There are many methods of testing breathability but they don't all give the same results making comparisons between different manufacturers difficult. Here's a summary of the various methods
The Upright Cup Method, ASTM E96, uses a cup of water, sealed with a fabric, which is weighed before and after to determine how much liquid has been lost. The weight of water lost per area of fabric per period of time gives a breathability rate in g/m²/24hrs.
The Inverted Cup Method, ASTM E96, works in the same way but the cup is inverted so that the water sits against the fabric. This produces a higher breathability figure, as the fabric isn’t buffered by an air gap, which can absorb a surprisingly large amount of moisture. This method may favour hydrophilic coatings (also applied to many microporous laminates) which work best when wet.
Results using this method are typically 5 times those of the Upright Cup!
The Desiccant Inverted Cup Method, JIS 1099, uses a liquid desiccant to move water through the fabric. Again the cup is weighed to produce results that can be more than 20 times those produced by the first method!
Evaporative Resistance can be measured using ISO 11092. This involves a “sweating hot plate” which is electrically powered and controlled by a thermostat to maintain a steady temperature. The amount of power it uses to maintain that temperature defines how much energy has been lost through the fabric covering it by evaporation. The results are read in m²PaWֿ¹, and the lower the figure the better the breathability.
This figure may also be converted to a percentage of the breathability of a standard piece of material, such as gauze.
The Dynamic Moisture Permeation Cell, ASTM F 2298, uses dry and water-saturated nitrogen streams passing over each side of the fabric. Controlling flow rates, temperature and humidity of the two flows, and measuring the humidity afterwards, gives figures for the water vapour transmission rate or the fabrics resistance.
Results may be 4 to 6 times those of the Upright Cup!
Problems with Breathability Testing
Apart from the Upright Cup Method, these tests are statistically correlated. However, the actual breathability rates can be dramatically different, so it is impossible to compare one company’s claims with another’s without knowing the test method and conditions.
Each of these tests relies on a controlled climate of temperature, humidity and flow. Small discrepancies in these factors can skew the results because the experiments must be carried out over relatively long periods of time. For example the ASTM E 96 methods must be run over 1 to 3 days!
The upright cup method has a substantially lower breathability because the air between the fluid and the waterproof retains a large proportion of the evaporated liquid. However this is analogous to the typical insulation worn beneath a shell and demonstrates the humidity capacity of insulation layers.
The main problem with these tests is that they usually don’t mimic real situations. Often a breathable barrier is cooled by water running over it, maybe at a temperature only a few degrees above freezing, with a significant wind chill and insulated from the main heat source (the body) by a thick mid layer! In such conditions breathability is reduced dramatically, sometimes to zero! However, tests are being developed using sweating mannequins in wind chilled rain rooms to better measure the performance of real garments.
As normal breathability testing mimmics a typical day in the Sahara, Manchester University are testing FurTech garments to research how they breathe in wet weather conditions, when breathability is really needed.
Here's a link to a site detailing some scientific user trials of waterproof breathables http://www.sailgb.com/sshop/tech_info.asp?ID=305
Thanks to SailGB and Julie Gretton for the above link.
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