jueves, 25 de septiembre de 2014

(vi) Tents and shelter. Hidden costs in humanitarian aid and emergency. THERMAL ISOLATION.

What is the purpose of a flysheet in a tent? (Second function)

As I said before, a flysheet in a tent is supposed to offer thermal isolation for the users, but it is really hard to find a tent that has a flysheet capable to meet this expected function.

The main reason is that to make it possible to create a thermal isolation it is needed to have a physical barrier between inner fabric, the body, and outer one, the flysheet.

The purpose is that the air surrounding the tent finds something between outer side and the interior, capable of reducing the thermal transmission, in cold weather avoiding inner air being as cold as outer one and in hot weather avoiding inner air being as hot as outer one.

In architecture and construction, in a simplified manner, there have been historically three main ways to reach this goal: first very effective way was to make a thick barrier, sometimes even one meter wide; second way adopted, not so effectively, was to reduce this thick walls in order to save money in the house building substituting it with lighter materials and using inner layers of isolation materials and leaving some space for creating an air barrier, and finally it was discovered that one third effective way to create a chamber of air in movement, between inner air and outer air, keeping this air chamber between two layers of material. This is the effect known as ‘ventilated façade’.

The ventilated façade works as follows:

When the sun shines, the air inside the chamber is heated and it starts to flow to upper areas. As the hot air goes up, fresh air occupies the space abandoned by hot air, bringing fresh air inside the air chamber, if the hot air has an escape way to flow to the atmosphere, this movement becomes continuous and the flow of air avoids the inner air inside the tent getting hotter.

The outer layer that creates the outer closing side of the barrier also avoids the sun to impact directly on the inner body and helps also, in this way, to reduce the inner temperature.

The air barrier between outer layer and inner layer creates a difference, a gradient, of temperatures, keeping the interior fresh.

When the weather is cold outside, the same effect is produced. But with some differences: If the hottest air is not allowed to escape the air chamber, the air inside the chamber maintains a higher temperature than interior and avoids the interior to lose temperature. In this way, the air inside the chamber is a barrier that avoids inner air to cool too much.

When we analyze most tents in the market, we can easily find that the immense majority of tents have a flabby flysheet (those which have one) that is not capable to be tightly separated from inner body, even many of them commonly touch frequently the inner layer. This kind of flysheets are not capable of keeping an air chamber functioning.

Thermally isolated tent CtentsTo have a real air chamber capable of producing thermal isolation it is needed to have inner and outer layer permanently separated. Otherwise, the free circulation of the air inside the chamber would be obstructed and the air chamber wouldn’t work.

To have a tight flysheet separated of the inner layer and to keep both layers stretched permanently it is not an easy task. Ctents is a tent that shows that it is possible to create a ventilated façade effect for a tent.

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