sheeps wool, old stone walls, do I need an air gap?

[Glenbirnie]

Im getting conflicting information from various high profile sources, sheeps wool manufacturer, historic Scotland, eco buiding expert in Ireland etc. But hopefully I'll find someone on here who has done it!

My house is 22" granite wall, suspended floor ventilated solum, slated roof on felt and sarking. At the moment the internal walls are studded on wooden pegs in the mortar joints and sheeted with old wood fibre board.

There is technically air gaps from the solum up the back of the internal wall lining (fibre board) to the attic space.

Im stripping them and building new stud partitions between the ceilings and floor joists just off the stone walls.

Plan is then sheeps wool between studs.

Its been suggested if I leave an air gap because my walls are so thick and non vented any moisture that gets from the room to the gap will condense and form moisture. If there is no air gap any moisture will just travel from one medium to the next.

Also some say I need a VC layer on the room side behind the plaster board I'll fit and some say it needs to be breathable!

Everything about the house is as dry as the proverbeal nuns chuff, timbers inside stone walls etc The house is cold though.

This project is costing a lot of money and I need your advice to avoid doing it wrong.

Would love to hear from someone who has actually done this or has decent advice.

Cheers

 

[flameport]

The old walls had an air gap because the wall boards were permeable, and any moisture passing through them would cause condensation if there was no gap.

For the new there are 2 options, either the same method as the old - an air gap for ventilation and use standard wallboards, or
install with no gap or ventilation, but the wall will need an impermeable layer on the back of the plasterboard, so that the moisture can't pass into the wall.

Personally, I'd use the older method and have an air gap behind the insulation. Reason being that VCLs are all very well until they are damaged by fixing items to the wall, then they are useless.

 

[mointainwalker]

Sorry I don't have a suggestion, just a question.

I have read in sales literature for sheeps-wool that it handles moisture differently ( better ) than ordinary fibre-wool.

What has been said about this by the people consulted so far ?

 

[jeds]

flameport is correct. Either have a ventilated space or no space and a vapour control layer. (well, even with a ventilated space I would still include the vapour control layer) The idea of having a breathable membrane is nuts so forget that. If it were me I would use a thermal plasterboard direct to the masonry surface. Much more efficient, easy to install and takes a lot less space.

 

[Glenbirnie]

Can you tell me the name of the thermal plaster board?

Ive looked at Pavadentro is it similar to this?

Thanks

 

[jeds]

Lafarge thermalcheck K or Kingspan K17/K18.