Knauf Insulation’s new, high-performance, glass mineral wool product, Crown® Rafter Roll 32, solves the problem of how to satisfy both ‘Part E’ and ‘Part L’ of the Building Regulations in warm roof constructions. To reduce flanking sound transmission, the Robust Details covering masonry separating walls in room-in-the-roof construction specifies that the roof must be insulated with a layer of mineral wool at least 100mm thick between the rafters. With its impressively low thermal conductivity, a layer of Crown® Rafter Roll 32 just 185mm thick also meets the Elemental U-value target of 0.20W/m2K set by ‘Part L’.

Many new homes are now built with a room-in-the-roof as a cost effective way of increasing their usable floor area and this means constructing a warm roof. Doing so commits designers to the requirements set by both Approved Document L for thermal insulation and, since July 2004, Approved Document E for sound insulation – and therein lies the problem…

For new-build warm roofs, ‘Part L’ specifies an Elemental U-value of 0.20W/m2K and, to achieve this, the industry has grown used to using rigid foam insulation products as they provide a good level of thermal insulation in a comparatively small thickness. The alternative is to use mineral wool products but this usually means having to use a relatively thicker layer and this doesn’t always fit within the depth of the rafters.

‘Part E1’ covers protection against sound from other parts of the building and adjoining buildings. In practice, many construction methods fail ‘Part E1’ as a result of ‘flanking sound transmission’ – sound travelling through flanking structural elements – and the Robust Detail programme both recognises and addresses this. Mineral wool is recognised as the material-of-choice for acoustic insulation and its use is both accepted and widespread. So, not surprisingly, those Robust Details covering masonry separating walls in room-in-the-roof constructions specify that a layer of mineral wool insulation, at least 100mm thick, must be installed between the rafters to achieve a desirable level of insulation.

“Yes, it is a dilemma!” said Stephen Wise, Knauf Insulation’s Insulation Academy Manager, at one of the company’s very successful ‘Part E’ Seminars. “In this instance, the material generally used to provide thermal insulation doesn’t meet the acoustic insulation regulations and this means having to use an alternative which is often thought of as being less practical – fortunately Crown Rafter Roll 32 solves the problem completely!”

Crown® Rafter Roll 32 benefits from having a higher density than other glass mineral wool products and this gives it an impressively low thermal conductivity of 0.032W/mK. This means that just a 185mm thick layer between rafters at 600mm centres, or a 200mm thick layer for rafters at 400mm centres, meets the Elemental U-value target of 0.20W/m2K and, of course, fully satisfies the requirements of ‘Part E-1’.

The roll is available in three thicknesses – 75mm thick in rolls 5.40m long, and 85mm and 100mm thick in rolls 2.70m long – and all are 1200mm wide so they can be cut to fit between rafters spaced at either 400 mm or 600 mm centres. Installation is quick and easy as the material’s semi-rigid nature allows it to friction-fit between the rafters with its ‘scrunchiness’ overcoming timber variations and preventing air gaps.

And the benefits don’t stop there! Apart from its impressive thermal and acoustic insulation properties Crown® Rafter Roll 32 is also totally non-combustible; a fact recognised by it having a Euroclass A1 fire classification, the highest rating possible.

Further information about Crown® Rafter Roll 32 is available in two Knauf Insulation brochures – Pitched Roofs: New Build & Upgrade and Loft Conversions. These are available free on request via Knauf Insulation’s website at www.knaufinsulation.co.uk or by calling 08700 668 660. Please quote KR 269 04.

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