How insulation can make or break a ground floor

Poorly specified and installed insulation can have a detrimental impact on ground floors. Rob Firman, Technical and Specification Manager at Polyfoam XPS explains the risks, and how different insulation types can affect performance.

When constructing ground floors in residential properties, it is not uncommon for errors to occur and there could be several reasons for this. Poorly specified insulation from the outset, failure to properly read the specification, or substituting an unsuitable product for one of the floor’s layers are just some of the issues, which could result in an incorrect floor.

But regardless of what causes the error, poorly specified and installed insulation is a significant concern for housebuilders. Using an unsuitable material risks the floor not meeting its intended U-value, which means it may not comply with building regulations. In the worst-case scenario, it could result in the entire floor failing, resulting in site delays and additional costs.

Beware of Misconceptions

Project quality and compliance with building regulations depends on using the right material in the right situation.  To ensure the correct product is selected and installed, it is vital that specifiers and housebuilders understand the different types of rigid insulation for ground floors and are aware of common misconceptions.

For example, it is often assumed that products which offer superior thermal performance are also the strongest, but this is not the case.

Different rigid insulation boards offer varying combinations of characteristics. Some are more thermally efficient but unsuitable for bearing high structural loadings. Insulation materials with a higher compressive strength may need to be slightly thicker 


to achieve the required U-value but are more robust and tolerant of harsher environments.

As each type of insulation is designed to be used in different ways within a floor construction, it should be specified and installed accordingly as highlighted in Approved Document C of the Building Regulations in England which states; 

Insulants placed beneath floor slabs should have sufficient strength to resist the weight of the slab and the anticipated floor loading as well as any possible overloading during construction. In order to resist degradation, insulation that is placed below the damp proof membrane should have low water absorption. If necessary, the insulant should be resistant to contaminants in the ground.”

Ground Floor Insulation – Key Differences

The most common rigid insulation boards are polyisocyanurate (PIR), phenolic (PF), and extruded and expanded polystyrene (XPS and EPS respectively).

Both phenolic and PIR foams derive some of their long-term thermal performance from facing materials that restrict gas loss in the foam structure. Protecting those facings is important to maintain the product’s performance which means keeping water away from the insulation boards.

PIR foam is a popular choice for domestic floors as it combines thermal efficiency with an economical price point.

Whichever product is specified, phenolic and PIR boards should always be installed above the damp-proof membrane. 

EPS has no facings that are susceptible to damage from alkalis or moisture, but its capacity for moisture absorption means it must still be installed above the damp proof membrane. While EPS is capable of much greater loadbearing capacity than phenolic and PIR foams, it is also offered in grades with lower compressive strengths. This variability means the correct grade must be specified for the correct application.


Uniquely, extruded polystyrene (XPS) insulation can be used in damp or wet environments without affecting its thermal performance. It can be laid directly on prepared ground, with the damp proof membrane laid over it also acting as the separating layer between insulation and concrete – saving time and membrane material.

In addition, its high compressive strength makes it ideal for installing below concrete floor slabs, to the outside of basement structures and – where required by national building regulations – surrounding swimming pool basins. 

It can also be used under floating floor coverings like screed or chipboard, above the ground floor slab. Alternatively, when used below raft slabs in residential buildings, XPS helps to enclose the building structure within the thermal envelope, achieving a greater level of building performance, comfort and energy efficiency.

Raising Standards

A ground floor has to meet several key requirements, including thermal performance and bearing loads, which all demand different qualities from an insulation product. 

Installing the incorrect insulation board in a ground floor may seem like a small risk, but the impact could be bigger than you think.  At a time when the construction industry is being scrutinised on how it delivers projects, such a risk should not be an option.

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