Due to the nature of energy efficiency regulations across the UK, it is impossible to state a definitive U-value that any single ground floor construction should achieve. Energy efficiency is measured by whole-building assessment methods that take into account many variables – of which building fabric performance, including the ground floor, is just one part.

U-value targets established by national building regulations are different depending on whether the project is new-build, or an extension or alteration to an existing building. The thickness of insulation required to achieve a U-value target also varies considerably depending on the size of the floor. Since that varies from building to building, project-specific U-value calculations really are essential.

A ‘typical’ XPS insulation enquiry

The Polyfoam XPS technical department is on hand to provide U-value calculations and specification advice for the use of extruded polystyrene (XPS) insulation on projects of all types.

Many of the enquiries that come through are for non-domestic buildings,  often with a U-value target of 0.22 W/m2K. To reiterate, this target is not a universal one, and therefore may not apply to your project. But it is a common request.

Also to reiterate: the size of the building, and the measurements of floor area and exposed perimeter, have a significant effect on the insulation solution proposed. To use one recent commercial project as an example, however, measuring the exposed perimeter and floor area generated a P/A ratio of 0.2.

Inputting the details into a U-value calculation gave an insulation solution of 50mm Polyfoam Floorboard, which was only 20mm more to achieve the same result as a much more expensive phenolic foam solution, as well as offering all of the other benefits that XPS provides over alternative insulants.

Thermal properties of the ground

Another significant variable that affects ground floor U-value calculations is the type of ground, or soil type, below the building. Calculation standards for floors describe three ‘categories’ of ground type, with an associated thermal value for each:

Category 1 – Clay or Silt.

Thermal conductivity: 1.5 W/mK

Category 2 – Sand or gravel.

Thermal conductivity: 2.0 W/mK

Category 3 – Homogeneous rock.

Thermal conductivity: 3.5 W/mK

When carrying out a ground floor U-value calculation, if the ground type is unknown then sand or gravel should be assumed. Unfortunately, this is where an issue commonly arises, because category 1 is often assumed and delivers more favourable results.

If the actual ground type does not fall into category 1 then the finished building will not achieve the result given in the calculation. At the worst extreme, this could mean the project doesn’t actually comply with building regulations. And while calculations do include the figure used as part of the output data, most readers of U-value calculations are unaware of the relevance of the figure and will not know to look to see what has been used.

One tool that the Polyfoam XPS technical department has started to use to try and improve the accuracy of ground floor U-value calculations is Soilscapes – an interactive online map that indicates the likely soil type that might be found in locations across England and Wales. It offers “soil overview information at a relatively basic level”, but can prove useful for the purposes of justifying the data in a U-value calculation.

Floor insulation specification

XPS insulation offers a variety of benefits that are particularly suitable for commercial and industrial applications, but which can also be employed to good effect in domestic construction. The high load-bearing capacity of XPS lends itself to projects with significant imposed loads.

The moisture tolerance of XPS means it can be used below the damp proof membrane, and also on external walls below damp proof course level – something that can help achieve continuity of insulation in Passivhaus projects where extremely high levels of energy efficiency are the target.

For more information on specifying XPS insulation in a range of ground floor constructions, contact us to book the new Polyfoam XPS CPD seminar about the benefits of XPS as floor insulation.