Lath & Plaster Attic Room Renovation

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Fantastic, I'll give that a go.

Regarding insulation, I can't remember the exact spec for insulation but I think I need something like 100mm between the rafters a 50mm gap and then 50mm running in the other direction to bring it up to modern building standards. Does that sound right?

The plasterboard would then be mounted on top of that and fixed to the rafters to secure it.
 
Yes but slightly different order.

You want the 50mm gap under the tiles/felt.

100mm between the rafters, (flush with the underside) then 50mm continuous under that in any direction with foil tape on the joints.
 
It's a 130-year old house so there is no felt as far as I'm aware, just roofing slates mounted to the rafters like this: https://www.sprayfoaminsulation.co.uk/wp-content/uploads/2011/05/No-Felt.jpg

So the plan would be:
  1. Rip out the plaster
  2. Rip out the laths
  3. 100mm foam board between the rafters
  4. 50mm (in any direction) with foil tape on the joints
  5. Board and skim over the top
I assume the 50mm gap is to deal with condensation - is there a way of ensuring a suitable air gap in my case?

The plan would be to re-roof in the next 10 years but it doesn't need it right now, nor can we afford it!
 
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The 'gap' is meant to be a ventilated air space to reduce the risk of condensation. You also need a continuous vapour barrier on the warm side of the insulation. Draw a cross section through the proposed roof construction showing the rafter depth and how everything is going to fit. 50mm air gap plus 100mm insulation ,that is 150mm ,how deep are the rafters ? You might need battens on the underside of the rafters to achieve the required depth.
p.s. being mid terrace won't affect the heat loss through the roof !
 
D

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