Minimizing energy use in buildings, and therefore improving the thermal performance of building envelopes, has become increasingly important in the drive for sustainability and energy security. We have seen the adoption of more stringent envelope thermal performance requirements in Building Regulations (Part L in the England and Wales), and voluntary certification schemes such as BREEAM and Passivhaus. These include requirements to reduce heat flow through the walls, roofs and floors. Adding insulation to the building is one obvious way to do this, but insulation is not effective if there are easy heat flow paths around it. This is why codes and standards are progressively moving to requirements based on Effective Thermal Resistance, which requires identifying and minimizing thermal bridges in the building envelope.
Thermal bridges can be defined as localized areas with higher thermal conductivity than the adjacent areas. A typical thermal bridge in a building envelope would be where a material of high conductivity, such as a structural attachment or metal flashing, penetrates the insulation layer. The presence of a thermal bridge in a building assembly would result in:
- Higher heat transfer through the assembly
- Colder surface temperatures on the warm side of the assembly
The possible consequences of these conditions include:
- Higher energy use for heating
- Higher energy use for cooling
- Noncompliance with Building Regulations
- Discomfort due to cold surfaces
- Condensation on cold surfaces, which could lead to:
- Corrosion of metal elements and structure
- Decay of wood-based materials
- Visible patterns on interior or exterior surfaces due to variations of surface temperature and drying potential
- Degradation of insulation performance (if condensation occurs within the structure)
- Mould growth and associated health concerns
A primary design goal for the construction of any building envelope in cold climates is to have a continuous and aligned layer of insulation, minimizing the number, size and impact of thermal bridges. Many designers are not fully aware of how significantly some common thermal bridges compromise the value of the installed insulation.
As shown later in this document, the heat transfer through common thermal bridges in a well-insulated building can equal the heat transfer through the insulated envelope (according to research by Oxford Brookes University). If designers do not consider the impact of thermal bridging, they will not meet the carbon emission targets in the Building Regulation Part L models used to establish compliance (SBEM and SAP).
Schöck provides product solutions specifically designed to mitigate or eliminate structural thermal bridges in commercial and multi-residential building construction. Schöck has over thirty years research experience, developing expertise both in the building physics of thermal bridging and bringing effective solutions to market.
The intent of this manual is to provide designers with:
- A better understanding of how heat moves through building assemblies and how this affects the surface temperatures and condensation control
- A confirmation of the minimum Standards and Code Requirements to ensure absence of damage
- Examples showing how the impact of thermal bridges can be mitigated during design, both in general and using Schöck Isokorb® thermal breaks
- Design guidance on how best to integrate Schöck Isokorb® thermal breaks for performance and code compliance
- Methods to calculate the impact of thermal bridges on the energy flows, temperature and moisture performance of building envelopes