BS 5250:2021 Management of moisture in buildings addresses the following four categories of floor construction.

  • Groundbearing floors with damp proof membrane (DPM).
  • Groundbearing floors without DPM.
  • Suspended floors.
  • Basement floors.

For the most part these categories deal with floors at ground level or, in the case of basement floors, below ground. However, the scope of the standard extends to all potential sources of moisture in a building, not just from the ground. ‘Suspended floors’ therefore also deals with intermediate floors, including those over unconditioned spaces.

To find out more about how the latest version of the standard differs from previous versions, read what you need to know about BS 5250:2021.

1. Groundbearing floors with DPM

This type of floor typically consists of a concrete slab that is cast onto, and supported by, a prepared base – usually sand-blinded hardcore. The DPM acts as a barrier to moisture from the ground.

The DPM should be positioned in the floor build-up based on the thermal insulation material used. Many thermal insulation materials are affected by moisture and therefore have to be installed above the DPM, in order to ensure that they deliver their intended performance.

Some materials, including extruded polystyrene (XPS), do not have their performance affected by moisture and can be installed below the DPM. This offers multiple advantages.

On some projects, a lack of understanding about the potential effects of moisture leads to unsuitable insulation materials being installed below the DPM. Specifying a material like XPS guards against the negative impacts of mistakes like this being made.

In addition, where insulation is installed above the DPM, an air and vapour control layer (AVCL) must then be installed over the insulation to limit the risk of interstitial condensation occurring.

Specifying and installing a material like XPS below the DPM means the membrane can serve a dual function by also acting as the AVCL – saving time and cost by only having to install one membrane layer.

The combination of insulation and DPM can be positioned above or below the concrete slab. Figures 3 and 4 of BS 5250:2021 clearly illustrate all of the ways in which this type of floor can be built up. The decision about where to place the insulation layer relative to the concrete slab should be made depending on the intended heating regime of the building.

2. Groundbearing floors without DPM

In modern construction, with the possible exception of some historic buildings, a new floor should not be constructed without a DPM. This type of floor is therefore generally only found in existing buildings, and is very unlikely to have any thermal insulation.

If a traditional floor with no DPM is to be replaced, the addition of thermal insulation and a DPM should be done with care, and with regard to potential knock-on effects. For example, a damp proof course (DPC) will likely be required in the external walls, if one is not already present.

BS 5250:2021 gives detailed guidance on the potential moisture risks that might result from replacing an existing groundbearing floor without DPM.

3. Suspended floors

Suspended floors separating a conditioned space from an unconditioned void or space should feature thermal insulation. At ground level, most new suspended floors are a concrete deck (such as a block and beam system, or a pre-cast slab), with timber joist floors being found in older properties.

The requirement for a DPM depends on the design of the floor and the ground conditions.

Because the thermal insulation is above a ventilated void or space, and not in direct contact with the ground, it is usually not necessary to consider the potential affects of moisture on the material’s properties. Therefore, most types of floor insulation can be considered suitable, with an AVCL typically required over the insulation layer to guard against interstitial condensation.

Again, BS 5250:2021 contains detailed guidance on potential moisture risks and suitable build-ups. 

4. Basement floors

The detailing of basement floors (and walls) depends on the ground conditions on site, whether the basement is new or existing, how the basement will be waterproofed, and the intended use of the basement (including moisture generation inside the room(s)).

Like groundbearing floors with a DPM, if the thermal insulation is to the outside of the basement structure and in contact with the ground, then it needs to be capable of retaining its thermal performance even when exposed to moisture.

Polyfoam XPS supplies floor insulation products suitable for use in groundbearing floors above or below the DPM, and in basements. View our Floorboard Standard and Floorboard Extra products or, for help in specifying the correct type and thickness of insulation in your project, contact us.