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Homework Help: Thermal conductivity of air

  1. May 28, 2013 #1

    I was hoping someone could enlighten me on the thermal conductivity of air. I'm aware the λ for air is 0.024. Meaning its a very good insulating material. Much better than the conventional insulating material we use for conventional buildings such as EPS and XPS which are at λ 0.030.

    My questions are as follows - what happens to the thermal conductivity of air under pressure? (not vacuum - but pressure.) logical answer would be more gases per sq mm. Therefore easier conductivity therefore higher λ value. But I've been unable to find a suitable answer to reference.

    Second question is: albeit a little silly one which I know is not true but I have been unable to answer this damn question as my physics knowledge is limited and unfortunately during my studies in genetics I didn't go into much detail - if air has such a good thermal conductivity why the requirement for insulating material in buildings? Again, logic explains it as - due to the good thermal conductivity of air it takes a long time for it to change temperature which explains why despite a strong sun during early mornings the temperature gradually rises towards mid-day. so I am to assume a different median is required to break that process such as a concrete brick work.

  2. jcsd
  3. May 28, 2013 #2
    The thermal conductivity of an ideal gas is independent of pressure.

    If we just used pure air as the insulating material between inner and outer surfaces of a wall, the heat transfer rate would be enhanced by natural convection of the air. With insulating materials, the air is still a big part of what is happening (it is present together with the insulating material), but the insulating material also acts to retard the air from circulating and enhancing the heat transfer rate.
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