Passivhaus Thermal Bridges and Psi Values
The following article contains details and information from our book Understanding Passivhaus – The Simple Guide to Passivhaus Detailing and Design and Details:Calculated.
This article also contains contributions from Tad Everhart.
Passivhaus Thermal Bridges
A thermal bridge, also known as a cold bridge, occurs where the conductivity of the thermal envelope changes substantially. For example, a highly-conductive material interrupts the insulation layer, or the thickness of the insulation layer changes.
Essentially, a thermal bridge is a conduction path where heat moves more rapidly through the building fabric than in surrounding areas. Thus, it is a bridge from the outdoors to the indoors over which heat can move quickly—out in winter and in on a hot summer day. Always from warmer to cooler, both in winter and in summer. Heat follows the path of least resistance: the thermal bridge.
A thermal bridge can have many negative effects on a building, but even more so in a Passivhaus. Investing in high levels of insulation, we expect excellent thermal performance. However, heat travels from well-insulated surrounding areas towards thermal bridges and slips around the superior insulation, wasting the investment.
Thermal bridges cause not only unwanted winter heat loss and summer heat gain, but also potentially cold surfaces where moisture condenses, supporting mould growth, and health risks. These cold spots can also lead to discomfort.
A Passivhaus aims to be thermal bridge free where possible, and to minimise all unavoidable thermal bridges.
Thermal Bridge Free
Passivhaus defines a building as “thermal bridge free” where the calculation of the heat loss from all the thermal bridges present in the construction doesn’t increase the overall building heat loss calculation based on the area of the exterior of the fabric.
Generally speaking, a junction in a Passivhaus is considered to be thermal bridge free if the insulation at the junction is continuous at a minimum of two-thirds the thickness of the insulation in the assemblies meeting at that junction.
PHPP slightly overestimates heat transfer through the envelope by calculating envelope area from exterior dimensions; thus, conservatively automatically offsetting typical minor thermal bridges.
Careful design enables construction of continuous insulation and avoids or minimises thermal bridges. If the designer adopts this approach early in the design phase, and the builder follows the design, they can ensure thermal bridge free construction.
Thermal bridge free construction is not only energy efficient, but it ensures occupants’ thermal comfort.
Calculating thermal bridges is a laborious task, and as such the Passivhaus method recommends a practical approach the thermal bridging:
- Following pre-calculated standard details will avoid or minimise calculations. These are available in catalogues, published plans, and for PHI-certified building systems.
- A junction in a Passivhaus is considered to be thermal bridge free if the insulation is continuous at a minimum of two thirds the thickness of the insulation surrounding that junction.
- Any repeating thermal bridges such as timber studs in a wall construction are accounted for in the U-value calculation for that assembly.
The effect of thermal bridges on the thermal performance of the building is quantified as a “psi-value” which is essentially a correction factor of the greater rate at which energy passes through a length of material at a thermal bridge than it does through the adjacent assemblies.
Psi-values account for thermal bridging
Remember that a thermal bridge is a place in the building envelope where the conductivity of the envelope changes substantially. This is typically where two or more assemblies are connected. These junctions often include structural materials which are selected for strength withstanding loads, sheer, and other forces; not insulation selected to slow conduction. These materials are often more dense than insulating materials, and because of that, more conductive.
The psi-value accounts for the greater amount of heat transferred through the thermal bridge than through the adjacent areas. Quantified by time, we speak of heat moving faster through thermal bridges than through the rest of the building envelope assembly.
Explore Psi Values and Thermal Bridges further
If you would like to explore this topic further, we delve into Psi values and thermal bridges in more detail in Understanding Passivhaus and Details:Calculated. Details:Calculated includes a large selection of Psi value calculations for Passivhaus constructions and key junctions. Some examples below.