Thermal Bridges: Fundamentals
What are Thermal Bridges?
Thermal bridges are areas in a building's structure where heat escapes more easily than in the surrounding parts, disrupting the insulation. This happens due to the materials used or the building's shape. For example, materials like timber framing or steel beams have higher thermal conductivity, allowing more heat to pass through them compared to the insulated parts of the wall. Geometrically, thermal bridges occur at corners or edges where there is more exterior surface area compared to the interior, leading to uneven heat flow. In these spots, the insulation is less effective, causing more heat to escape and making those areas colder. This uneven heat flow can increase heating costs and lead to moisture issues, such as condensation and mould growth.
Significance in the Building Code
With the growing demands for the insulation of building envelopes, the particular significance of thermal bridges as thermal weak points will need to be recognised beyond Passive House, or New Zealand Green Building Council's Homestar Rating. To ensure the overall high quality of an insulated building, special attention must be paid to thermal bridges. This consideration is not only due to energy aspects, but also because thermal bridges have a direct impact on the hygienic conditions inside the building.
What are the consequences?
Due to the higher heat flux density in the area of thermal bridges, the surface temperatures of the component on the room side decrease. This has various consequences:
Comfort:
The stronger cooling on the room side of the construction can lead to restrictions in comfort, as the component is perceived as cold.
There can also be radiation asymmetries between other adjacent components, which are perceived as uncomfortable.
Energy Losses
The higher heat flux density of the thermal bridge also means higher energy losses in this area.
Moisture Problems
Due to the lower temperature level of the cold component, the relative humidity in the boundary layer of the air increases, leading to the condensation of water vapour on the component.
At a relative humidity of 80%, there is a risk of mould growth. Thermal bridges often lead to a hygienic and possibly health problem.
Types of Thermal Bridges
1. Material Thermal Bridges
Timber Framing: In an insulated wall, timber studs can create thermal bridges because wood has higher thermal conductivity than the surrounding insulation material.
Steel Beams: Steel has a much higher thermal conductivity than most building materials, so any steel beams that penetrate through the insulation layer can create significant thermal bridges.
Concrete Columns: Concrete columns or slabs that extend through the insulated envelope of a building can also act as thermal bridges due to their higher conductivity compared to insulating materials.
2. Construction Thermal Bridges
Window and Door Frames: Improperly insulated window and door frames can allow more heat to pass through compared to the adjacent insulated wall sections.
Balconies: Cantilevered balconies can act as thermal bridges since they protrude from the main insulated envelope and create a direct path for heat to escape.
Penetrations for Utilities: Holes or gaps for pipes, wires, and ducts that pass through the building envelope without proper insulation can become thermal bridges.
3. Geometric Thermal Bridges
Corners of Buildings: External corners where walls meet at a right angle create thermal bridges due to the increased exterior surface area relative to the interior.
Wall to floor connection: The intersection where the wall meets the floor slab wall creates a thermal bridge.
Roof Eaves: The intersection where the roof meets the exterior wall creates a thermal bridge.
Common Thermal Bridges in Buildings
Window connections
Interior wall connections to exterior walls, floors, or ceilings
Floor slabs to exterior walls and foundations
Floor slabs integrating into exterior walls
Interior and exterior corners of façades
Connections of cantilevered components, such as balconies or canopies
Corner formations of exterior walls at ceilings and roofs
Penetration points of service lines
