On the surface, choosing the right floor insulation may seem a relatively straight forward task. But, warns Mark Thompson, Knauf Insulation’s Foam Market Development Manager, get it wrong at your peril – it can be an expensive mistake!

Wrongly specified or just badly laid, dealing with the results of ‘getting-it-wrong’ can be a nightmare; furniture and equipment will have to be removed, partitioning ‘dismantled’, expensive floor coverings ripped up, reinforced concrete and screeds dug out, underfloor heating and buried services torn out and alternative accommodation provided. Then, of course, there’s the remedial work and, after all that, the process of rebuilding, refurnishing and putting everything back in place…

And when you consider that ‘marginally specifying’ the floor insulation may only save around half the cost of a material that, in itself, probably only accounts for just 5% of the floor’s total material cost, saying it’s a ‘false economy’ is a bit of an understatement! Clearly, getting it right ‘first-time’ is the only cost effective way to insulate a floor.

Insulation specifications for most building elements usually concentrate on meeting either thermal or acoustic standards – the question of ‘strength’, and how the chosen solution contributes to the structural performance of the building, rarely comes up. But floor insulation is an area where it just has to be considered. All materials compress under load so installing low-strength floor insulation increases the risk of failure and limits the potential use of the building – using a high-strength one reduces that risk, providing more flexibility in both constructional design and ongoing use. Recognising this helps create a floor that is solid, capable of withstanding specified and future design loads and will last the life of the building.

Building design for housing and most industrial construction uses loading guidance given in BS6399:1996. This expects, for example, domestic floors to accept a UDL (‘Uniformly Distributed Load’) of just 1.5KN/m2 (2.0KN/m2 in bathrooms) – but, by definition, this figure is based on a load being uniformly distributed whereas the usual cause of floor failure is point loading. To address this, designers are forced to build-in safety factors, working to provide a solution that balances acceptable compressive behaviour over the design life of the floor with economics and the need to keep floor thicknesses from building up unnecessarily.

Rigid-board is today’s material-of-choice for floor insulation and, in theory, there’s a number of potential types to choose from; ‘thermoset’ polyurethane (PUR), polyisocyanurate (PiR), phenol-formaldehyde and urea-formaldehyde along with ‘thermoplastic‘ expanded polystyrene (EPS) and extruded polystyrene (XPS). Some, expanded polystyrene for example, are quite straightforward to make; polystyrene beads are ‘melded’ together under pressure and heat to form a board – others are more complex and use ‘insulating gases’ to create a foam which, as it sets, trap a multitude of micro-bubbles in a rigid structure. (From the beginning of 2004 the foams used must be 100% Ozone Friendly.)

Thermally, foam boards have much better insulation properties than expanded polystyrene board and their use makes achieving the Elemental U-value of 0.25W/m2K prescribed by ‘Part’ L of the Building Regulations or Part J of the Scottish Technical Standards relatively easy. But, and here’s the ‘but’; other properties – especially mechanical strength – must be considered when selecting a suitable material for floor insulation…

Strength varies greatly; from ‘low-strength’ expanded polystyrene and polyisocyanurate at one end of the scale to ‘high-strength’ extruded polystyrene at the other. To compare this objectively, the quoted compressive strength of standard polystyrene flooring sheet (EPS70) is just 70kPa whereas for extruded polystyrene it’s between 200 and 500kPa and that, in the real world, means being able to use a much thinner layer of extruded polystyrene to achieve the same structural stability.

Another important consideration – as the insulation is being used at, or below, ground level, and often in an exposed situation – is moisture. Approved Document C of the England, Scotland and Wales Building Regulations locates the damp proof membrane (DPM) between the insulation and any potential external moisture but, apart from the risk of getting rained on, moisture will be introduced by many floor finishing systems and this can become trapped in the insulation. In addition, having the flexibility to place the insulation below the DPM is a valuable design option in some forms of construction. Fortunately, unlike all other foam insulation-board solutions, the closed-cell structure of extruded polystyrene is unaffected by moisture making it both site and user friendly – a ‘real-world’ building product.

So, what’s best? The ideal floor insulation material has to be high-strength, moisture resistant, thermally efficient, lightweight and competitively priced and, considering these as ‘musts’, it’s easy to see why the winner, ‘The Best Floor Insulation Material’, can only be – extruded polystyrene.

Knauf Insulation have designed specific products to allow building designers to develop an optimum floor insulation solution. Polyfoam® Floorboard is available in a variety of thicknesses across three grades: Standard; for general domestic and light loading, Extra; for commercial loading and Super for very high loading situations such as cold stores. Not surprisingly, the products are manufactured from high-performance extruded polystyrene and, using the material’s outstanding benefits of high-strength, light-weight, water resistance and versatility, offer a real thermal and ‘structural’ insulation solution.

To help designers, Knauf Insulation offer a comprehensive Polyfoam Insulation brochure – Floor Insulation – along with a series of product specific datasheets and application brochures containing a wealth of information. And, of course, expert one-to-one advice is also available from the Knauf Insulation Technical Advisory Service on 01744 693885.

For more details about Knauf Insulation products, or to obtain copies of Polyfoam Insulation literature, please visit www.knaufinsulation.co.uk or call 08700 668 660 quoting ID: PH 06603.

® Polyfoam is a Registered Trademark of Knauf Insulation

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