Ground floor edges, at the abutment with external walls, are one of the most common areas for breaks in continuity of a building’s thermal envelope to occur.
However, as we seek to further reduce carbon emissions from buildings, and as our understanding of moisture behaviour improves, regulations like Part L 2021 are starting to provide more specific guidance on addressing them.
Floor edges are difficult to detail because the loadbearing external wall has to continue to foundation level. This usually prevents a connection between the ground floor insulation and the chosen external wall insulation, unless very specific, high-performance solutions are selected.
How does Part L 2021 address continuity of insulation?
Solutions to lengthen the path of the thermal bridge at the floor edge have been established for some time, because eliminating the thermal bridge entirely is difficult in typical construction methods – but it’s important to be aware of how that detailing needs to adapt.
Compared to Part L 2013, Part L 2021 sets out more detailed technical guidance on how continuity of insulation must be addressed.
Approved Document L1A 2013 said that the building fabric should be constructed “to a reasonable standard so that the insulation is reasonably continuous over the whole building envelope.”
Approved Document L1 2021 – which, depending when you are reading this, takes or took effect in June 2022 – maintains the “reasonably continuous” wording, but goes on to provide specific guidance in a number of new areas.
Designers, specifiers and installers need to be aware that the insulation layer should be identified on drawings, and that a review of drawings should take place to ensure the layer is “continuous, buildable and robust”. An on-site audit should then take place, before work is concealed, to assess that the designed details have been constructed.
What new floor and foundation requirements are in Part L 2021?
These requirements are not specific to ground floor edges – but there are also a couple of new requirements relating to floors and foundations. These aim to address the thermal bridge at the floor edge, and to further lengthen it, over and above existing measures.
The first point specifies that the perimeter upstand insulation be a minimum thickness of 25mm. Judging by the details we see, this is already commonplace in detailing. Nevertheless, it remained possible that people were specifying and/or installing less than 25mm, or even none at all.
The second point says, “moisture-resistant insulation should be fitted below damp-proof course level and extend to the foundation block/structure.”
How extruded polystyrene (XPS) insulation can help to meet Part L 2021’s requirements
Even before publication of Approved Documents L1 and L2 in December 2021, at Polyfoam XPS we were seeing increased demand for insulation solutions in the base of masonry cavity walls.
Specifiers wanted a moisture-resistant insulation that could be used to improve thermal performance below the DPC, without any risk that the thermal performance at the junction would be compromised if the insulation came into contact with moisture. Compared to other lightweight, rigid foam insulation boards, XPS has the lowest moisture uptake.
Unlike expanded polystyrene (EPS) and polyisocyanurate (PIR) products, for example, XPS can be used as the ground floor insulation layer below the damp proof membrane (DPM). Installing it in the base of cavity walls to reduce thermal bridging is therefore a natural extension of the product’s capabilities.
To support the increased use of XPS in wall constructions below DPC, Polyfoam XPS has published a white paper detailing how extruded polystyrene can be specified and installed, and providing psi values for common ground/floor external wall details to demonstrate the thermal benefits that can be achieved.