Thermal Bypass: What is it and why does it matter?

Thermal Bypass, so what is it? When we are looking at retrofitting buildings, Thermal Bypass is the transfer of heat (or heat loss) that bypasses lower conductive materials that are designed to limit such heat loss.

The diagrams below show the 3 most common methods of Thermal Bypass in construction:

  1. Wind Washing. This is where external air is forced into the building element from outside penetrating the insulation layer and forcing warmer air out. A good way to visualize this is to think of being on a mountaintop on a windy day wearing a woolly jumper. While the jumper would provide some comfort, adding a windbreaker jacket would make you much warmer as it would prevent a large amount of that wind from circulating around your wool jumper. This is why external wind barriers are an important part of the puzzle when designing new insulation measures.
  2. Infiltration or Exfiltration. This is very similar to the above situation but in this case, air from outside will brace the entire building element reaching the internal conditioned space (Infiltration) and lowering the temperature. The reverse can also happen here, where warm conditioned air is forced outside creating large heat losses (Exfiltration)
  3. The final and perhaps most overlooked example of thermal bypass is that of Convection within an enclosed space, as is most often the case in cavity wall construction. Here as warm air rises, it forces colder air to be pushed downwards and a convection circle can occur. This can lead to areas of the internal wall becoming much colder creating unwanted cold spots and greater heat losses.

But why does this matter? In some cases, depending on the materials used, the design implemented and the level of installation, significant variances in the performance of the fabric elements can occur. For example, if an air gap is present between newly introduced insulation/existing substrate, and if there are joints that open to the external environment at the top and bottom, heat loss can increase by up to 520% ¹ depending on the size of the gaps present.

Put in other terms, imagine retrofitting a property, thinking you were going to achieve a U value of around 0.2 with new insulation. Then the insulation is installed poorly, leaving gaps behind it and above and below and your final U value ends up being in the region of 1.0!

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 QA 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.

Pete Marsh
Pete Marsh, Technical Director

About the author

After graduating with a BEng (Hons) in Civil and Coastal Engineering, Pete diversified his skills and trained as a Passive House Designer and OCDEA. As VOR’s technical consultant, Pete focuses on technicalities of the retrofit design process.


  1. Hens H., Carmeliet J. (2002). Performance prediction for masonry walls with EIFS using calculation procedures and laboratory testing. Journal of Thermal Envelope and Building Science; 25; 167