Thermal Bypass

by | Apr 16, 2024 | Glossary | 0 comments

The Definition and Facts about Thermal Bypass

Thermal Bypass is the unintended transfer of heat across building elements, circumventing insulation or other thermal control layers.

In other words, in the UK, thermal bypass occurs when cold air penetrates to the warm side of the insulation layer, or warm air penetrates to the cold side, in any construction. It is caused mainly by wind and by convection within the building fabric.

Studies have shown that gaps on the warm side of the insulation can increase the heat loss through the wall by 300% thus lower the actual U-value of that element.

Air leakage through the building fabric and ‘wind washing’ are both forms of thermal bypass. Learn more about air leakage test by clicking here.

How to tackle Thermal Bypass?

Therefore, on any retrofit project that introduces new insulation measures to the fabric it is critical that:

  1. The correct insulation is specified and designed accordingly.
  2. The existing element is prepared in a way that allows the correct installation of any new measures
  3. The installation is done in strict accordance with the manufacturers guidance, making sure that all joints and seams are sealed, and that the insulation adheres to the substrata correctly.
  4. That there is sufficient quality control on site to make sure the 3 points above are achieved.


The above shows what huge impacts thermal bypasses can have on our buildings and in turn what a large affect these can have on heat losses, carbon emissions and the general health and comfort of the building’s occupants. Contact us today to learn more.