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| Essay:vapor barrier during roof renovationConsequently, better thermal insulation must be provided and new problems of moisture distribution arise inside the roofing system, as higher quantities of water vapour are present in the inside air. Commonly, a vapour barrier is introduced at the bottom of the insulation layer to prevent water condensation in the structure and a membrane is placed at the top of insulation as an additional protection from water leaking under the tiles due to poor roofing maintenance.Unfortunately, in many cases, this solution - which is theoretically sound -creates a range of problems: ??the vapour barrier allows moisture through if it is not correctly laid; ??the ageing of the barrier, or tampering in time, creates gaps allowing the access of moisture. In both cases, the vapour is trapped under the insulating material and can lead to: ??increased moisture in the underlying timber structures with possible rotting; ??decreased overall thermal resistance of the roofing; ??condensation, with the formation of stains and surfacing on the internal side of the roofing. The most up-to-date suggestions to prevent these problems address the use of membranes, that are waterproof yet permeable to vapour, placed on top of the insulating material. Furthermore, an under roofing ventilation may be considered, even if the suitability and the modality of this sort of intervention is currently debated (Kščnzel and Grobkinsky 1989).
The industry has developed several types of permeable membranes offering a wide range of mechanical and permeability features. These materials include Tyvek?, which provides good tear resistance and vapour permeability, with permeability considerably higher (~ 400X greater) than those offered by the more common micro-pore polyethylene film membranes. These features appear unaltered over the medium to long term. This membrane was chosen for the present intervention.
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