Condensation in Tents & the Vapor Pressure Differential (VPD)

[Ryan Jordan]

Condensation processes (including vapor pressure differential aka VPD), campsite management, condensation management, and tent/shelter design were the key topics from last night’s webinar: Your Tent, Your Campsite, Your Comfort.

(The recorded version of the webinar will be available through the weekend to all members, FYI).

One of the takeaways from last night is that as temperatures approach freezing (assuming negligible wind) on clear nights (and you’re not camped under tree cover), the use of a tent’s “little vents” (e.g., like the eyebrow vents at a shelter’s peak) start to be overwhelmed by the fact that the dew point is just too close to the actual temp, and the RH% is too high (all this points to very low VPDs in the < 0.5 range).

Questions:

1. Just deal with the resulting condensation in the morning?
2. Open up all the tent doors/windows (even on cold nights) and get as much ventilation as you can to mitigate?
3. Some other strategy?

[Bill Budney]

This was an especially excellent vid.

What about a candle hanging near the vent to stimulate air movement on calm nights? I’ve never tried it because I’m chicken about sleeping in a nylon cocoon with a flame burning for some reason. However, some people love it.

It has occurred to me that maybe we could do something similar with electrical devices. Perhaps a small fan? Or even an LED as a small heat source? Of course that would require some way to recharge the battery (probably daily). Maybe a solar charger? But the ounces add up…

So most of us probably wind up just toughing it out when the weather doesn’t cooperate. Clear skies mean sunshine to dry things out in the morning. As you say, keep the doors and windows open whenever you can.

Shrug… I always thought that condensation was just part of being outdoors. Stuff happens, and that’s part of the fun. :)

[Ryan Jordan]

For sure, one night is no big deal. I’m interested in the multi-day, no-sun, cold-weather-high-humidity scenario (granted, a worst-case scenario).

[Bill Budney]

That Stephenson Warmlite sealed-inner-wall tent design sounded interesting. I look forward to your review. ;)

IFF I faced the problem that you describe, I would try a fan at the top vent (instead of a candle). (Aim it to exhaust moist air from the tent, of course). (Worry about recharging later; after solving the condensation problem).

EDIT: Here’s a 40mm USB fan with 3-step speed controller for $12. Plug it into your power brick and see if it reduces condensation. If it does, then you could fine-tune the design with smaller fans that are intended for battery power, such as laptops.

[Jon Fong / Flat Cat Gear]

OK, If I understand this correctly, if you can tolerate the cold, using a tarp with high ground clearance would be superior to a single or double walled tent.  On fact, it should be vastly superior in all conditions.  Am I on track here?

[Scott Emmens]

I have just watched most of the webinar, so informative, thank you.

FYI I have purchased a couple of these as a cheaper (but much heavier) alternative to the Kestrel Drop, at NZ$25 a much easy to digest investment.

Would much of the finding in this webinar, especially in-regards to radiant heat, dew points and vapour pressure differential be relevant to apparel too, specifically WPB garments?

[Scott Emmens]

I might get roasted for this, but here goes…I know some people carry a tarp for shelter through the day, or as extra dry space in camp. Would pitching a tent UNDER a tarp help with the radiative heat loss? In the absence of trees for example?

[Bill Budney]

Would a Zippo or HotHands hand warmer work like a tent candle?

[Jon Fong / Flat Cat Gear]

A candle is only about 25 watts, not too powerfull.  My 2 cents.

[Bill Budney]

Yes, that’s what made me think about an LED heat source earlier.

Maybe I should ask: Do we know, for certain, that tent candles work to reduce condensation? It seems like common knowledge; the “usual way” to combat condensation. But I’ve never tried it. Roger Caffin says that silnylon doesn’t ignite, which is a plus. If it didn’t work for some reason, then CO would be another concern.

The candle is pretty well enclosed, and might blow itself out if it fell. So maybe not a real fire hazard (although it could melt your expensive gear). It’s just that I can see the flame, which feels creepy to me.

The hypothesis is that it stimulates air movement from wherever to the top vent (on an otherwise calm and condensation-prone night).

[Jon Fong / Flat Cat Gear]

Maybe someone should develop a UL snorkel to vent your breadth outside to keep the internal RH down.

[bjc]

Jon, a tarp’s superiority in terms of condensation and hence comfort certainly has been Ray Jardine’s contention.

[John S.]

The snorkel thing has been done, I think.

[Matthew / BPL]

Here are some tangentially related threads

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