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condensation in tents


  • water condensation can be problematic for campers as it may:
    • form condensation on the outside of the tent which not only makes it wet but the moisture can sometimes wick inside if one touches the inner surface
    • form condensation on the inner surface of the tent mainly from poorly ventilated exhaled humid air which then drops back onto the camper's sleeping bag and clothes making them wet and cold
    • if frost forms it may stop your zippers working until it melts
  • too little ventilation can make the camper and their sleeping bag/clothes cold and wet
  • too much ventilation on cold nights reduces camper comfort from:
    • wind chill on exposed skin
    • lower inspired air temperatures - when this drops below 10degC the risk of cold-induced bronchospasm with coughing or asthma attacks is increased.
  • bring a spare towel to mop up any troublesome condensation

The science of condensation

  • when air temperature drops below its dewpoint temperature, water will begin to form dew or condensation on the colder surfaces where there is a colder layer of that air and if this temperature falls below zero degC then it becomes frost (in air it becomes fog or cloud).
  • the dewpoint is a measure of the amount of water content in an air mass and its measurement is mainly determined by the temperature of the air mass and it's relative humidity (RH) - the greater the RH, the closer the dewpoint comes to the dry bulb temperature.
    • dewpoint = dry bulb temp - (100-RH)/5) and thus RH = 100 - 5(dry bulb temp - dewpoint)
  • exhaled air contains 0.034g water per litre air (at 37degC, 1 liter of air is full saturated at 0.0436g water content) and generally has a temperature in the range of 31.4-35.4 °C and a RH of 40-100% (not always 100% as many state)1) and this results in some 100-150mL of water being exhaled into the tent overnight per person.
    • at 33degC and at 80% RH, the dewpoint will be approx. 29degC so it is easy to see that this is a major risk for condensation as most nights have air temperatures well below 29degC
    • if this exhaled water is not vented outside (eg. a fully sealed fabric inner canopy of a tent), the RH of the tent will increase quickly and the dewpoint will approach the tent temperature which will generally be much warmer than the outside air temperature. This will inevitably result in condensation on the inside of the tent fabric as this will be cooler from the outside air.

The science of evaporation

  • general equations for rates of evaporation of “open” water bodies:
    • evaporation rate in kg/hr = (25 + 19 x wind velocity in m/s) x surface area in sq.m x (Xs-X)
      • Xs = maximum saturation humidity ratio of air at the temperature of the water surface
      • X = humidity ratio of the air (depends upon air temperature and relative humidity)
      • humidity ratio = weight of water in the air-water mixture / weight of air in the air-water mixture
      • one can determine the maximum saturation humidity ratio for a given dry bulb temperature by using a dry bulb temp vs humidity ratio Mollier chart and looking at the 100% RH curve which gives the saturation point for a given dry bulb temperature 2)
      • example Xs values:
        • at water surface temperature 25degC, the maximum saturation humidity ratio in the air above the water surface is around 0.02 kg/kg
        • at water surface temperature 20degC, the maximum saturation humidity ratio in the air above the water surface is 0.014659 kg/kg
        • at water surface temperature 15degC, the maximum saturation humidity ratio in the air above the water surface is around 0.0105 kg/kg
        • at water surface temperature 10degC, the maximum saturation humidity ratio in the air above the water surface is around 0.0075 kg/kg
        • at water surface temperature 5degC, the maximum saturation humidity ratio in the air above the water surface is around 0.0053 kg/kg
      • example X values:
        • in the tropics for example, if air temperature 29degC and 80% relative humidity the humidity ratio in the air is around 0.02 kg/kg
        • if air temperature 25degC and 50% relative humidity the humidity ratio in the air is 0.0098 kg/kg
        • if air temperature 15degC and 50% relative humidity the humidity ratio in the air is around 0.0055 kg/kg
        • if air temperature 5degC and 50% relative humidity the humidity ratio in the air is around 0.0025 kg/kg
      • the higher the air humidity, the less the differential of X from the Xs value and thus minimal evaporation is possible, hence your tent and towels may take a long time to dry out unless they are heated by the sun or exposed to strong winds
  • cooling effect of water evaporation
    • heat loss due to evaporation = evaporation heat (enthalpy) of water for that temperature x evaporation rate
    • evaporation heat (enthalpy) of water at temperature at 20degC is 2454 J/g.
    • as canvas tent fabric can hold a greater mass of water than nylon tents, canvas is better at cooling the tent down during the day by wetting it, but it also takes much longer to dry out due to the increased mass of water that needs to evaporate
    • this is one reason you do not want to get wet in cold weather and why wind chill is a problem - you rapidly lose body heat as your body heats the water compared to the surrounding air and evaporation results which causes you to cool down rapidly
    • getting wet also substantially reduces any insulation you have and increases general heat losses to the environment

Ventilation and wind chill

  • wind chill equation for low temperatures at low humidity:
    • Wind chill index = 13.12 + 0.6215T – 11.37 (V0.16) + 0.3965T (V0.16)
      • where T is temperature in deg C and V is wind speed in km/h at height 10m
      • corrects the officially measured wind speed at 10m height to the wind speed at face height, assuming the person is in an open field
      • Windchill temperature is defined only for temperatures at or below 10 °C (50 °F) and wind speeds above 4.8 kilometres per hour (3.0 mph)
  • Australian apparent temperature “Feels Like” equation for higher temperatures and humidity:3)
    • based on a mathematical model of an adult, walking outdoors, in the shade, in dry clothing to give the temperature, at the reference humidity level, producing the same amount of discomfort as that experienced under the current ambient temperature and humidity.
    • AT =Ta + 0.33w - 0.7v - 4.00
    • where:
      • Ta = dry bulb temperature (°C)
      • w = water vapour pressure (hPa) = (rel.hum /100) x 6.105 x e(17.27xTa)/(237.7 + Ta)
      • v = wind speed (m/s) at an elevation of 10 m

hot humid nights

  • when temperatures are above 24degC, the apparent “feels like” temperature rise due to humidity tends to outweigh wind chill
actual dry bulb temp relative humidity no wind: feels like temp wind speed to feel like 24degC 40% RH
24degC 60% 25.9deg C 10kph
24degC 80% 27.9deg C 20kph
24degC 90% 28.8deg C 25kph
28degC 60% 31.5deg C 40kph
28degC 80% 33.9deg C 50kph

mild-cool nights

  • essentially wind chill has a linear relationship with wind speed (temp drops 0.9degC for every 10kph wind speed) and equation suggests limited effect of RH changes although anecdotally, one does seem to feel colder and have more trouble staying warm in fog which may have more to do with specific heat and mass of water compared to dry air
  • coastal camp sites tend to have high RH of over 60% and often over 95%
actual dry bulb temp wind speed Feels like (if 40% RH) Feels like (if 60% RH) Feels like (if 80% RH) Feels like (if 95% RH)
24degC 0 kph 24deg C 26degC 28deg C 29degC
24degC 10 kph 22deg C 24degC 26deg C 27.5degC
24degC 20 kph 20deg C 22degC 24deg C 25.5degC
24degC 40 kph 16deg C 18degC 20deg C 21.5degC
12degC 0 kph 10deg C 11degC 12deg C 12.4degC
12degC 10 kph 8deg C 9degC 10deg C 10.5degC
12degC 20 kph 6deg C 7degC 8deg C 8.5degC
12degC 40 kph 2deg C 3degC 3.8deg C 4.6degC
6degC 0 kph 3deg C 4degC 4.5deg C 5deg C
6degC 10 kph 1deg C 1.5degC 2.6deg C 3deg C
6degC 20 kph -0.5deg C 0.2degC 0.6deg C 1deg C
6degC 40 kph -4.5deg C -4degC -3.5deg C -3deg C

Preventing condensation inside tents

camping site selection

  • camping close to a water body such as a river or lake, particularly in a region where night temperatures fall significantly and there is little wind to keep the air near the fly moving is a recipe for lots of condensation on the outer and inner surface of the fly as RH is higher and the temperature is more likely to fall below dewpoint overnight
  • high humidity days or regions are also likely to increase risk (and also prolong the ability for the tent, towels and clothes to dry out during the day

tent design and camper management of ventilation

  • condensation is particularly a problem for single wall tents (most hiking tents are dual wall - inner canopy and a fly)
  • the prime method is to ensure there is adequate ventilation of external air passing through the inside of the tent and then venting (preferably near the ceiling
  • mesh tent design with the inner tent “canopy” made mainly of mesh will theoretically have the best ventilation and least amount of condensation issues however:
    • if the fly is not well ventilated, water can condense on its inner surface and drop back through the mesh onto the camper
    • this mesh design has the greatest wind chill on the camper hence many tents have a fabric bottom half and a mesh top half as a compromise
  • mainly fabric inner tent design should allow the camper to vary the degree of ventilation depending on conditions as:
    • they may prefer to reduce ventilation for some time to keep themselves warm
    • wind chill inside is an important comfort factor at night and this relates to the outside air temperature and the speed at which it passes over the camper
      • most tents are designed for the outside air to enter UNDER the fly (or through an open vestibule doorway) then pass upwards to escape through a ceiling vent
        • this will usually suffice to keep the inner surface of the fly dry but becomes more complex for the inner tent “canopy” when this is mainly fabric such as nylon rather than mesh.
        • in these situations, the camper will need to open (preferably insect proof meshed) vents to ensure adequate ventilation of the inner tent canopy and to minimise wind chill on the camper, the entry vent for the inner canopy is often high such as the top part of the door and the exit vent is either high on the opposite door or a ceiling vent.
australia/tent_condensation.txt · Last modified: 2021/07/06 07:44 by gary1