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Moisture in ground floors: what is going on below the floor finish?

Dealing with unwanted or excess moisture in a floor can be challenging, particularly when decisions may be out of your remit. Rob Firman has some solutions…

A FLOOR finish is only as good as the quality of workmanship in the ground floor build-up below it. Whether your responsibility is specifying the desired finish, or installing it, the quality of your work can be impacted by decisions made during construction of the floor.
Often, those decisions may be out of your remit and not in your control. Dealing with unwanted or excess moisture in a floor is a good example of when such a situation could occur.

If there’s an issue with the floor finish, any remedial work that repairs or relays the finish might only be addressing the end result of the problem, rather than the root cause of it. The nature of ground floors means that digging up the floor is highly disruptive and therefore undesirable – so it’s best to have confidence that the floor build-up has been correctly specified and constructed from the outset.

What are the different sources of moisture in a ground floor?
There are three main sources of moisture that we can think about in this article: moisture from the ground; moisture in construction materials; and moisture from internal sources.

Moisture from the ground
A requirement of national building regulations is to resist the passage of moisture into buildings, and that includes from ground sources.

The extent to which a floor might be subject to moisture from the ground depends on hydrostatic pressure (which typically requires specialist measures), moisture load in any existing foundations or below-ground structure, and how rainwater is drained away from the building.

Dealing with moisture in the ground requires the appropriate specification of, and correct installation of, a dampproof course (DPC) in the external walls and a damp proof membrane (DPM) in the floor.

Moisture in construction materials
Concrete slabs (including pre-cast units) and screeds require sufficient time to dry out, as their moisture content is high.

As noted in BS 5250:2021 Management of moisture in buildings – Code of practice, water released during drying can cause degradation of floor finishes and high humidity in the building.

The position of slabs and screeds relative to a damp-proof membrane (DPM) needs to be considered as part of giving sufficient time for drying out, before installing the finish.

Moisture from internal sources
Moisture is naturally generated by people and the way they use buildings. Obviously, this can have a direct, immediate, and short-term impact on floor finishes, such as through leaks or spillages.

Moisture vapour can also pass into the construction, which has the potential to cause a long-term unseen condensation risk. The level of risk depends on the placement of the DPM or air and vapour control layer (AVCL) relative to the thermal insulation, as well as the quantity of moisture generated internally.

What are the implications of moisture on wider floor performance?
The DPM, and its interaction with the thermal insulation layer in the floor, has wider implications for the performance of the floor as a whole.

If ground moisture can penetrate the layers of the floor build-up and cause an issue with the floor finish, it is likely that it can also come into contact with the insulation.

Thermal insulation materials have different moisture absorption characteristics, and this dictates their placement relative to a DPM. Those that can absorb moisture have to be protected, as their thermal performance will worsen if they become wet. If a floor fails to achieve its intended U-value due to the effect of moisture in the build-up, the overall energy performance of the building will be impacted.

As a manufacturer of ground floor insulation, we’re frequently asked questions about the correct placement of floor insulation and DPMs. Some of the situations we see risk poor performance of the floor, but without a likely knock-on effect to the floor finish. Others could have a detrimental impact on the quality of the floor finish installation.

Robust specification and construction of ground floors is essential to give all parties the peace of mind that moisture is being managed correctly, that the intended thermal performance will be delivered, and that floor finishes will not be adversely affected.
How can XPS insulation help to ensure high quality floor finishes?

Extruded polystyrene (XPS) insulation is a robust and moisture tolerant insulation product that can be installed above or below the DPM, without any loss of thermal performance. It therefore provides specifiers with a flexible solution to suit a range of different building types.

If XPS is installed below the DPM, the membrane can serve the dual function of also acting as the AVCL. Not only does this reduce installation time and material costs, it reduces the potential for mistakes in detailing and installation that could otherwise compromise the moisture resistance of the floor.

The specification of XPS in a floor does not change the fact that slabs and screeds need the right amount of time to dry out – something that is not always provided. Nor does it change that internal moisture generation and humidity control needs to be properly managed. But it can play an important and reliable role in guarding against moisture risk from the ground and protecting high quality floor finish installations.

To find out more about XPS insulation and moisture risk in ground floor build-ups, visit the below website.

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