Key considerations of insulation solutions for domestic ground floor design

Karen Everitt at RAVATHERM UK explains best practice methods during insulation specification and installation of ground floors.

During the design of any new build or major refurbishment project, careful attention must be paid to the ground floor insulation specification. As well as contributing to the thermal efficiency of the finished building, it must also provide a durable and robust solution.

The Insulation Market

The recent shortage of polyisocyanurate (PIR) insulation has impacted many housebuilding projects. Caused by insufficient supply of Methylene Diphenyl Diisocyanate (MDI), the main constituent of PIR, severe production delays have resulted in a market-wide shortage and extended lead times – not to mention price increases – leading housebuilders and developers to seek viable alternative insulation products.

Although not traditionally a first choice for domestic projects, extruded polystyrene (XPS) insulation is one of those alternatives, and there are some key considerations for specifiers and installers if XPS is specified. For instance, slightly thicker XPS board will likely be required, meaning a little more digging out at the start of construction.

As when using any unfamiliar construction product, it is recommended that housing developers liaise with a reputable XPS manufacturer with a knowledgeable technical team to ensure the key differences of an alternative material against the original specification can be explained.

Design Considerations

During design stage, consideration needs to be given to the applied floor loading and the position of the installed insulation. Ground bearing floors can include insulation either below or above the concrete slab, depending on the choice of the designer. Insulation installed below the slab increases the thermal capacity of the building, helping to maintain steady internal temperatures. If insulation is installed above the slab, the building responds more quickly to an intermittent heating system.

Compressive Strength of Insulation

Insulation materials used under slabs, screeds and chipboard should be capable of accommodating the applied loads with the minimum of compression. It’s best practice for housing developers to refer to the guidance and recommendations contained within BS EN 1990:2002 +A1:2005 and BS EN 1991-1- 1:2002, which should be followed when making sure the insulation is able to accommodate the designed loads.

In reality, the loads in a domestic property are not likely to be significant. However, some circumstances such as basement constructions or swimming pools impose greater loads, and benefit from the insulation being laid directly on the ground. Basement projects are increasingly popular in London where many people are ‘building down’ due to space restrictions to build around the home.

In these instances, XPS insulation really comes into its own and delivers a durable solution thanks to its compressive strength and low moisture absorption.

The Position of the Insulation

As briefly mentioned earlier in the article, the position of the insulation in the floor has an influence over the thermal characteristics the floor brings to the building. Ground bearing floors can include insulation either below or above the concrete slab, and there are factors the designer needs to be aware of when choosing either method.

Where the insulation is below a slab, screed or timber boards for example, the entire load is acting on the insulation. Point loads are spread by the layers above the insulation so that the load acting on the insulation is lower than the load applied to the floor surface.

In most domestic projects, an ‘over slab, under screed’ insulation solution is ideal for intermittent heating regimes, where a homeowner may only turn on the heating twice a day. In this scenario, the insulation should be installed with a vapour control layer over it. Ideally, the insulation should be positioned above the damp proof membrane. Yet in some circumstances – due to cost savings or traditional thinking about installation – this is not always the case. However, if installation doesn’t accord with best practice, the durable qualities of XPS insulation means it is resilient enough to still perform as expected.

The versatility of XPS insulation means it’s ideal for domestic projects, and even when installed below a damp proof membrane, it can still offer its declared thermal performance, which is reassuring for specifiers considering switching specifications for future projects.

Calculation of U-Values

Unlike insulation specified for walls and roofs, the heat loss through a ground floor depends on the size and shape of the floor, as well as the soil type. The calculation methodology in BS EN ISO 13370 should be followed, which uses the ratio of the exposed floor perimeter to the floor area because more heat is lost at the edges of a floor than in the centre.

The measurement of the perimeter and area should be to the finished inside surfaces of the perimeter walls that enclose the heated space; however, unheated spaces such as porches or garages should be excluded. In the instance where extensions to existing buildings need to be considered, the floor dimensions of the complete building may be taken, including the extension.

Making the Right Choice

During the design of any building, it’s important to ensure the correct specification of insulation material, and to follow the recommendations of current guidance and Building Regulations. Liaising with a trusted XPS insulation manufacturer, offering technical and specification advice during design stage, will ensure the building performs as per the specification.