How to insulate a sloping roof on an edwardian house?

[Triggaaar]

I've done a loft conversion and new build sloping roof before, where I've used a breathable membrane on top of the rafters, put (say 50mm) celotex between the rafters, and ventilated above the celotex, and then added further celotex underneath.

This time I want to improve the insulation on an old property. It has lath and plaster underneath the rafters, presumably with felt on top of those, underneath the tiles.

Could I just put say 50mm celotex underneath the lath and plaster, then a vapour barrier, and then plasterboard and plaster?

Thanks

 

[MMUK]

I take it you mean effectively lowering the ceiling height?

Personally I would remove all of the lathe and plaster, fit 50mm Celotex into the joist voids and then use 30mm thermal p/board (9.5mm plasterboard with 20mm polystyrene bonded to it) and skim over.

I hate lathe and plaster with a passion - I refuse to patch it up and will always replace it with board and skim. It's so much cleaner and easier to work with.

 

[Triggaaar]

Thanks for the reply.

I take it you mean effectively lowering the ceiling height?

Yes.

Personally I would remove all of the lathe and plaster, fit 50mm Celotex into the joist voids and then use 30mm thermal p/board (9.5mm plasterboard with 20mm polystyrene bonded to it) and skim over.

That's roughly what I'd do on a new build, but I'd be using a breathable membrane over the rafters instead of old felt. My concern with any approach is that I'm tampering with an old building that's used to breathing. If I follow that route, would I need to add any ventilation to the ridge or eaves? That would would require expensive scaffolding, as it's a tall building.

 

[MMUK]

An old property like this one will already be well vented at the eaves, that's how they were designed.

 

[Triggaaar]

Thanks.

Cutting the lath and plaster down will be very messy, and cost money - and fitting celotex between the rafters and underneath will also be more expensive than just fitting it underneath the existing. So, is there anything wrong with just fitting it underneath the existing plaster?

For example, it there a danger that there'll be small air gaps between my new insulation, and the lath and plaster, that vapour could condense in? Or should foil backed celotex be enough to stop that happening?

 

[Triggaaar]

Personally I would remove all of the lathe and plaster, fit 50mm Celotex into the joist voids and then use 30mm thermal p/board (9.5mm plasterboard with 20mm polystyrene bonded to it) and skim over.

By the way, why would you use plasterboard with polystyrene bonded to it, instead of celotex (or Kingspan etc)? Polystyrene isn't as thermally efficient is it, so what advantages does it have?

 

[Student]

Regarding MMUK's comment

"I hate lathe and plaster with a passion - I refuse to patch it up and will always replace it with board and skim. It's so much cleaner and easier to work with."

Whatever you do, don't say that on the Period Property forum!

 

[MMUK]

Personally I would remove all of the lathe and plaster, fit 50mm Celotex into the joist voids and then use 30mm thermal p/board (9.5mm plasterboard with 20mm polystyrene bonded to it) and skim over.

By the way, why would you use plasterboard with polystyrene bonded to it, instead of celotex (or Kingspan etc)? Polystyrene isn't as thermally efficient is it, so what advantages does it have?

I use thermal plasterboard AS WELL AS Celotex :mrgreen:

 

[MMUK]

Regarding MMUK's comment

"I hate lathe and plaster with a passion - I refuse to patch it up and will always replace it with board and skim. It's so much cleaner and easier to work with."

Whatever you do, don't say that on the Period Property forum!

Maybe I'd better not post any pics on there of the Georgian property in Suffolk I renovated a few years ago then :lol: