I am building a cold roof loft and am planning a robust vapour control layer at ceiling level so there shouldn't be much moisture getting in to the loft. Loft space will house a couple of water tanks for the open vented systems and these will be enclosed in a thick insulation tent. I was planning on making the roof unventilated under the 'felt' and double battening the tiles and ventilating under them. I was going to use vapour permeable underlay (SolitexPlus) taped and sealed all round. I am now reading that vapour permeable only underlays may not be good enough as the NHBC are recommending ridge ventilation where they are used. An alternative is vapour and air permeable underlay (like Roofshield) but then there will be more air movement in the loft negating the purpose of sealing the space. I suspect having open vented water tanks in the loft may generate some vapour but what would be the best choice of underlay?

Vented dry ridge systems are good and easy to fit as are over fascia vents ,they help set out the sprocket/eaves tile angle. I'd use the even if ventilation wasn't an issue

Posted By: CWattersPretty sure cold lofts must be ventilated.

BCO was happy with my non vented cold roof, but had well sealed ceilings, breathable membrane and battens/counter battens.

Ironically the (old school) roofer was determined to vent it, so ended without taping the membrane and with a potential vent at the apex. Builder sealed the soffit as requested (no vents in it). Outcome there is some external air movement in the loft, but not a lot and the 500mm of mineral wool does not suffer wind wash.