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AIM Overlay and Underlay System

AIM Rock Wool Warm Roof System

Introduction

The AIM Overlay and Underlay System is used to produce a 'Warm Roof' type construction in buildings with pitched roofs in excess of 15 degrees. The System provides an effective means of insulating the roof space at rafter level, thereby creating a useful warm room within the attic. The Overlay and Underlay boards are vapour permeable and should be used in conjunction with a breather membrane on the outer face, thus preventing the need for ventilation of the roof void. They can be installed directly on a timber rafter construction, or the construction may include profiled steel or timber decking.
Overlay boards may be used without Underlay boards, depending on the U Value requirement. However it is preferable not to use Underlay without Overlay, to avoid any potential for condensation on the tops of the rafters.

Design Considerations

It is important to take account of potential condensation problems, when designing a Warm Roof construction. The Overlay & Underlay boards themselves have a vapour resistivity similar to air and will therefore 'breathe' satisfactorily, provided the underslating membrane is a breather membrane and not of the impervious type. It is good practice to incorporate a vapour control layer on the warm side of the insulation, particularly in areas of high humidity. If used, the vapour control layer should be overlapped at its edges and adequately sealed.

OVERLAY BOARD

Strength
The Compressive strength of Overlay Board exceeds 100kPa at 10% compression (i.e. more than 1 tonne per square metre). However AIM Overlay Board is NOT a structural board suitable for foot traffic.

Design Notes
AIM Overlay Boards are not designed to add to the structural stability of the roof. The roof structure should be properly designed and fully braced, independently of the insulation.
AIM Overlay Boards will resist normal wind and snow loads. However, the fixing system is very important and the type and spacing of the fixings should be specified by a reputable fixing supplier.

UNDERLAY BOARD

Board Size
The Underlay boards are supplied 1140mm wide to enable them to be cut on site to suit the rafter spacing - typically into 2No. 1200 x 570mm slabs for rafters at 600mm centres, or 3No. 1200 x 380mm slabs for rafters at 400mm centres.

Click here for full data sheet (pdf format)