The transfer of heat energy from a building into the ground is very different to heat transfer through an external wall or roof. As a result, the way in which U-values are calculated for ground floor constructions is different to how they are calculated for other building elements.

For architects and specifiers, it isn’t necessary to have a detailed knowledge of the standard that describes this different method. What is worth knowing, however, are some of the factors that influence the calculation outcome. Armed with this knowledge, ground floor build-ups can be specified with the confidence that they will deliver their designed performance once in service.

Before getting into the detail of ground floor calculations, you might like to read our general introduction to calculating U-values, including BS EN ISO 6946:2017 and the combined method.

What is BS EN ISO 13370:2017?

Because of the thermal properties of the ground, it used to be the case that ground floors were not insulated. The continued development of energy efficiency requirements in building regulations means that option no longer exists.

BS EN ISO 13370:2017 Thermal performance of buildings – Heat transfer via the ground – Calculation methods sets out how thermal transmittance can be calculated for slab on ground floors (often called ‘solid floors’), suspended floors (such as timber joist or beam and block constructions), and basements.

A basement is defined as a usable part of the building that is partly or wholly below ground level, and the calculation method includes basement walls as well as basement floors.

Generally speaking, a ground floor U-value calculation starts by calculating the total thermal resistance of the layers of the floor construction. This is done as per the method described in BS EN ISO 6946:2017, so all the layers need to be of consistent thickness and properties, with any repeating thermal bridges taken into account.

What variables should you be aware of when requesting ground floor U-value calculations?

The variables associated with ground floor constructions impact on the accuracy of the U-value calculations, and therefore the contribution of those U-value calculations to SAP and SBEM calculations.

Heat loss from a ground floor is greater at the edge of the floor than at the centre. Arguably, therefore, the most significant variable is the size and shape of the floor, expressed by the total floor area and the length of the exposed perimeter.

The temperature of the ground in the UK remains in a fairly narrow window throughout the year, but the variability in the external climate at the edge of the floor increases the rate of heat loss. Where the edge of a floor meets the external air, or an enclosed space that is unheated, that edge is classed as part of the ‘exposed perimeter’.

Two buildings can have the same length of exposed perimeter, but be a completely different shape. The perimeter therefore bounds a significantly different heated floor area. The relationship between exposed perimeter and heated floor area is essentially an expression of how simple (or not) a building’s form is.

How does the type of soil affect a ground floor U-value calculation?

A ground floor U-value calculation is affected by the type of soil on which the building is constructed. BS EN ISO 13370 describes three categories of ground, each with a different thermal conductivity.

CategorySoil typeThermal conductivity (W/mK)
1Clay or silt1.5
2Sand or gravel2.0
3Homogeneous rock3.5

While soil in the UK is typically clay, if the ground type has not been confirmed for a project-specific calculation then it should not be assumed that the lowest thermal conductivity can be used. The standard addresses this, saying: “If the ground type is unknown, category 2 should be used.”

Assuming the wrong soil type for a ground floor can lead to a significant difference between the result of a U-value calculation and the actual performance of the floor once constructed.

About Polyfoam XPS

A ground floor U-value calculation from Polyfoam XPS is backed by our technical team’s many years of experience in the construction industry and insulation manufacturing. This blog post only offers an overview of some of the factors affecting ground floor calculations.

For example, there are a range of factors that influence suspended floor calculations and basement calculations.

Whenever we receive a U-value calculation request, we work closely with the designer or specifier to ensure that the result we produce is appropriate to the project and an accurate reflection of the proposed design.

Request a calculation from us today by contacting us about your project, find out more about standards on our blog, or view our short online CPD session all about ground floor insulation.