Introduction
It seems widely accepted that elevated air permeability (the ease with which ambient air can penetrate jacket fabric) in a lightweight windshirt can provide effective moisture vapor removal during backpacking and hiking. In prior studies, I have made the case that activities that occur at low speeds, such as backpacking, do not provide sufficient air pressure on the face of a jacket to support significant convective cooling or adequate moisture removal by means of windshirt fabric air permeability. Rather, in the absence of winds, one must rely on ventilation provided by jacket openings such as pit zips or the front zipper.
In this study, I compare the performance of four jackets made with fabrics that span a wide range of air permeability rates and moisture vapor transmission rates (MVTR). The study shows that significantly greater moisture removal can be achieved as a function of jacket MVTR than jacket air permeability. In fact, the ratio of moisture removal in the jackets tested due to MVTR exceeds that of air permeability by a factor of nearly 7 to 1. This means that when we select a windshirt or even a waterproof breathable shell, we should pay close attention to vapor transmission characteristics if we wish to obtain effective moisture removal. This study also demonstrates that a high MVTR waterproof/breathable shell can provide better moisture removal than a typical windshirt. This means that you can have a single layer that functions as both a rain jacket and a windshirt. In short, MVTR is a performance characteristic that should receive lots of attention when selecting your next wind layer or rain jacket.
Background
During exercise, your body will eliminate excessive heat by sweating. How effectively your clothes allow sweat to be removed will determine how well sweating accomplishes its cooling function. Effective elimination of moisture from sweat will also avoid accumulating condensed water vapor in various clothing layers.
Sweat must evaporate to provide cooling and the resulting water vapor must be then removed from all garment layers. Water vapor removal is typically accomplished through convection and/or moisture vapor transmission.
Convection describes moisture vapor removal by means of air circulation within a layer or through a layer. Convection can be enhanced by garment ventilation features such as pit zips, openings at the neck, sleeve, or hem, or air movement through pores in the garment fabric. Convection is driven by air pressure differences across the garment layers.
Moisture vapor transmission describes the transfer of moisture through one or more garment layers. It can occur at the garment ventilation features mentioned above. Moisture vapor transmission also can take advantage of tiny openings in garment fabric, both pores and spaces between fibers, to expel moisture. Vapor transmission is driven by the vapor pressure difference between the skin and the ambient environment. Vapor pressure is a function of both temperature and relative humidity. A high temperature at the skin, combined with high humidity will produce the pressure gradient necessary to expel moisture vapor to an ambient environment that has a lower temperature and/or humidity.
The ease with which air can move through a garment is termed air permeability. Air permeability is typically characterized by measuring the volume of air that can pass through a fabric at a known air pressure differential across the fabric. The ability of a garment to support moisture vapor transmission is often termed breathability or vapor permeability. Of course, the term breathability is sometimes used by some to include air permeability, so confusion can be expected when breathability is not clearly defined.
There are many ways to measure a garment’s ability to transfer moisture vapor. Two widely used measurement approaches produce very different and not necessarily comparable measurement data: moisture vapor transmission rate (MVTR) and evaporative resistance. MVTR is measured as grams/meter2/24 hours. MVTR test methods tend to promote evaporation of water from a reservoir, through a piece of fabric, to the ambient environment. The other main approach is often called the skin method. It uses a device called a sweating guarded hot plate and produces results in units of Evaporative Resistance. Both general approaches are guided by several available test standards. The results of different test standards are not necessarily in good agreement and will almost always result in different magnitudes of vapor transfer rates. In this study, MVTR is determined by measuring the quantity of moisture that passes through a garment at a vapor pressure differential of 0.3 psi using devices that I’ve designed called permeation kettles.
The relative effectiveness of air permeability and MVTR for removing moisture from garments has not received a great deal of attention here at Backpacking Light. In this study, I look at the relative effectiveness of both simultaneously.
In a previous study published in 2001 at Backpacking Light, the author went on runs at similar exertion levels and conditions using different shells. The shells were utilized under sealed conditions or ventilated conditions. The subject athlete wore a wool base layer under the shells for each run. At the end of the run, he weighed the base layer and compared its weight to the dry weight of the base layer. The weight difference, of course, was sweat accumulated in the base layer. The author reasoned that less accumulated sweat in the base layer indicated improved moisture transfer for the test garment. Based on his tests, he concluded that higher exertion exercise could overwhelm the ability of any waterproof/ breathable jacket or any windshirt to remove moisture. He found that only a combination of ventilation and adjustment of insulation or activity level could effectively ensure adequate removal of moisture during higher exertion exercise.
I decided to try a similar study, but the jackets would span the extremes of air permeability and vapor transmission levels. I would then apply statistical measures to attempt to parse the impact of these and other characteristics on the jackets’ abilities to remove moisture. One of the nice features of the 2001 Backpacking Light study and my test methodology is that anyone with fairly rudimentary equipment but high enough motivation can conduct their own version of this test.
Test Design
Four jackets were selected for this test. Table 1 below provides their characteristics.
Table 1: Test Jacket Characteristics
| Jacket | Fabric | weight (grams) (see note 1) | Air Permeability (CFM/ft^2 @ 0.5" wc) (see note 1) | MVTR (grams/m^2/24 hr) (see note 1) |
|---|---|---|---|---|
| Montbell Peak Dry Shell | Gore Shake Dry | 237 (see note 2) | <.43 | 3370 |
| Patagonia Houdini | Dense weave nylon | 107 | 0.6 | 2250 |
| Patagonia Houdini Air | Dense weave nylon | 121 | 14.3 | 3120 |
| Arcteryx Squamish 2019 | Dense weave nylon | 157 | 11 | 2580 |
Table Notes:
- All measurements made with in-house instruments
- Extra weight due to the addition of custom pit zips.
The Montbell Peak Dry Shell is constructed from a waterproof, breathable Gore Shakedry fabric and is not typically considered a windshirt. It is virtually air-impermeable with air permeability that is lower than I can measure. It has the highest MVTR of any waterproof breathable (WPB) garment I have tested. The Patagonia Houdini Air ranks seventh highest out of 19 windshirts or windshirt fabrics that I have tested for air permeability. It is near the top of general-purpose windshirts in terms of air permeability and MVTR while still offering some wind protection. The 2019 Patagonia Houdini has very low air permeability and has the second-lowest MVTR of the windshirts I have tested. The 2019 Arc’teryx Squamish has somewhat middle-of-the-road air permeability and MVTR performance.
The base layer worn for the test is a long sleeve shirt made by Xoskin. This garment is constructed using a nylon 3D seamless knit fabric with embedded PTFE and copper in the fibers. This garment is skintight which means that perspiration cannot easily drip down the skin; rather, it will be absorbed into the fabric until saturation is reached. This garment offers some of the best wicking/drying performance of any base layer I have tested, making it an ideal base layer for this test. The dry weight of the Xoskin shirt is 6 ounces (167 g).
Each of these jackets was worn during a series of runs. Four runs were conducted for the Squamish. Three test runs were completed for each of the other three jackets. The run takes place on a 4.9-mile (8 km) circular trail located in a large open space. The trail has minor elevation changes. During the runs, all zippers, hems, and cuffs were closed to minimize pumping air exchanges. The hoods were worn and tightly sealed. During the run, a Garmin Fenix (version 5) along with a heart rate monitor chest strap was used to collect physiological data. The average MET level for each run was calculated using average heart rate data and results of metabolic testing I underwent at the University of Colorado Sports Medicine and Performance Center (Boulder, CO). Weather data was obtained using NOAH statistics from the Vance Brand Airport, located approximately 2 miles from the center of the running loop. The data is published online at approximately 15-minute intervals. The weather data corresponding to the beginning and end of the run are averaged.
Water retained in the base layer was weighed on an A&D SJ-2000HS digital scale. The scale resolves 1 gram.
The runs cover a range of temperature, humidity, and wind conditions. Of course, this being Colorado, the highest humidity during a run was only 67%. The range of environmental conditions can be seen in the test results table below.
Test Results
Test results are presented in Table 2. The results for individual runs are listed by date. The critical measured data for each run is the water weight gain of the Xoskin base layer, shown in column 3. Performance data for each run is shown in columns 4, 5, 6 and 7. Columns 8-12 show environmental data for each run.
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