What causes condensation in a tent?

Though you may never have experienced tent condensation, you will nevertheless be familiar with other scenarios where condensation occurs: on the inside of your car windscreen; on the outer surface of the glass when you prepare an ice-cold drink; when a fog forms on your glasses or binoculars. In each case, condensation occurs for the same reasons as it does inside a tent – it's just water obeying the laws of physics. With modern life indoors, we seem to have eliminated, or at least controlled, condensation problems – except on bathroom mirrors. Perhaps then, when considering the best way to handle condensation in harsher environments, we should look at how NASA scientists solve condensation issues in deep space. But, guess what? … They get problems too! What causes condensation? Condensation is water; and water comes as solid (ice), gas (water vapour), or liquid (water). Though it takes freezing temperatures to create solid ice, the real problem with water is that, given half a chance, it needs very little encouragement to change from a gas to a liquid state, and vice versa, often creating condensation along the way. Scientists call the move from liquid water to water vapour 'evaporation'; and 'condensation', scientifically speaking, is just the return journey – water vapour back to liquid. Looking at these processes will help to understands what is going on. Evaporation As a gas (water vapour), water is invisible; so, as a kettle boils, the steam given off is evidence that the heating process is causing evaporation and turning the liquid water into gas. Water vapour is absorbed into the air until so much accumulates that the air becomes saturated and unable to hold any more. When this point is reached, water vapour will quickly turn back to liquid water. Air is flexible too, and warmer air won't hold much water vapour, whereas cold air holds much more. Condensation Whilst raising the temperature creates evaporation, a fall in temperature will prompt condensation, causing water vapour to revert back to liquid. A classic case of condensation occurs inside a tent when warm humid air (which is loaded with water vapour) hits the cooler tent wall when it has been chilled from the outside by much cooler night air. When these conditions occur, the rapidly cooling internal air forms water droplets on the tent wall as it condenses from gas to liquid. Why is there so much moisture inside a tent? Tent moisture is the result of human occupation. Sleeping overnight, for example, each person will exhale an average of one litre of water. Being warm, this exhaled air is also moisture-saturated – with two sleepers, this results in the equivalent of more than a kettle-full of water sloshed around the tent to greet you as you wake up. Cooking, wet clothes, and equipment also contribute humid, moisture-laden air to aggravate the problem. In addition, cooking and heating also raise the internal temperature, which means the air becomes saturated with water vapour even more quickly. Perhaps less obvious, camping beside water means humid daytime air cools as darkness approaches, producing an atmosphere typically saturated with water vapour – perfect condensation conditions. Condensation control Motorists encountering windscreen condensation quickly engage the heater-blower to circulate warmer air across the screen, clearing the condensation. Should this be insufficient, drivers open a window to assist circulation. In extreme conditions, a last resort is to wipe away moisture as it collects on the screen. Appropriately adapted to a camping context, the above procedure summarises the basics of controlling tent condensation: air must be allowed to circulate to avoid high humidity levels; tent walls and roof must be prevented from cooling and becoming condensation traps; and additional moisture accumulation must be avoided, for example by cooking outside if possible. Effective condensation control inside a tent thus involves recognising when condensation is likely to form, and taking steps to prevent, or minimise, its occurrence. Tips to limit internal condensation 1. Air circulation - take advantage of prevailing winds by pitching so the breeze blows through the vents. - take advantage of insect screens to open windows and doors. - keep the rain fly and vestibule fully, or partially, open wherever possible, using clips or ties. 2. Insulation - a double-wall tent has an insulating layer of air between the walls, which helps keep the inner layer warmer and less condensation-prone. - a single-walled tent will benefit from a separate insulating fly sheet erected over the main tent. - a groundsheet will prevent moisture entering the tent from below. - larger tents generally suffer less condensation. 3. Prevention - choose your site carefully, avoiding low-lying areas beside water, and take advantage of breezes. - ventilate whilst cooking, or cook 'alfresco' where possible. - keep wet material out of the tent if you can. - wiping tent walls with a pack towel is an effective removal strategy.