What is Thermal Bridging?

‘Thermal Bridging’ is a term that is being thrown around lately but what is it?

Imagine wearing a pair or oven gloves that have copper wires on the outside penetrating through the insulating material to your fingers……. Now pick up a hot oven tray.

Thermal bridging occurs when a more conductive material allows an easy pathway for heat flow across a thermal barrier. The wire in the oven glove is a thermal bridge.

Now, lets put this into the perspective of your home. The exterior walls of your home form part of the building envelope and are insulated,  (click here to be reminded what a thermal envelope is). The intent of this insulation is for your home to stay warm during the winter months and cool in the summer. Insulation is measured as an R-value which is its capacity to resist heat flow.

Now consider a typical 90mm timber framed wall assembly with fibreglass insulation batts placed between the framing members, (plasterboard lining on the inside and light weight cladding on the outside). The insulation batts have an R-value of R2.5 and the timber frame has an R-value of R0.9. The timber frame allows heat to travel through the wall assembly nearly 3 times faster than through the insulation. (2.78 for those of you doing the sums).

The timber frame acts as a ‘Thermal Bridge.’ When you consider a wall frame can be about 25% of the wall area, this thermal bridging reduces the overall R-value of the wall assembly to R2.04 which is about 30% lower than the assumed wall system R-value of R2.84 without taking thermal bridging into account. (28.17% for those of you doing the sums). Now plug a steel frame into the equation and watch your Wall system R-value drop to R0.8.

Now that you’ve learnt about a poorly designed oven gloves and how wall framing conducts heat, have a think about how much heat might be transferred through a window frame or a single pane of glass. Here’s a clue. A typical single glazed, aluminium framed window has a U-value of 6.5 which is equal to an R-value of R0.15. Even our thermally bridged timber framed wall with R2.5 batts achieved a total R-value of R2.04…..The thermal resistance of the window is only 7.35% of that of the wall. Remember too, thermal bridging not only decreases the thermal comfort of the building, it also contributes to moisture problems, particularly in the way of condensation.

Lastly, don’t panic. There are things we can do about thermal bridging, and being aware of it will help us make better informed decisions when we come to design our building envelopes. As I keep saying, it’s not enough to just specify insulation. We need to have a good grasp of building physics and take a ‘whole house’ approach to design in order to create safe, healthy, durable, comfortable & energy efficient buildings.


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