In this tutorial we're going to look at how you define thermal bridges in the model. Thermal bridges are elements of the building's fabric with a significantly worse energy performance than the general material. An example might be a concrete lintel over a window, or some kind of steel fixing which runs all the way through a wall. In low energy buildings thermal bridges can be a significant source of energy loss, so if at all possible you should design them out. However, if you can't there are some options to define losses through thermal bridges in the spreadsheet.

In the thermal bridges sheet we have an option to define linear thermal bridges, like a lintel for instance, or point thermal bridges like steel fixings through a wall. In both cases we can define there are no thermal bridges. However, if we do have thermal bridges we define first a loss through that thermal bridge. In the case of a linear thermal bridge that's a loss per meter. We define the length of the thermal bridge. If we're defining a lintel over a window for example this would be the total length of all the lintel. Similarly for a point thermal bridge we define the loss. In this case it’s a loss per point, and then we define the number of points. So for instance the number of those steel fixings through a wall.

Once we've defined thermal bridges we'll have a summary of the the flow through thermal bridges for space heating and for space cooling. These are broken down by linear or point thermal bridge, and by months of the year.

If we have no specific values for thermal bridges but we know our design isn't thermal bridge free, we can apply just an approximate value which is based on the area of the building fabric. SAP10.2 recommends that for each square meter of building fabric you allow for a thermal bridge loss of 0.2 watts per kelvin. There is a tool to do this calculation in the Calculators sheet. You can then copy the value, and paste it as a single point thermal bridge. In this case we’re effectively using the the point thermal bridge to define total thermal bridges losses through wall fabric.

Like for some of the other building fabric input sheets, if we increase the complexity level we don't actually have any additional inputs to define, but it does give us more detail on the calculations that have been completed.