What is meant by Direction of Heat Flow?

Heat flow or Heat transfer, is the movement of thermal energy from a region of higher temperature to a region of lower temperature. This process continues until thermal equilibrium is reached, that is, until the temperatures in both regions become equal.

Put in simple terms heat flows from something that is hot to something that is cold (elements loose heat not gain cold).

There are three main ways heat flows:

  1. Conduction: This is the transfer of heat within a solid or between solids in direct contact. The heat flows from the hotter part to the cooler part. Examples would be direct fix cladding transferring solar radiation to the internal environment or a window exposed to the internal and external temperatures “conductive heat loss through the window frame”.

  2. Convection: This is the transfer of heat in fluids (liquids and gases) where the heated fluid rises and the cooler fluid sinks due to differences in density, setting up a circulation pattern known as a convection current. This regularly gets referred to as “stack ventilation” or “hot air rises”.

  3. Radiation: This is the transfer of heat in the form of electromagnetic waves, primarily infrared radiation. Unlike conduction and convection, radiation can occur in a vacuum, such as the heat from the Sun reaching Earth.  In building world we use a term thermal mass to describe the ability of a material to hold heat, e.g. solar action through the day will allow the thermal mass to gain heat and then as the air temperature drops at night it radiates the heat back to keep us warm.

It’s important to note that while we often talk about heat ‘flowing,’ it is actually the energy that is moving, not the material itself (except in the case of convection).

So if it is energy moving think about your power bill at home! We will use the words heat loss, this is another ways of saying wasting money.

In building world when describing where a control layer (insulation layer or air barrier) should be installed you will hear people say “on the warm side” this is talking about the direction of heat flow (hot to cold) and is dependant on your climate;

  • If we are talking about bulk insulation it is best located as close to the warm side of the building envelope.
  • If we are talking about an air barrier that restricts air movement, then we want it on the warm side of the building envelope.
  • If we are talking about a vapour barrier that inhibits moisture movement, we want it on the warm side.
  • If we are talking building materials and in a cold climate we want to keep things warm so they don’t reach dew point temperature, we want to reduce or avoid thermal bridging to avoid elements getting cold and reaching dew point temperature.
  • Hot humid climates, the warm side is usually on the outside, hence we are trying to stop heat getting in, trying to stop vapour from the outside, with air conditioning keeping the building cool, there is a high risk of interstitial condensation. Vapour or Air barriers on the outside of the building envelope will reduce the risk.
  • Cooler climates the warm side is usually on the inside, if it is somewhat airtight the vapour level will be higher on the inside, hence temperature or vapour is trying to get out of the building.  Showering and breathing will add moisture vapor to the air, there is a high risk of this vapour going through plasterboard and condensating within the walls. We want Vapour or Air barriers on the inside of the building envelope to stop the potential of this occurring.
  • Temperate climates can be in both directions, usually heating is more dominant hence both heat flow and the vapour drive will be from the inside to the outside similar to cooler climates.

Using the BCA to help the discussion, review BCA Volume 1 J4D4 Roof & Ceiling Construction

  • Climate zones 1 to 5 (middle to top of Australia) it says “downward heat flow” we are adding R value to stop downward heat flow or stop the heat getting in (heat gain).
  • Climate zone 6, (more the bottom of Australia) it says “downward heat flow” we are adding R value to stop downward heat flow or stop the heat getting in.
  • Climate zones 7 & 8 (Tasmania & Snow regions) ) it says “upward heat flow” we are adding R value to stop upward heat flow or stop the heat getting out (heat loss).

If you would like to know anything further, please contact us here!

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