Thermal lag is the time delay for heat to be conducted through a material. A material with high heat capacity and low conductivity will have a high thermal lag.

Thermal lag times are influenced by:

  • Temperature differentials between each face.
  • Exposure to air movement and air speed.
  • Texture and coatings of surfaces.
  • Thickness of material.
  • Conductivity of material.

Thermal lag can be used to ease out internal/external diurnal temperature variations. In temperate climates, external wall materials with a minimum time lag of 10 to 12 hours can be very effective to reduce internal/external temperature variations during day and night. If it is required to keep the heat for a longer time, a layer of insulation can be added to slow the rate of heat transfer and moderate temperature differentials.

Units and Measures

Thermal lag can be measured as the amount of time taken for a material to absorb and then release the heat energy.

Units for measuring thermal lag is generally Hours

Example Units

The following table indicates the relative thermal lag of some common building materials.

  • Insulated Brick Veneer - 5.0 Hours
  • Concrete (250) - 6.9 Hours
  • Concrete (250) - 6.9 Hours
  • Double Brick (250) - 7.0 Hours
  • AAC (200) - 7.0 Hours
  • Adobe (250) - 9.2 Hours
  • Rammed Earth (250) - 10.3 Hours
  • Compressed Earth Blocks (250) - 10.5 Hours
  • Sandy Loam (1000) - 30 days

Useful References

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