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Floating floors: alternatives to wet screeds

Rob discusses the relationship between thermal insulation and floating floorcovering options.

MANY ground-floor constructions are built-up by installing a screed over the thermal insulation layer. There’s no bond between the screed and the insulation, and as such the screed is said to be ‘floating’.

Some projects, however, don’t lend themselves to a floating screed. On these occasions a thin rigid board, which is installed over the thermal insulation layer without being screwed, nailed or glued to the sub-floor, may be preferable.

Where are floating floorcoverings used?
Floating floors are well suited to refurbishment or conversion projects where space is limited. For example, a domestic garage conversion is ideal – the insulation and board covering can be installed over the existing slab and are more likely to help meet thermal targets and match the existing floor level.

The typical approach is to use a layer of timber-based particle board as the floorcovering, ready to receive the floor finish such as tiles or carpet. The floating covering may also be the floor finish, like a laminate, but should be installed in accordance with manufacturer’s recommendations.

From a contract flooring perspective, a floating floorcovering is more likely to be an alternative to timber-based boards.

As a completely dry operation, floating floors are generally easier, quicker and cheaper to install compared to a screed, and can be walked on immediately. That makes them attractive for projects with tight timescales, or where it’s impractical for large areas of floor to be inaccessible to various trades engaged on the project.

As with any floor construction, a vapour control layer (VCL) should be incorporated into a floating floor build-up in accordance with best practice.

Timber-based floating floors
The terminology surrounding timber-based floorcoverings can be confusing. Typical options are chipboard – which is particle board, or low-density fibreboard (LDF); medium density fibreboard (MDF); oriented strand board (OSB), also known as sterling board; or plywood.

The first three are types of fibreboard and all four are classed as an engineered wood product.
Of those four options, chipboard is the option most commonly referenced by insulation manufacturers – but is more likely to be used in domestic scenarios where it’s most cost-effective. MDF is denser than chipboard (LDF), meaning it’s harder-pressed and more consistent throughout.

That makes it heavier, free-of-air voids, and a better acoustic solution. MDF is generally cheaper than plywood, although for the extra expense it is possible to obtain a quality of plywood that, appropriately treated, could also be used as the finished floor.

OSB, meanwhile, is typically specified as a sheathing board over timber framed walls and some pitched roofs to provide racking strength, so is probably stronger than needed in many floating floor applications.

One downside of timber-based boards is the effect that moisture and humidity can have on them – if they get wet the coverings can expand and buckle. Fibreboards absorb moisture and swell if not protected, though they’ll not warp. In rooms such as kitchens and bathrooms, a moisture resistant board should be used.

Because it still has a grain, plywood can warp when the moisture content is different on either side of the board – for example, where one side is exposed to direct sunlight, or a high level of humidity.

Alternatives to timber-based boards
Instead of using timber-based boards, gypsum-based or cement fibre boards are popular on larger commercial projects.

In some cases, they’re referred to as ‘dry screed’ boards, reflecting their intended strength, durability and versatility. Like their ready-mixed wet screed counterparts, dry screed boards perform consistently well without the time-consuming wet processes associated with traditional screeding.

Benefits of these products include acoustic performance and compatibility with underfloor heating systems.

While they are perhaps intended for intermediate or separating floors more than ground floor constructions, they can be laid over insulation boards that have been routed to accommodate underfloor heating pipes.

Where these products are intended for use with underfloor heating systems, manufacturers are keen to stress the high thermal conductivity and low thermal resistance of their particular offering. This can seem rather counterintuitive when we are used to seeing claims for the most thermally efficient products.

However, being less thermally efficient compared to timber-based boards or traditional screeds means less of a barrier to the heat from the UFH system, so the benefit is felt more quickly in the space being heated.

Floating floors and thermal insulation
Much like a regular wet screed, floating floorcoverings need to be selected based on the performance expected of the floor construction as a whole.

If extruded polystyrene (XPS) has been chosen as the insulation layer because of its load-bearing capability, then a thin timber covering is unlikely to be appropriate because it will not be able to bear the same loads. By contrast, dry screed boards are available in different strength grades comparable to the performance of XPS insulation products.

If the floating floor is being used with UFH and XPS insulation has been selected because to act as a ‘tray’ for the underfloor heating pipes, then timber may still be an option.

On domestic or smaller projects, timber joists or battens may be installed across the concrete floor base. Insulation is fitted between the timbers, then the floorcovering is laid over and attached to the battens and joists. Fixing the ‘floating’ covering may be preferred because it’s considered to give a more ‘solid’ feel, but because the battens or joists bridge the insulation layer, it requires a thicker insulation layer to achieve the same thermal performance.

Wooden floors flex even when attached to joists, so some movement should be expected from a floating floor. A small expansion gap is allowed around room perimeters, and the area of covering is restricted.
Boards should have tongue-and-groove edge jointing which can interlock and be glued.

While all of these measures help, movement can still be noticed, particularly at the floor edges. At the far end of the scale, the distribution of loads, or excessive differential movement between the covering boards and the insulation, may cause glued joints to crack or separate.

As with any component of the floor build-up, selecting the right product for the right application is essential. Timber floating floors and their alternatives can be used successfully with an extruded polystyrene insulation layer, but contractors are advised to speak with the manufacturer of the board covering and the insulation for project-specific advice.

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