HOW CAN CONDENSATION RISK AT GROUND FLOOR EDGES BE REDUCED?
Enhancing the insulation specification at the base of masonry cavity walls, below damp proof course (DPC) level, can reduce thermal bridging and ensure there is no risk of surface condensation occurring. At the same time, it can also help to guard against ingress of moisture from the ground, thereby satisfying multiple requirements of building regulations.
Increasingly, architects, housing developers and other specifiers are looking to use a different insulation solution below DPC than they do in the cavity above DPC level. This is leading them to consider extruded polystyrene (XPS) in the base of the cavity, with a standard full fill or partial fill solution in the rest of the wall.
We discuss the regulatory drivers, including moisture resistance, behind this change, examine the certification requirements for cavity wall insulation, and detail best practice XPS installation in separate blog posts. This post looks specifically at how the solution also helps to remove any risk of condensation.
Thermal bridging and condensation risk in floors
Energy efficiency requirements in national building regulations require thermal bridging at junction details to be minimised. Psi values – the rate of heat loss through linear thermal bridges such as the ground floor/external wall junction – are entered as part of SAP calculations.
This has been the case for nearly twenty years, but the introduction of updated regulations like Part L 2021 has placed even greater emphasis on addressing thermal bridging heat losses.
In addition, as part of its guidance on achieving continuity of insulation at the floor edge, Part L 2021 also makes specific reference to “moisture resistant insulation”. This, in part, is what has increased the number of specifiers looking to XPS insulation as an appropriate solution.
In addition to Building Regulations, NHBC Technical Standards outline the need to achieve appropriate levels of performance at junctions.
A poorly detailed junction is also a risk for surface condensation internally. Without sufficient insulation at the junction, the rate of heat loss compared to the surrounding wall is considerably higher and the surface temperature of the wall at the junction locally much lower.
Using surface temperature factors to assess condensation risk
The same thermal modelling that calculates psi values also calculates the surface temperature factor, or fRSI, for the linear thermal bridge.
The fRSI shows the internal surface temperature at the junction detail, thereby predicting the likelihood of condensation and mould growth occurring. To avoid a risk of condensation occurring, surface temperature factors should exceed 0.75.
As part of our white paper about wall insulation solutions below DPC we thermally modelled all of the enhanced details and insulation solutions included as guidance.
The heat flux diagrams and calculation outputs provided in the white paper can be used to give an idea of the likely performance of a junction detail featuring Polyfoam XPS insulation below DPC level. However, they should not be used as a substitution for project-specific details and calculations, as floor and wall specifications will differ.
Download the white paper from our technical support section, or contact us to discuss the floor/wall junction detailing on your current project and how XPS could help to meet the requirements of Part C and Part L.