Three questions about calculating ground floor U-values
Many ground floor U-value calculations feature a simple build-up of materials, in layers that have consistent thickness and performance characteristics. However, structural requirements (such as foundation design) or on-site variables can introduce complexity to U-value calculations, and lead to doubt about the correct way of establishing a floor’s thermal performance.
The Polyfoam XPS technical helpdesk is often asked the following three questions about calculating ground floor U-values, and we’ll explore each in turn.
- Does the ground/soil type make a difference to the calculated U-value?
- Does a step in the floor slab mean I should calculate U-values for two separate slabs?
- Should I account for pile caps in the U-value calculation?
Does the ground/soil type make a difference to the calculated U-value?
The type of ground a building is constructed on does affect the calculation of the ground floor’s U-value. This is addressed in BS EN ISO 13370, the international standard that sets out the method of calculating U-values for elements in contact with the ground (including floors and basements).
BS EN ISO 13370 describes three categories of ground. Each has a different thermal conductivity, so it’s important to reflect the site-specific ground conditions as accurately as possible in a calculation. If the soil type is unknown (or not confirmed), for the purposes of producing a calculation the standard says category 2 should be used.
| Category | Soil type | Thermal conductivity (W/mK) |
| 1 | Clay or silt | 1.5 |
| 2 | Sand or gravel | 2.0 |
| 3 | Homogeneous rock | 3.5 |
However, the majority of soil in the UK is clay. It’s therefore not uncommon for some offering calculation services to default to category 1 regardless of the information available. This can lead to situations where two otherwise identical calculations are being compared, with a different U-value calculated due to the soil type assumed.
“Customers naturally gravitate to the better-performing calculation if it means they can buy thinner insulation,” said Rob Firman, Technical and Specification Manager at Polyfoam XPS. “But if the final specification is based on the wrong soil type there can be a significant difference between the calculation and the eventual in-service performance of the floor.”
At Polyfoam XPS, we always ask for the ground conditions as part of the information supplied about the project. If the detail is not given or unknown, we use an online tool called Soilscapes to gain an understanding of the likely ground conditions at the project location. If there seems little doubt about it being clay then we will use that category; otherwise we use category 2 as directed.
The preference, however, is for the person requesting the calculation to tell us the soil type.
Does a step in the floor slab mean I should calculate U-values for two separate slabs?
This depends on the reason for a step being introduced into the slab and the nature of the space(s) above. For example, does the step coincide with a change between a heated and unheated space, or a change between two areas requiring different U-values?
In the vast majority of cases where we get this question, there is no change in the heating regime of the spaces above the slab at the location of the slab step. The slab therefore needs to achieve the same U-value across the different thicknesses.
Where the density of floor slab concrete is high, conventions say the concrete’s thermal performance can be disregarded in the U-value calculation. As such, the step in the concrete, and the increased slab thickness, is therefore making no additional contribution to the final result.
The required insulation thickness therefore remains the same, and the main thing is to ensure continuity of the insulation around the step so as to avoid a linear thermal bridge.
If you have a different situation with a stepped slab, such as the step occurring at a junction between a heated and unheated space, we recommend that you contact us to discuss your query in more detail.
Should I account for pile caps in the U-value calculation?
Where a floor slab bears onto pile caps and the thermal insulation has been positioned below the slab, then pile caps will disrupt the continuity of the insulation layer and their thermal bridging effect should be taken into consideration.
Depending on the depths of the insulation and the pile caps, insulating down the face of the pile cap may help to mitigate the thermal bridging effect to some degree. The precise extent can only be established through numerical modelling of the pile cap detail.
The picture is complicated by the complex heat flows through ground floors. More heat is lost at the edge of a floor than the middle, so pile caps located along the perimeter of a building are likely to act as a worse thermal bridge.
One way to avoid this issue could be to insulate above the slab instead, though the suitability of this solution depends on a variety of factors.
What to do next
If this blog post has provided useful information about ground floor design and insulation specification, then you can learn more by downloading a copy of The Polyfoam Orange Book of XPS Insulation. Created with designers and specifiers in mind, the book is a comprehensive guide to extruded polystyrene (XPS) which explores ground floors and other applications in more detail.