no membrane in roof next to tiles

yes you can put insulation in between the rafters But in you are to convert the loft you will need to put felt under the slate to safe you money in the long run If there is ever a leak it will cost you and you won't know till the damage is done Thank fascia medics

if their is insulation between the floor joist why don't you just increase it to current regs 300mm, insulation between the trusses is really a bad idea if their is no under felt as it will hide any water ingress till their a lot more damage done which will cost you more in the long run , if you are thinking of using it for storage you can get legs to heighten the boards above the 300mm insulation required . good luck alex

Glass wool insulation between rafters is to ideal as would stop air flow if you where going go down this route you need use Kingspan

You need a 50 mm air flow between roof covering and insulation vented at both eaves with a clear flow vented ridge if you cant achieve through flow hope this helps