Are Thermal Bridges a Problem?
by Paula Hugens, eZED Ltd | 8 June 2014
Thermal bridges are a huge problem in our buildings. Each time there is a gap in the insulation a thermal bridge is created, here heat losses can occur relatively unimpeded. Highly conductive building materials such as metals are also a problem if they are not thermally isolated.
You can see the effects a gap in the underfloor insulation has in this thermal image, here there is no perimeter insulation to the concrete slab on grade. See the heat glowing from the concrete perimeter, it looks like there’s an underfloor heating system pumping out the heat. This energy loss is much greater than the thermal bridging of the timber wall studs which are a feint vertical outline.
The floor is effectively heating the outdoor perimeter of the house which is great for frost prevention, but a huge waste of energy and costly to run.
To design your building to be free of thermal bridges start by ensuring there is a continuous line of insulation from walls to roof to floor. If you trace a line around the insulation layer in a building cross section you can check that the insulation is indeed a continuous line. Window lines can be included as part of this insulation line but leave a gap at the window frame if these are standard aluminium.
Have a look at the example cross-section I prepared from a Passive House project in Auckland. There is a nice green line enclosing the building so we don’t have any obvious thermal bridges. I look at the junctions at a bigger scale to really make sure there are no thermal bridges. A great way to do this is through isothermal analysis.
The wall to slab junction was a particular risk element for this building. Here we have an isothermal image we created of this junction to check that the detailing was satisfactory. You can see there is very little reduction in surface temperature right at the internal corner junction.
A concrete nib was introduced under the wall bottom plate to ensure there was adequate ground clearance to the exterior ground line as required by Code, however, this also introduced a thermal bridge. The problem was resolved by placing a strip of XPS insulation against the concrete nib. This is ok for a milder Auckland climate but it won’t work for any regions further south.
Having the insulation layer above the concrete slab can be very useful, it means that any conventional structural slab solution can be adopted. In this case a waffle slab was required due to highly expansive clay soils.
A thermal bridge free construction is obviously important for any energy efficient project however it is also very important if you want to avoid problems with structural decay and mould.
What should really concern you is that a surface temperature below 12.6°C at the corner junction will allow the formation of mould, as the Relative Humidity at this point will be raised above 80%. Left unchecked this will lead to structural decay in the bottom plate. Surface condensation will occur when the surface temperature drops below 9.6°C, you don’t need surface condensation for mould to grow, just moisture laden air. You can learn more if you read our case studies on Mould and Underfloor Insulation.
Copyright eZED Limited 2014