The underside of the roof sheathing will dry rapidly to the exterior through the slate due to ventilation drying and diffusion.

Under slate roof insulation. Kingspan with a flat roof this issue does not arise but the u value constraint and the amount of insulation needed is just the same. If you create an air space under the roof sheathing boards as dana suggests there is no need to have any soffit vents or ridge vent. Any gaps in the foil will also reduce its performance. Sarking is a pliable membrane that sits under your roof tiles like a protective second skin.

The smartest place to install reflective foil is in walls or as sarking under the roof as it reflects radiant heat away from the interior of the building in summer. For maximum effectiveness reflective insulation needs to remain clean and dust free as dust build up will reduce reflectivity. You may have to build out on the inside a bit to get enough insulation. It is vapor permeable and air leaky.

Protects homes and buildings against radiant heat. A strong tear resistant pliable building membrane designed for use as sarking under concrete and terracotta tile roofs. On new build or replacement pitched roofs the ideal build up would be insulation fitted over and then between the rafters giving a big boost for airtightness image credit. Install wood between the rafters maintaining a 1 gap between the wood the underside of the roof deck.

A well ventilated roof space is good this stops your timbers rotting. Check the depth between your trusses and the slates then install the correct thickness of kingspan in the gaps allowing for a 25mm air gap between the kingspan and the slates. Then use spray foam insulation after that finish the ceiling how you would like. Slate is a wonderful forgiving roofing.

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