A slab edge is one of the most common thermal bridges we see in New Zealand. In a traditional timber framed building on a concrete slab, the perimeter thermal bridge will often have a large impact. “ISO13370: Thermal performance of buildings — Heat transfer via the ground — Calculation methods” provides the steps to calculate it and take into account the impact of the ground. An example is shown below.
To do this calculation, we use the characteristic dimension of the floor slab (B’), defined as the area of the floor divided by half the perimeter (also found in cell H20 on the ground sheet in PHPP 9). Half of B’ is used as the width of the slab for the thermal bridge calculation—see the example with a B’ of 4m. In general, for residential buildings, we find B’ in the 3-5m range. A B’ of close to 8m would have to be a large slab with a good geometry (eg a square shape).
To understand how much impact B’ has, we considered three slab edges with varying XPS edge insulation (10mm, 30mm and 50mm). The example is a common Passive House detail: 140mm insulated timber wall, 45mm insulated service cavity and 150mm continuous EPS underneath a 100mm concrete slab.
As the graph shows, there isn’t a large difference in PSI values for different B’. The PSI value only very slightly improves as B’ increases. For individual cases, it’s important to follow the standard and use B’ in the calculation. However in general, for thermal bridges that are going to be used for multiple buildings, we recommend using a B’ of 4m and adding some margin.