Insulation – Heat Loss Slab On Ground

Why is there less fuss about insulation under a concrete raft slab than ceilings and walls?…………well here are a few interesting facts:

  1. A thick layer of earth provides a reasonable amount of insulation.
  2. The soil contributes to the thermal mass of the structure which helps smooth out any temperature variations.
  3. The temperature of the ground below the surface varies much less than the air temperature. For Victoria a ground temperature range in the order of 13 degrees in winter to 22 degrees in summer is typical.

As a consequence the main heat loss from the slab is only from the edges of the slab rather than from the middle.

Heat Loss Calculation

When estimating the heat loss a key factor is the ‘ Perimeter to Area Ratio’ (PAR). Examples are:

For a 10m x 10m slab the PAR = 40/100  = 0.25

For a 20m x 5m slab the PAR   = 50/100  = 0.5

The  table below provides some values of ‘U’ for the total structure for various values of the ‘PAR’ .



















So for a typical single storey house of 20m x 10m

The PAR = 60 / 200 = 0.3

From the table ‘U’ is  0.49

The Heat loss from the slab  =   Area x ‘U’   =   200 x 0.49   =   98 watts/degree C

The ‘U’ value for this standard slab is similar to an Insulated  Brick Veneer  Wall.

A Waffle Pod Slab will have a slightly better insulation value but have a lower thermal mass.

If you want to install slab edge insulation see Insulating Your Slab.


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