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Unventilated warm roof?

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New build in progress. One section has warm roof. Roof covering details being planned.

Fully air permeable roofing membrane. Certificate states: "For roofs with an insulated ceiling, ventilation above or below the underlay will not be required provided that the passage of moisture by diffusion and by convection is controlled, eg by a continuous envelope of insulation with a high vapour resistance"


Roof covering layers:

Cambrian slate
Completely air permeable roofing underlay (Roofshield)
Rafters with 100mm Celotex or similar insulation between
50mm Celotex or similar taped across ceiling
Foil backed plasterboard
3mm skim coat

Qu: Would it be too unwise to do away with the traditional 50mm ventilation gap below the underlay?


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I like using counterbattens or drape the membrane so any moisture under the slates can run down the roof and escape , battens touching membrane can lead to rot/crap building up.

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The better method is make it a proper warm roof.  Put the insulation ABOVE the rafters then the membrane counter battens, battens then tiles, and full fill insulation between rafters.

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