A requirement of national building regulations throughout the UK is that the building fabric, and the occupants of the building, are protected from moisture. That includes moisture from the ground, putting the onus on the construction of the floor in contact with the ground.

Arguably, the thermal performance of a ground floor or the loadbearing capability of a floor construction are likely to be given more consideration than this aspect of regulations, but it is every bit as important when considering the performance of the floor as a whole.

Which parts of building regulations apply to groundwater?

Part C of the Building Regulations in England sets out performance requirements in respect of moisture from the ground. Approved Document C sets out technical guidance, including different types of floor construction, to help designers, specifiers and installers comply with the requirements.

Technical guidance in Wales (Approved Document C also) and Northern Ireland (Technical Booklet C) is identical or very similar to that in England. The 2019 Building Standards technical handbooks in Scotland also detail different types of floor constructions and give guidance on how to resist moisture from the ground.

To give an idea of the nature of the guidance, paragraph 4.2 of Approved Document C in England says the following:

“Floors next to the ground should:

    1. Resist the passage of ground moisture to the upper surface of the floor.
    2. Not be damaged by moisture from the ground.
    3. Not be damaged by groundwater.
    4. Resist the passage of ground gases.”

It goes on to set out four alternative ground-supported concrete floor build-ups that may be taken as meeting the requirement to resist moisture from the ground (provided the ground is not chalk, limestone or gravel, the permeable nature of which would likely subject the building to water pressure).

What is Radon?

The excerpt from Approved Document C mentions ground gases. The slight difference between ground pressure and internal air pressure drives gases from the soil into the building naturally, via the stack effect, through cracks and gaps in the floor.

Radon in particular merits focus in building design and construction. It is a naturally occurring radioactive gas, possessing neither colour nor odour. It occurs everywhere, but concentrations vary – much of the time the level is low and of no danger to public health.

Increased exposure, however, can lead to a higher risk of lung cancer. For new construction, there are three categories of measures depending on the level of risk: no protection, basic protection and full protection. More information is available from Public Health England.

Thermal insulation placed in contact with the ground

Different types of thermal insulation are designed to be used in different ways within a floor construction, and should be specified and installed accordingly. To use Approved Document C in England as an example again, paragraph 4.10 says the following:

“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.”

Extruded polystyrene (XPS) insulation is unique among widely-available thermal insulation materials for its tolerance of damp or wet environments, and can therefore be installed below the damp proof membrane (DPM) with confidence.

We often come across situations where other types of rigid foam insulation have been incorrectly placed below the DPM, with little awareness as to the potential consequences on the performance of the insulation, or the performance of the floor as a whole. For more information about the benefits of XPS as floor insulation, contact us to book the new Polyfoam XPS CPD seminar.