Attic Insulation and Flooring when there is no roof membrane under the slates.

Calibos

Early 20th century mass concrete house/shop premises with flat on the first floor. Want to insulate the attic over the flat and there is no membrane under the roof slates and most of the dot/dab lime plaster at the slate overlaps has disintegrated and is now effectively sand lying on the ceilings between the joists.

No roof membrane (bitumen nor breathable) I presume means all the usual attic ventilation and insulation ventilation gap rules don't apply. There is no eaves soffit overhang. The roof beams sit on top of the wall plate with wood trim nailed to the end of the beams flush with the wall of you get me. Semi vaulted ceilings inside. In other words, if I drilled a 100mm hole in the trim I could push my hand up inside and tap on the roof beams either side, tap the sloped ceiling, tap the underside of the slates.

If the beam/joist depth is about 100mm, Should I just buy 100mm solid foam insulation (foil backed?) and cut to width to fit between the roof beams/joists, cut the ends of the pieces to the appropriate roof slope angle such that when I push the foam pieces down the vaulted ceiling sections they butt up full contact to the backside of the exterior wood trim on the end of the beams/joists. Once the vaulted ceiling sections are done around the periphery of the roof, then fill between the ceiling joists with 100mm fiberglass insulation rolls and and then 75mm insulation backed attic flooring across the top of the ceiling joists.

ie. With no roof membrane there is no need for a 50mm ventilation gap from the eaves soffits (there are no eaves soffits!!) to cross ventilate the attic. The whole attic is effectively ventilated because there is no roof membrane under the slates. Hence why I assume I can just completely fill the vaulted sections with insulation right up to the undersides of the slates and why I assume that solid foam insulation is the best thing for these vaulted sections because they can be pushed down into the vaulted sections whereas fibreglass roll could only be stuffed down the vaulted sections and the lack of membrane would just mean the air was blown/sucked out of loose insulation rendering it useless anyway.

I'll try and take some photo's if you can't visualise what I am describing.

Side Question: Need to fit a Ventilation fan to the bathroom. As I said, the walls are 10inch thick mass concrete and a nightmare to drill a 100mm hole through for a fan. Could I fit a fan in the flat ceiling over the shower before laying the fibreglass insulation or insulated attic flooring in the attic above and run the flexible ducting down a vaulted section between the joists and cut out a 100mm hole in the wood trim and fit the exhaust vent on the exterior of the trim. Or does the exhaust vent need to be a vent roof tile above the exhaust fan in the ceiling in which case I couldn't DIY it and would need a roofer to fit the exhaust vent tile for me.

Calibos

Researching this a bit more.

Would I be correct in treating the vaulted ceiling sections around the perimeter as if I was creating a warm attic and insulating between roof rafters rather than ceiling joists. ie. if the space is 100mm, then just push 50mm foam insulation down those sections between the rafters and leave a 50mm ventilation gap between the solid foam insulation and the underside of the (No membrane) slates. Then on the flat, just insulate between the ceiling joists with Fiberglass roll and foam insulated attic flooring across the ceiling joists as described above.

From the perspective of heat loss, if I have 100mm Fiberglass and 75mm insulated attic flooring above the flat ceiling which is 90% of the attic floor area but only 50mm insulation around the perimeter of the attic ceiling space in the vaulted sections, does the 50mm around the perimetre negate the 175mm over 90% of the ceiling?