Managing condensation: a shelter case study

Z-35-RnHcDc29osUdHBfNGBRz6WAgB8bEIIqhMcAr9FmcuBWlyxCDSw2b7CeizKrjoRD8shcJodVGA_eM_TUuBtlwnDccY0yVFe5IFsMSilTN6VP078B3x7H9wUPKR4h6qCkTFKA23TqRSim2C7BopNfCiZyF4dc2kD7uaGF_7_l_VYnPRNqfLv7SRwKA5gl2dt9uY6VdyV1mMqmzQipv3oyZZXTxWlFlGThe new Sierra Designs High Route tent, which manages condensation via interior air volume and venting, at the expense of vaguely compromised weatherproofing.  Photo by Andrew Skurka.

Condensation is a fact of life when camping in the backcountry.  Under certain circumstances condensation will occur in any shelter.  The art is in mitigating and avoiding condensation, hopefully to the extent that it never becomes a major issue.  If condensation is bad, especially for several nights on end, insulation in clothing and sleeping bags can become very damp.  Damp insulation does not work well, and with no chance to dry it, the loss of potential warmth could become problematic.

Condensation occurs when water vapor changes to liquid, as the air temperature drops below the dew point.  To keep things simple, we’ll allow that condensation is controlled by two factors, the amount of moisture in the air inside and outside the shelter, and the dew point within and outside the shelter.  These factors can be altered via campsite and shelter selection.

The most common and obvious way to manage condensation is to reduce the amount of moisture within the shelter.  The number one way to do this is via ventilation and shelter volume.  As these are the methods applicable to single wall tents, pyramids, and tarps, which are in turn of primary interest to the lighter weight traveler, I’ll discuss them in detail later.

R0013183The Sierra Designs Tensegrity, a single wall tent with exceptional ventilation, and thus near ideal condensation control.

Manipulating the dew point is a less obvious, but very effective, way to manage condensation.  It is why double walled tents with fabric inner tents are generally the preferred tool for humid, condensation-prone environments.  At night infrared radiation, collected from the sun during the day, is given off by the earth.  The combination of this and moving air typically results in the outer surface of a tent or tarp being colder than the ambient air temperature.  This means that the outer surface of a shelter often goes below the dew point well before the air at large, and is why condensation on a shelter can often be far more severe than on objects in the immediate vicinity, such as vegetation.  A double walled tent with a solid fabric inner is an excellent example.  The space between the inner and outer holds air in a semi-still state, which creates a modest insulating effect.  This in turn significantly reduces the super-cooling of the outer tent surface, which reduces condensation.

The downside of double wall tents is simple: weight.  While there are several new double walls whose weight approaches or even goes below 2 pounds, these all have such significant compromises that they hold no interest for me.  Fly and floor fabrics with low hydrostatic head, cramped interiors, and all-mesh inners are the most prominent of these.  When design and technology allow for some of the benefits of a fully realized double wall in a light package it will be an appropriate object of excitement.


R0001564The Seek Outside BT2, a smaller pyramid/tipi which favors weatherproofing over ventilation.

In any shelter the quality and especially quantity of ventilation is with respect to necessity inversely correlated with wind speed.  Truly strong wind circulates air through the shelter whether you like it or not, and makes venting less necessary.  In the top picture of the BT2 we had a steady 25-30 mph wind coming down valley all night, and even with the BT2 zipped tight and M and I both inside, we had no condensation.  Contrast that with the bottom picture, when I slept solo near a creek on a still night with the doors both open a foot at the bottom, and woke to plenty of frost inside.  This is what Seek Outside means when they say the BT2 is optimized for alpine conditions; it is one of the most exceptional wind shedders I’ve ever seen, but in the name of weight and simplicity it has only modest ability to vent, especially when rain protection is necessary at the same time.  Compromises always.

The Tensegrity 2 person lies at the opposite end of the spectrum.  It allows for near 360 degree venting, even during a steady rain.  The doors have fabric covers, but the foot and head are always only mesh, making it a cold tent in a strong wind.  And the big flat panels catch a lot of wind and move a lot, though with sufficient tension the tent is far more stable than one might think.  It has a similar, slightly smaller footprint than the BT2, but much more interior volume and useable headroom.  A great option for mild, warmer, humid weather, but not functional on windy autumn ridges.





4 thoughts on “Managing condensation: a shelter case study

  1. Always enjoy your writing. It’s so very to the point and helpful, especially as I start reassessing all the shit I buy and start thinking about what I actually need for not only what I do, but what I hope to do (which doesn’t involve serious mountaineering despite what my closet would indicate, lol).

  2. Hi Dave, Any thoughts on adding screens to the BT2? I have one, w/o screens, and out East here it always seems to be humid in the summer and I was thinking it might be nice to have screens. I asked if they could just do one, to save weight, but they said they could only do two. Maybe I’m doing what you say here, kind of overestimating my need for lightness, even with two screens weight is just over two lbs.

    1. I haven’t, but it seems like it would be a worthwhile investment. If you need ventilation, cross ventilation is certainly the way to go. Since this post I’ve been thinking of a clean way to give it ventilation and rain proofing at the same time. Cords on the tieouts to pitch it off the ground is the obvious answer, but sometimes you want still more.

      1. Cool. I appreciate the feedback. I’m going to have to play around with the elevated pitch, but I still think I might try to sell my current one and get one with screens. I did purchase a nest for it (that no longer seems to be made), but it’s just not an easy to use piece and doesn’t offer what I was hoping. I’m mainly thinking of areas with a decent bug threat, but where when pitched to the ground most mosquitos are kept out. When it’s all closed up then, it gets warm and sticky in their quick during the summer.

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