Thermal Bridging

What is Thermal Bridging?

Any break in the thermal continuity of the building envelope.

Thermal bridging is when materials that conduct heat – typically with low thermal mass- pass through the building envelope and transfer heat and cold through those elements both in and out of the building, effectively bypassing the building’s thermal envelope.

Thermal bridges are the weak points in your building where heat can escape quickly.

There are 3 types so Thermal Bridges: Clear Field, Linear, and Point Thermal Bridges

Clear Field – a regular array of point bridges over a defined area of the envelope (façade anchors, insulation fasteners). Represented by Chi Value (Watts per meter kelvin or W/mk) or de-rate in R-Value to assembly or insulation layer.

Linear – Found along the length of an assembly or junction of two exterior surfaces – easy to see as a line on the thermal image. Represented by Psi Value – measured in W/mk.

Point – penetrations of the envelope (steel column through the floor, pipes through roof or walls, etc.) Represented by Chi Value – W/mk.

Heat Exchange – The example image to the right highlights one a Linear type of thermal bridging with the wall studs and windows.

Common Culprits:

Material Changes – Windows/Doors, insulation to adjacent material.

Repetitive Bridges – studs, floor joists, rafters, especially metal.

Penetrations – pipes, fasteners, especially metal.

Assembly junctions – roof to wall, wall to floor, wall to wall, balconies, canopies, structural connections.

Corners and parapets.

Metal and Concrete/CMU are the most problematic heat transfer materials – but good detailing can prevent and minimize loss.

Calculation Methods:

Concept: TB (thermal bridges)

Software is needed to give you heat loss data. Always assume (in northern US) an exterior temperature of 14° F. and an interior temperature of 68° F.

Solution: Good Detailing and Thermal Breaks

Thermal Breaks – You want thermal breaks. Windows and fasteners and connection points with thermal breaks provide an interruption for the culprit. They provide a major difference in thermal bridging points and heat loss.

Continuous insulation is mandatory for energy codes and thermal breaks and supplementary.

The featured image of the post (above) demonstrates this clearly: the insulation layers should be planned in such a way that one should be able to outline the minimum insulation thickness (20 cm for the Passive House) of the whole external envelope within the insulation layers using a pencil, without a break. This is commonly referred to as the pencil test.

Additional Resources:

Simulation tool “Therm” – simple to use to explore options for points of interest. Requires external calculations for psi value calculations. Look for irregularities or “bulges.” Helps show where dew point and low temperatures may occur on the inside of an envelope and cause mold.

https://passipedia.org/basics/building_physics_-_basics/what_defines_thermal_bridge_free_design

Here is a guide for high-performance walls: https://www.swinter.com/wp-content/uploads/High-Performance-Walls-2019-reformatted.pdf

https://thermalenvelope.ca/catalogue/

https://www.peelpassivehouse.ca/thermal-bridge-modelling