Renovating a slate roof on large house by stripping ; creating perfect opportunity to put insulation over rafters creating warm roof. The second floor is currently converted into some rooms ( rudimentary insulation) and a large attic space. Previous renovations have created damp problems because of ventilation issues. to create the warm roof I'm looking at a wood fibre sarking board, although I would like to avoid between rafter insulation as I wish to retain the intricate shape of a couple of hipped elements with Rafters on show. My architect is suggested foil backed insulation to avoid any great increase in roof height although I'm concerned about it's impact on permeability ( actis triso super10)

Does pavaboard or other boards offer much insulation on their own ? or do they need additional insulation ? Will raising the roof cause problems with valleys and hips ( large pitched roof with two bays either end with hipped roofs. ? I assume I need to be air tight -so will I need to raise hidden gutters at eaves ? ( these are behind a parapet which sit above the solid walls)

Will the walls be impacted ? As I cannot see a way to insulate them. The house although not listed is locally important and I cannot cover the external brick or stone and internally I cannot see how I can increase internal walls without undermining the cornice and window detailing.

Any advice or comments gratefully received

If you make it a warm roof then the permeability of the insulation becomes irrelevant as the timbers are moved out of the Dew Point zone. We often use the Warm Roof build up below for Passive House renovations;

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Are we saying that the Wood fibre systems do not offer building reg levels of insulation ? ''

Well they do if they are fat enough. Probably around 250-275mm to meet Part L 0.18 U value for sloping roof, based on Lambda value of 0.040.

Nick