VENTILATED COLD ROOFS

For many years the hard metal standing seam roof using copper, zinc, stainless steel and aluminium has worked on the cold roof ventilated system.

This comprises a plywood or metal deck, or concrete roof structure onto which timber spacers to the depth of the insulation plus 50mm are fixed (alternatively the rafters of the building can form this spacer).  A vapour barrier is either fixed below the timber spacers or draped so that it fits over rafter sections; this is often fixed to the underside with plasterboard.

A low density glass fibre or Rockwool insulation is then laid between the spacer/rafter and a 50mm air gap is allowed above the insulation. Over this a plywood deck or slatted timber with air spacers is laid, as necessary a separating layer is applied and then the hard metal sheet formed to provide a standing seam is installed.

A ventilated cold roof has the risk of interstitial condensation forming within the insulation because of the difficulty of installing an effective vapour barrier. In addition, a ventilated space, if provided with good air flow, allows rodents, birds and insects to enter the system; if virtually closed it does not effectively ventilate.

The changes in the Building Regulations in 2002 and further updated in 2006 call for the avoidance of cold bridging and control of the airflow and heat loss through the roof fabric. This is difficult to achieve with a ventilated system.  When interstitial condensation occurs in the construction there is a risk that the timber or metal will rot/corrode leading to earlier failure of the roof

Heavy condensation underneath the metal cladding. The cause: humidity-loaded air flow condenses on the «cool» surface.		In addition, it has been proved that: 1. CONDSENATION OCCURS ALMOST DAILY In highly insulated roofs, when the metal cools off - which happens virtually every night - the roof below is continually subjected to the effects of dripping water. 2.FASTENERS WHICH PENETRATE THE SYSTEM REDUCE THE CALCULATED THERMAL PERFORMANCE BY OVER 50% In practice, the standard k-values of 0.3-0.2 W/m²K can never be achieved because of the effects of cold bridging, created by fasteners which penetrate the insulation. In a twin-skin system this can result in a reduction in thermal performance of 20 and 50%. Increasing the insulation thickness theoretically calculated by more than 5cm provides only a partial remedy. Increased depth of build-up means increased costs and this is only rarely acceptable.
Condensation drips, in this case, directly into the permeable insulation. A deterioration in the thermal insulation properties and problems of building performance are inevitable.
Insulation damaged by dripping condensate. This leads to the destruction of the fibre cohesion and loss of compressive strength of the insulation material. Moreover, the exposure to moisture of the load-bearing structure, ultimately causes corrosion and failure.

*Generally, it is recommended that construction elements remain under the maximum value of 0.1m³ (m² h) relative to air tightness.