I posted the following post on another forum recently and thought that perhaps it could help others here in regards to using Reflectix for insulation under a hammock and the issues with condensation.
"To get slightly more technical... Reflictix has a perm rating of 6.3 (see below) making it a class 3 vapor retarder (ASTM E 96) barrier that does allow some moisture vapor to pass through it. It does not have any R-value as Juany118 stated above. The reflective surface does reflect the long wave radiation both while in contact with your skin and when there is an air barrier but the material is most effective when you allow that radiation to warm the retarded air space between you and the barrier. Since Reflectix has no R-value most of the warming action from the long wave radiation is reduced dramatically by the conduction of the material when it is in contact with you. You may actually loose more heat via conduction than what is being radiated back at you (I have not tested this). It is this fact (no r-value) that presents our issue. Having a moist warm air space that is retarded by a low or no r-value barrier with a high temperature differential between the exterior and interior spaces will cause condensation.
Examples of similar perm ratings to 6.3 :
100 sheets of 6mil Polyethylene plastic sheets-aka Visqueen (.06 perm each)
Building paper, asphalt-saturated felt 15# (5.0 perm)
OSB Sheathing 1/2" (perms 1-7)
Nine sheets of Plywood, exterior, 1/4" (.7 perms each)
Now let's further consider the fact that cotton cloth (we will assume underwear or t-shirt material) has a perm rating of .347 at 12°F so that it will give us a reference that most of us are familiar with along with the Visqueen material for allowing moisture to pass through it.
Assuming the Chicago weather today at 71% humidity at around 34°F and using the formula "WVT = A x T x Delta P x Perms" we can determine that if we have a theoretical ideal air space of 4"x40"x72" (960 sq ft) under us (3.5" loft plus reflectix), an exterior humidity of 71% RH (32°F) and an interior space of 40% RH (at 72°F being preferred) we get a vapor pressure of .19415 {(71x.1803)-(40x.7912)=.19145 VP}, and so... 960(A)x1(T)x.19415(VP)x6.3(Perms)=1174.2 WVT (grains of water) or 5.961 lbs of water or 1.5 pints/.75 quarts per hour trying to get out of our space that is being retarded and trying to condense on us. You would feel better if the reflectix was on the ground and have far less condensation to deal with. Please correct me if my math was off
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Knowing how the moisture is retarded, how much retardation (impermeability) we have, how much moisture there is, and how the interior and exterior temps affect it will help us better understand the problem so that we can address the issue directly. This is why those expensive quilts are made to breath but only at a certain desired rate. Too much and your cold. Too little and your wet. Also remember that different humidity levels alone at the same temperature can cause you to feel "colder" or "hotter" but that is a different story...."
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