Condensation

[Grey Mouse]

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 .

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...."

[Tyroler Holzhacker]

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 .

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...."

Here is the DC Metro area we have a lot of moisture that is retarded/impermeable. LOL!

[Grey Mouse]

Here is the DC Metro area we have a lot of moisture that is retarded/impermeable. LOL!

I have lived near there. I agree

[BillyBob58]

Since condensation occurs when warm vapor contacts air or a surface that is colder, at the dew point( i.e. car windshields, breath in cold air, inside breathable down quilts if it is cold enough for the dew point to be inside the outer shell, etc), what happens with the condensation if that Reflectix is kept warm by being inside of insulation, rather than on the colder outside?

I've never used Reflectix, so I do not know the answer, though I have a suspicion.

[Grey Mouse]

The reflectix itself is not warm due to a zero or low R-Value. Any heat coming into contact with it will quickly conduct either through the material or into any moisture droplets that form on it's surface. Those droplets have the potential to add up to make a moisture film on the material or if enough moisture retardation is happening the moisture will most likely appear in the material beneath you such as your under quilt. Remember that our quilts do not make an 100% encompassing environmental envelope. For example, if the reflectix is under your shoulders and head area the top quilt is not present to complete an envelope of conditioned space thus allowing the increasing moisture to cool at the hammock or underside of your pillow depending on what it is. That moisture would quickly wick into the lower materials (under quilt) via gravity and wet to dry absorption rules.

The way that reflectix warms you is by long wave radiation that is normally lost bouncing back at you. The vapor retarding barrier helps stop the wind and allows your heat loss from your underside to warm the air beneath you. Since little air is going through the reflectix it helps keep that pocket of air in the empty space and you will be warmer almost like having a second quilt there. The down side to a vapor barrier/retarder is the condensation that builds up and slowly lowers the r-value of your under quilt through the night. You may also experience a "clammy" feeling.

[OlTrailDog]

Probably what you are dealing with is the impermeable material reflectix is made out of does not let your body moisture, i.e. sweat aka perspiration escape into the atmoshphere. This is different than condensation. Condensation has been succinctly described and most likely would occur when you moisture laden breath condenses on a a surface below the dew point. For hammock camping this is most likely to happen above your body and not below.

i'd suggest using a material that is semipermeable or perhaps encapsulating the material in a material liner, e.g. sleeping bag liner may help. Some folks have this problem even with using pads in their hammocks. This is also highly dependent on your individual physiology and air conditions, e.g. humidity and temperature. Personally, where I live and my body physiology it hasn't been much of a problem using inflatable pads except perhaps in under unique combinations of conditions.

[Grey Mouse]

I agree trail dog. Your mouth puts out a ton of moisture. I read a study where hikers wore mask and lost 50% less moisture and save 118Kcal per night thus allowing them to carry less food on longer trips. It's amazing the amount of water that we produce. The condensation mentioned above was really simply heated air under us that released it's moisture and condensed via dew point temps on the vapor impermeable barrier due to the perm rating and almost zero r-value of the material.

[BillyBob58]

I was thinking about the statement from the OP "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.", as I was hanging in my HH Super Shelter, no tarp, 39-40F, 99% humidity, 15 mph winds.

As usual, I had a space blanket under me. I got to wondering what the perm rating of that space blanket might be, as well as the vapor barrier clothing that I sometimes sleep in? I am guessing that the space blanket and VB clothing are even less breathable than the reflectix? (SP?)

[Grey Mouse]

I was thinking about the statement from the OP "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.", as I was hanging in my HH Super Shelter, no tarp, 39-40F, 99% humidity, 15 mph winds.

As usual, I had a space blanket under me. I got to wondering what the perm rating of that space blanket might be, as well as the vapor barrier clothing that I sometimes sleep in? I am guessing that the space blanket and VB clothing are even less breathable than the reflectix? (SP?)

You could consult ASTM D737 (Standard Test Method for Air Permeability of Textile Fabrics) for some of the info.

Also: https://www.scribd.com/document/3632...rial-EEM-00259

The thickness of your mylar will determine it's perm rating: http://usa.dupontteijinfilms.com/wp-...Properties.pdf

[BillyBob58]

You could consult ASTM D737 (Standard Test Method for Air Permeability of Textile Fabrics) for some of the info.

Also: https://www.scribd.com/document/3632...rial-EEM-00259

The thickness of your mylar will determine it's perm rating: http://usa.dupontteijinfilms.com/wp-...Properties.pdf

Wow! All very technical, thanks for the links! I'm betting the space blanket or my VB clothing is going to be about like 6 mil polyethylene, which is 0.06.
Or maybe even more impermeable, more like 10 mil. I don't think I am going to be able to find out exactly. But I can not tell that either space blanket or VB clothing allows any vapor or liquid(sweat) to make it into my insulation. Not in the last 12 years at least.

And since I keep those VBs on the warm side of my insulation, any issues with condensation have so far been minimal at worst case. Mostly none. ( But, not the case when I messed up and had a VB on the cold side of the insulation, with no inner VB to keep the vapor from contacting the outer, cold VB! Soaked! )