Humidity, or water steam, is the worst foe to ideal thermal insulation. Water as a main heat conductor destroys the insulation value of any material.
Water is enemy number one of any insulating system. The problem lies in water in its natural status as water steam, that is to say, humidity. Steam is a constant condition in our atmosphere and therefore it will reach by any mean through the air.
Hot air absorbs humidity
The higher the air temperature, the bigger the capacity to retain humidity. It is due to the ability of hot air to absorb steam. When air is heated it expands and makes “room” for more water molecules, so air is able to “absorb” more water (like a sponge). This process is known as “relative humidity”.
When relative humidity (RH) is zero, it can absorb all the water, but when it is 100%, it is unable to absorb even an extra drop.
“Dew point” is the temperature when the RH is of 100%. In this case, air is not able to hold water and it starts to fall, first in tiny drops known as “dew” and, afterwards, as fog (it happens when there is a sharp fall on temperatures, when there is a lot of humidity in the environment).
Temperature difference is in fact a pressure difference. Nature tends to balance this condition. Cold air is heavier than hot air. When the door of a cold room opens, cold heavy air slips from the lower part meanwhile hot air circulates through the upper part.
Hot air which contains steam comes into contact with cold surfaces and immediately it loses its ability to host humidity.
It leads to condensation: humidity is laid over the surface and insulation is exposed to its disastrous effects.
Taking into account the previous explanation, the ideal insulation would be one with the maximum gas content, air for instance, encapsulated and without the possibility of circulating.
These insulation principles have been widespread for a long time, but they have not been definitively assimilated until very recently.