To test the thermal insulation properties of a blanket, the first step is to test the material's R-value. R-value is a measure of relative thermal resistance. However, there's more to measuring the thermal resistance of the blanket than simply testing its R-value, because blankets need to endure conditions that thermal wall insulation, for example, would never have to encounter.
Measuring Thermal Resistance
Thermal resistance is measured by the R-value. A high R-value indicates more effective insulation. The factors that work together to determine the R-value of the type of material are its density and its overall thickness. To test the R-value of your blanket, you'll need a swatch of the blankets you are testing, some mason jars filled with warm water, some twine to secure the fabric swatches around the jars, and a freezer. You'll also need a control material with a known R-value from which to form the basis of your comparison. Place all of the wrapped jars into the freezer, testing their temperature at equal intervals and noting the differences. Then you can perform calculations based on your control in order to determine the R-value of each given material.
Stress Testing Blankets
Thermal blankets need to withstand a variety of different conditions, not all of which are ideal. They also need to be able to protect people even in the worst-case scenario of being wet. To test the effectiveness of your blankets, take swatches as used in the basic thermal resistance test and wrap jars filled with warm water in the cloth after it has been submerged in cold water, again testing the temperature loss over time. Some fabrics perform differently depending on whether they are wet or dry. Natural fibres such as wool are able to better retain their insulation value when wet, while synthetic options often lose their insulating properties when wet. This isn't always the case though, as evidenced by a neoprene wetsuit.
Qualitative Analysis and the EN 13537 Standard
Beyond their basic thermal resistance values, the human factor also needs to be measured. A standard called the EN 13537 Standard was designed by the European Outdoor Group in 2002 in an attempt to better communicate the ideal operating standards of sleeping bags and other thermal blankets. The statistics offered by this new standard take into account the temperature differences of the male and female bodies, with women being slightly colder. The standard measures the upper-limit temperature, which is the maximum at which an average adult man is able to sleep comfortably wrapped in the blanket without severe sweating. The comfort rating is based upon the average temperature at which a woman would find the blanket comfortable. Also measured is the lower temperature limit at which an average male's ability to have a comfortable night's sleep is affected. The standard also measures the extreme survival rating, which is the blanket's ability to protect the average woman from hypothermia in extreme conditions. It is important to note that the survival rating is a measure of extreme usage and should not be relied upon for general use.