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  1. #1
    Senior Member bcaron's Avatar
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    Do I Need a Vapor Barrier

    I tested out my new AHE Potomac last night in the back yard. Temps dipped down to -3c or about 26f with very little wind. Used a warm down bag as a TQ, and my HH asym tarp as an undercover. Everything worked very well and I was toasty warm. There was a fair bit of moisture on the bottom of the Potomac that had condensed on the top of the undercover. It wasn't enough to wet the insulation, and dried quickly in the morning after removing the undercover. Given that the Potomac is synthetic insulation should I worry about a bit of condensation? If so what's the best way to deal with it? When the temps aren't going below freezing I won't use the undercover. A vapor barrier just under the hammock would likely solve the problem, but then I'm adding yet another layer to the system.

    Blair
    " I have not yet begun to procrastinate!"

  2. #2
    Senior Member PuckerFactor's Avatar
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    The tarp/undercover probably acted as a vapor barrier and condensed the WV after it had made its way through the Potomac. If you need an undercover, I'd use something not waterproof, or put a vapor barrier, like you said, next to the hammock. A space blanket would work well, and add a bit of warmth.

    PF
    It's better to have it and not need it than need it and not have it.

    Formerly known as Acercanto, my trail name is MacGuyver to some, and Pucker Factor to others.

    It's not procrastinating, its proactively delaying the implementation of the energy-intensive phase of the project until the enthusiasm factor is at its maximum effectiveness. - Randy Glasbergen

  3. #3
    Señor Member wisenber's Avatar
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    Just remember that a vapor barrier between you and your insulation can leave the vapor on you...which is not always a good thing. Vapor barriers tend to come into play in more extreme cold when your insensible perspiration continues to cause evaporative heat loss.

  4. #4
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    Quote Originally Posted by PuckerFactor View Post
    The tarp/undercover probably acted as a vapor barrier and condensed the WV after it had made its way through the Potomac. If you need an undercover, I'd use something not waterproof, or put a vapor barrier, like you said, next to the hammock. A space blanket would work well, and add a bit of warmth.

    PF
    That's the way I see it also and will add that I would not use the tarp/undercover without first having something as a vapor barrier between me and the underquilt.

    Another way to say it is if I were to use only one vapor barrier it would always be the one between me and the underquilt. I might add a second one like the tarp/undercover, but I would never use that without something as a vapor barrier between me and the underquilt. That vapor barrier can be a space blanket, a sheet of plastic, a closed cell foam pad, self inflating one, etc, or some type of clothing that you wear.

    If you have a large tarp that can block the wind, it diminishes the usefulness of the tarp/undercover. Every little bit can help with insulation once you have the wind issues solved, but direct hits by wind can be a biggie.

    I haven't used a vapor barrier on both sides on an underquilt very often as I typically use a large tarp in cooler conditions. But one time when I did I noticed that I had some ice form on the inside of the one on the outside of the underquilt that was acting as an overcover, not much but some. Not only was I using a vapor barrier between the hammock and the underquilt but I was wearing a vapor barrier as well.. so I scratched my head on where the moisture came from that turned into ice on the inside surface of the overcover.

    I'm guessing that it was moisture from the ambient air that was trapped between the two vapor barriers and that it condensed like dew does when the ambient temperature dropped 20, 30, or 40(?) degrees during the night. As I understand it that is how water appears in mustard jars-- we open them and they take in air at room temperature that has moisture in it. Then we put the lid on the jar and put it in the cool refrigerator whose temperature is below the dew point of that trapped air. So when that trapped air cools it can no longer hold all that moisture and it condenses as water on top of the mustard in the jar.

    And those gasoline additives that are methyl alcohol that are used to get moisture out of your gas tanks (that some of us use as fuel for our alcohol stoves), I figure that the moisture gets in when we don't fill our gas tanks all the way when we get gasoline by way of the trapped ambient air cooling below its dew point at night or whenever the temperature drops enough.
    Youngblood AT2000

  5. #5
    Senior Member BillyBob58's Avatar
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    Quote Originally Posted by Youngblood View Post

    I haven't used a vapor barrier on both sides on an underquilt very often as I typically use a large tarp in cooler conditions. But one time when I did I noticed that I had some ice form on the inside of the one on the outside of the underquilt that was acting as an overcover, not much but some. Not only was I using a vapor barrier between the hammock and the underquilt but I was wearing a vapor barrier as well.. so I scratched my head on where the moisture came from that turned into ice on the inside surface of the overcover.

    I'm guessing that it was moisture from the ambient air that was trapped between the two vapor barriers and that it condensed like dew does when the ambient temperature dropped 20, 30, or 40(?) degrees during the night. As I understand it that is how water appears in mustard jars-- we open them and they take in air at room temperature that has moisture in it. Then we put the lid on the jar and put it in the cool refrigerator whose temperature is below the dew point of that trapped air. So when that trapped air cools it can no longer hold all that moisture and it condenses as water on top of the mustard in the jar.

    And those gasoline additives that are methyl alcohol that are used to get moisture out of your gas tanks (that some of us use as fuel for our alcohol stoves), I figure that the moisture gets in when we don't fill our gas tanks all the way when we get gasoline by way of the trapped ambient air cooling below its dew point at night or whenever the temperature drops enough.
    That sounds just like some of the problems that some people have reported with the HH SS. Problems that had me also scratching my head because not only had I not experienced those problems, but also I couldn't even figure out how they had happened. How did the moisture get past the VB?

    You presented this theory before in one of those SS problem discussions, and I thought it sounded reasonable. It sounds more reasonable with your elaboration in your above post.

    Of course some folks also had moisture problems on the warm side of the VB, another problem I have never had. But at least I could see how that might happen. But I could not understand how this "body moisture/vapor" was getting past the VB in order to condense in the insulation. But I think your theory might well account for it.

    Two thoughts/questions:
    1: Would the amount of condensation be somewhat limited, due to only a relatively small amount of moisture being in the air compared to what a body can pump out? (unless fresh air is coming in?
    2: Would a snug fit help, by reducing the amount of trapped air which contains moisture, thus reducing the moisture available to condense?
    For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us....that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. For we know that the whole creation groans and suffers the pains of childbirth together until now.
    Romans 8:18,21-22

  6. #6
    tbone's Avatar
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    I think the reason you are seeing the condensation and ice and so forth is . You are creating zones. You have your indoor air inside the hammock and you have your outside air. You are generating vapor (humidity) with your body, this creates vapor pressure which will always seek equilibrium. So now your vapor pressure is escaping your inside zone and coming into contact with surfaces outside of the zone , if the surface temperature of whatever it comes into contact with is lower than the dew point temperature then condensation will form. To avoid all of this you would need to equalize your zones so that you don`t have higher concentrations of humidity in your inside zone. So basically you need to "burp" your system which leaves it open to allow for airflow.

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