EXECUTIVE SUMMARY – You want the highest possible HH to begin with because silnylon degrades rapidly from handling. You can restore the HH head close to the “when new HH” by applying a coating of silicone diluted with mineral spirits. The optimal ratio was not tested for.
I updated my original post to include an unused 2011 MLD Serenity Shelter’s silnylon in my test results plus a photo of the test head at its HH limit. The 2011 MLD silnylon fabric tests 1,195 mm H2O. This test, as well as all others, was repeated three times on widely different parts of each shelter. Only the average value is given. The standard deviation for three different areas averaged only~70 mm and so the average value is most relevant.
Yes, I have experienced "leakage" in field conditions which some portray as "condensation" and some as "misting". My most vivid memory, but by no means my only silnylon leakage memory, was in my then new DuoMid which I ordered 10/12/09. About one month later, after it arrived, I took it on a coastal CA kayak trip during the peak of the winter storm season. The forecast called for heavy rain and wind resulting in the group's head count dropping from about 20 people down to 2. I thought what great weather to test out my new DuoMid. I was one of the two people who spent a week out in the very stormy weather. I was sheltered by the DuoMid with a Titanium Goat bivy inside. The bivy’s mosquito net was over my face but still the cold water hitting my face through the netting prevented me from sleeping until I strapped my wide brim Tilley hat over my face and breathed through its side ventilation.
The manufactures’ spec on average is 1,200mm H2O HH for silnylon. Jim Wood and Roger Caffin’s prior tests seem to concur with the validity of this “when new” value. The “when new” values caused me concern and my “when used” values caused me even more concern.
-The consequences of misting-
It is desirable to avoid condensation and there have been many BPL articles explaining ways to minimize this phenomenon. Basically the articles explain that it wets the surface of your clothing or sleeping bag if you rub against it. Misting and its consequences seem to be less well understood. A fine mist flash evaporates after absorbing the heat from your warm micro-climate, effectively reducing the ambient temperature. One gram of water mist evaporation gives the same cooling effect as the melting of seven grams of ice. Worst case it can cool the local air temperature significantly. Misting in a cool weather shelter is the antithesis of thermal efficiency. In simple words, “IT IS UGLY AND IT SHOULD NOT BE ALLOWED” (smile).
-Rain never causes enough pressure to cause silnylon leakage explanation-
Three pieces of information appear to be at odds with this explanation.
1) When I conduct hydrostatic head tests, micro droplets form on the side opposite the pressure. During a rain storm, it would be impossible to differentiate from a micro droplet of moisture caused by condensation versus a micro droplet of water from pressure leakage.
2) Raindrops commonly vary from about 0.6mm to 6mm. Uncommonly, the rain drops are in the 9 – 10mm range. The larger the raindrop size then the higher is its terminal velocity and its’ resultant kinetic energy. The common range of 0.6mm to 6mm raindrop kinetic energy variance is greater than a 5,000 fold difference. I will avoid the resultant force calculations because the math is quite complex because the variables are the thickness of the water film, the angle the raindrop hits, the peak versus average force, and how much the silnylon moves from the force.
3) Conventional double wall tent manufactures determined by experimentation that if the fly has a HH of at least 1,500mm, then the inner tent prevented any customer complaints.
-The HH figure provided by your shelter manufacturer is rarely tested by the user when new (they typically don’t know what HH they started with) and it will always decrease with use (they don’t know what it is currently)-
1) In all my years of reading BPL I cannot recall a shelter review in which the manufacturers’ HH claim was ever verified with calibrated equipment. In some cases, even the shelter manufacturer doesn’t do a calibrated HH test; they just specify what their fabric supplier specifies. As a result, we don’t know what HH we start with.
2) Every time you stuff your shelter in your pack and then un-stuff it from your pack, the HH will be reduced from whatever it started at.
The Belgian Army has conducted an extensive long-term study of parachute wear on systems used by the school. They were very interested in the effect of high altitude deployments possibly accelerating the performance degradation they were experiencing on their main canopies. From the first part of their study, they concluded that there was no direct relationship between the exact number and type of jumps and the degree of increase in porosity measured, though the porosity increase was great enough to affect performance. They also concluded that the handling during packing was much more detrimental to the parachute than the actual deployment and use.
This led the Belgian Army to question their beliefs about their reserve parachutes, "which we assumed would stay brand new forever." In checking some reserves in their systems, they found porosity readings as high as 18 cfm in some areas of a parachute after only 30 repacks had been performed. A full 32% had porosity readings of more than 9 cfm... The original mil specification, now a PIA specification, for the fabric used on this particular parachute requires 0-5 cfm when new.
-I am as confused as many of you-
*Although I now know the HH of my shelters now, I have no idea what they were when new.
*I don’t know the degradation curve with use for the materials used in my various shelters. Furthermore I don’t know the relative contribution of stuffing versus UV degradation.
*I don’t know what the most efficient remedial action is to add HH to a degraded shelter. Ideally the HH of a flat tarp should be measured when new and then periodically monitored until either the degradation stabilizes or reaches 50% of the new value. One foot square should be marked with a magic marker to compare remedial actions. Examples to test include: Silicone in light dilution, medium dilution, and heavy dilution; and all other possible coating products.
*How does the degradation curve of a rejuvenated fabric compare with a new fabric?
*One of the related forum posts said, “If people are using shelters with a HH head of ~500mm in storms and experiencing few problems then it appears that HH is not terribly important”. R value for pads, EN 13537 rating for sleeping bags, and HH ratings for rain shelters may not be important because people are experiencing few problems. Yes, if it is not broken, I should not waste my time trying to fix it… time to go hiking.