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Advice you can’t ignore

Rob Firman explains how to use BS 5250:2021 as a starting point for flooring design and installation to ensure every part of the floor – including insulation and finishes – can perform at its best.

CONDENSATION is arguably the most talked about moisture-related issue in buildings. This could be because it’s a particularly visible phenomenon – such as moisture on windowpanes – while other damp problems can often occur hidden from view.

Alternatively, it could be because the risk of condensation is closely tied to heat loss. Condensation risk analyses often accompany U-value calculations, and we’re all aware of the importance of energy efficiency regulations.

We’re starting to see a shift away from this focus on condensation, however. Moisture risks in buildings go well beyond ‘just’ condensation, and there’s increasing understanding of the need to take a whole-building approach to moisture management.

As a result, the relevant British Standard, BS 5250, underwent a significant overhaul in 2021.

Previously titled control of condensation in buildings, the heavily revised version introduced a wider scope that is reflected in it now being the code of practice for management of moisture in buildings.
Management of condensation specifically, or moisture more widely, might be perceived as having little relevance to contract flooring and floor finishes.

However, the updated BS 5250:2021 shows we all need to have more awareness of moisture risks. In this article, we’ll look at the new standard, and what implications it might have for floor constructions – including floor finishes.

Why was BS 5250 updated?
As buildings have got more airtight and more energy efficient, changes to ventilation provision have not kept pace. The moisture content of indoor air is a significant issue: warm, moist air combined with poor ventilation and thermal bridges in building fabric, all add to the risk of condensation occurring.

Alongside that is the question of whether we’re constructing buildings that are resilient for the future. With climate change comes increasingly severe and frequent extreme weather, and greater flooding risks (which has implications for floors).

Buildings are therefore being subjected to more moisture, more often, internally and externally.
In the bigger picture of climate change and legally-binding net zero targets, the retrofit of existing buildings – domestic and non-domestic alike – is a significant issue with which we must get to grips.
Retrofit isn’t simply a case of insulating existing buildings to reduce their energy consumption; any retrofit design must also consider the need for ventilation and the moisture balance of the existing structure.

Applying energy efficiency solutions to existing building fabric must consider factors like driving rain, and whether moisture might be pushed to other parts of the structure that aren’t currently affected.
Changes to existing floor structures and upgrades of floor insulation need to be done sympathetically, and without making any other part of the building worse off than it was before. If they’re not already, floor finishes need to be specified and installed as part of a long-term, whole-building approach.

How has the advice of BS 5250:2021 changed?
The previous version of BS 5250, from 2011, already represented a comprehensive source of advice for designers, specifiers, and installers. The widened scope of the updated edition means it’s naturally more detailed, although its depth is also enhanced by a new approach.

Large portions of the standard now provide substantial design guidance for floors, walls, and roofs. The array of construction types covered under each element has been expanded, dealing with a much wider variety of materials and build-ups, and providing specific advice for each.

Each construction type is assessed in terms of various potential moisture sources, and possible risks are identified. The text is careful to distinguish between design intent (‘as designed in theory’) and potential real-world issues that could be encountered (‘as built/in service’).

Constructing floors with moisture risk in mind
The following four categories of floor construction are discussed in BS 5250:2021, with very clear diagrams showing how air and vapour control layers (AVCLs) and damp-proof membranes (DPMs) should be positioned relative to the insulation layer and concrete slab positions.

  • Ground bearing floors with DPM
  • Ground bearing floors without DPM
  • Suspended floors
  • Basement floors (floors below ground)

Correct positioning of membrane layers is essential to prevent moisture vapour from passing into the floor construction and causing a long-term, unseen condensation risk.

As an insulation manufacturer, we’re often asked 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.
If there’s an issue with the floor finish, any remedial work might only be addressing the end result of the problem, rather than the root cause of it. The nature of ground floors means taking up the whole floor is highly disruptive and undesirable – so it’s best to ensure these things are correct from the start.

DPMs, along with damp-proof courses (DPCs) in the external walls, are a critical to dealing with moisture in the ground. This is addressed directly in the Building Regulations. The extent to which a floor might be subject to moisture from the ground depends on hydrostatic pressure, moisture load in existing foundations or below-ground structures, and how rainwater is drained away from the building.

Getting floor build-ups right also means giving construction materials enough time to dry out. Concrete slabs (including pre-cast units) and screeds have a high moisture content, and BS 5250:2021 is clear that water released during drying can cause degradation of floor finishes.

Benefits of moisture-resistant insulation in floors
Something else addressed by BS 5250:2021 is the role of insulation that isn’t affected by moisture. Thermal insulation materials have different moisture absorption characteristics, which dictates where they can be positioned in a floor relative to the DPM. Those that can absorb moisture have to be protected, as their thermal performance will worsen if they become wet.

If ground moisture penetrates the layers of a floor build-up and comes into contact with the insulation, then the floor is unlikely to achieve its intended U-value and the overall energy performance of the building is negatively impacted.

Extruded polystyrene (XPS) insulation is robust and moisture-resistant, meaning it can be installed above or below the DPM, without loss of thermal performance. If XPS is installed below the DPM, the membrane can serve the dual function of also acting as the AVCL. This reduces installation time, material costs, and the potential for mistakes in detailing and installation.

While XPS provides a flexible solution to suit different building types, its specification doesn’t change the fact slabs and screeds need the right amount of time to dry out. Nor does it change that internal moisture generation and humidity control needs to be properly managed.

However, it can play an important and reliable role in guarding against moisture risk from the ground and protecting high quality floor finish installations.

Decisions made about the design and construction of floor build-ups, the specification of thermal insulation and floor membranes, and the management of moisture and ventilation should all be rooted firmly in the advice of BS 5250:2021, to ensure every part of the floor – including insulation and finishes – can perform at its best.
Rob Firman is specifications and technical manager at Polyfoam XPS

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