mm (2 mil) unplasticized PVC |
39.0 |
0.68 |
| Paint |
| 2 coats asphalt paint on plywood |
23 |
0.40 |
| 2 coats aluminum paint |
17-29 |
0.30-0.50 |
| 1 coat latex VDR paint |
26 |
0.45 |
| 3 coats white lead/oil paint on wood siding |
17-57 |
0.30-0.99 |
| 3 coats white lead/zinc oxide/oil paint on wood |
51 |
0.88 |
| 2 coats oil-based paint on plaster |
91-172 |
1.58-2.99 |
| Insulation |
| 25 mm (1 in.) extruded polystyrene |
23-92 |
0.40-1.60 |
| 25 mm (1 in.) polyurethane |
69 |
1.20 |
| 25 mm (1 in.) expanded polystyrene |
115-333 |
2.00-5.79 |
| 100 mm (4 in.) rock wool |
1666 |
28.97 |
| 100 mm (4 in.) cellulose fibre |
1666 |
28.97 |
| 100 mm (4 in.) glass fibre wool |
1666 |
28.97 |
| Other Materials |
| 100 mm (4 in.) glazed tile masonry |
6-9 |
0.10-0.16 |
| 3 mm (1/8 in.) cement board with oil paint |
17-29 |
0.30-0.50 |
| 19 mm (3/4 in.) board (wood) |
17-232 |
0.30-4.03 |
| 6.4 mm (1/4 in.) CDX plywood |
40 |
0.70 |
| 100 mm (4 in.) brick |
46 |
0.80 |
| 200 mm (8 in.) concrete block |
137 |
2.38 |
| 6.8 kg (15 lb.) tar paper |
230 |
4.00 |
| 3 mm (1/8 in.) hardboard (standard) |
630 |
10.96 |
| 19 mm (3/4 in.) plaster on metal lath |
860 |
14.96 |
| Spunbonded polypropylene |
884 |
15.37 |
| 12.7 mm (1/2 in.) insulating board |
1150-2875 |
20.00-50.00 |
| Sheathing paper |
1170 |
20.35 |
| Gypsum drywall |
2860 |
49.74 |
| Tyvek® HomeWrap® |
4620 |
77 |
| CommercialWrap® |
1680 |
28 |
|
|
Equivalent Wind Speed |
Equivalent Load |
| Total Pressure Difference (Pa) |
Metric
(km/h) |
Imperial
(m/h) |
Imperial
(lbf/ft²) |
|
| 200 |
65 |
41 |
4.2 |
| 500 |
104 |
66 |
10.5 |
| 1000 |
147 |
92 |
21 |
|
CONVERSION TABLE
1mm = 0.039 in
1 cm = 0.393 in
1 m = 3.27 ft
1 ft = .3048 m
1 m²= 10.76 sq.ft.
1 sq.ft = 0.092 m²
1 US fl oz = 29.57 mL
1 US gallon = 3.785 L
1 L = 33.87 US fl oz
1 g = 0.0353 oz
1 oz = 28.35 g
1 lb = 16 oz = .4536 kg
1 lb = 0.4536 kg
500 Pa = 10.4 lb/ft²
1 RSI = 5.68 R R x 0.176 = RSI RSI 0.8 = R5 1 Pa = 0.0208 lb/ft²
1 L/min = 33.87 US fl oz/min
1 L/sec/m² = 1.46 US gal/min/sq.ft.
1 perm(permeance)= 57.45 ng/(s.m².Pa)
°C = (°F-32)/1.8
DD degrees C = DD degrees F x 5/9
DD degrees F = DD degrees C x 9/5
Barrel: A volumetric unit of measure for crude oil and petroleum products equivalent to 42 U.S. gallons.
COEFFICIENT OF THERMAL EXPANSION OF EXPANDED POLYSTYRENE (IN./(IN.)(F) .000035
The dry-bulb is a thermometer that is dry. The D-B temperature is the temperature that you hear on the weather report or that you read on a thermostat in your house.
The wet-bulb is a regular thermometer with a wet (distilled water) muslin wick covering it, and brisk air flow across the wick (600 feet per minute). As a result of evaporative cooling, the W-B temperature will be cooler than the D-B temperature.
The larger the difference between wet- and dry-bulb temperature, the lower the RH.
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