WHAT IS THE DIFFERENCE BETWEEN PERIMETER UPSTAND AND EDGE INSULATION IN GROUND FLOORS?
Although the two terms are sometimes used interchangeably, perimeter upstand insulation and edge insulation are very different components of ground floors –
– the former being designed to address thermal bridging heat losses, the latter able to replace ground floor insulation layers in certain types of floor construction.
In their own ways, however, both address that heat is likely to be lost at the edge of a floor construction, making design and detailing of the junction between ground floors and external walls particularly critical. Updates to national building regulations in 2022 have highlighted the importance of ensuring continuity of insulation, especially at the floor/wall interface, which we wrote about here.
Defining perimeter upstand insulation
Specifying and installing upstand insulation to the perimeter of ground floors is an established method of reducing thermal bridging heat losses in buildings. A strip of insulation cut to the depth of the concrete floor slab or screed, and often no more than 25mm thick, is installed between the edge of the slab/screed and the inner face of the external wall (item 11 on the following CAD detail).
The upstand is designed to address the fact that continuity of insulation between the ground floor and external wall is impossible to achieve, due to having to continue the wall structure down to foundation level for structural purposes. The upstand therefore lengthens the path by which heat energy has to travel, reducing the impact of the linear thermal bridge.
Due to its position, the perimeter upstand insulation does not contribute to the U-values of either the floor or the wall. Instead, the junction detail must be calculated as a linear thermal bridge, with the upstand insulation contributing to a reduction in the psi value of the junction.
What is ground floor edge insulation?
A standard called BS EN ISO 13370:2017 Thermal performance of buildings. Heat transfer via the ground. Calculation methods describes how to carry out U-value calculations for ground floors and basements.
It is this standard that defines edge insulation, and it illustrates two possible arrangements for edge insulation – one horizontal, and one vertical. At the Polyfoam XPS technical helpdesk, the arrangement we have seen most often is horizontal, though occasionally a project has proposed vertical edge insulation instead of a full ground floor insulation layer.
Unlike other construction elements like walls and roofs, heat loss from a floor in contact with the ground is not even across the entire element. Instead, due to the effect of the ground conditions, more heat is lost from the edge of the floor than from the centre of the floor.
For certain types of floors, therefore, it’s possible to insulate only part of the floor area – around the edge – and still achieve the target U-value. This is typically only the case for particularly large floors with a low perimeter to area (P/A) ratio, restricting the use of edge insulation to warehouses or other large commercial or industrial buildings with a simple form.
Is ground floor edge insulation acceptable under building regulations?
Increasingly low target U-values make it harder than ever to specify only edge insulation. Where the right combination of circumstances makes edge insulation a feasible option for meeting the U-value requirements, it must be questioned whether failing to provide continuous insulation across the whole floor would constitute a failure to meet other aspects of guidance in the building regulation documents.
If and when edge insulation is capable of meeting the target U-value and being acceptable in terms of continuity of insulation, it is calculated as part of a standard U-value calculation. The edge insulation acts instead of a typical ground floor insulation layer, with the calculation procedures adjusted accordingly.
To find out more about ground floor insulation and U-values on your project, contact us to discuss how XPS can help your project to achieve better floor edge detailing.