IWI Advice

[Heatseeker]

I want to insulate this room, which is approx 300mm solid stone, mostly external walls.

 

It has what appears to be a cement 'render' in places - probably more to repair the stonework than anything.  It is dry and we never had any problems of damp with the previous polystyrene backed plasterboard.

Internally, the wall is not flat, with the top falling backwards by about 50mm compared with the bottom.  As the room is not large, we do not want to lose any more space than necessary.  There are a lot of sockets

to bring through the new work.  There is a water pipe - could that be run at high level in the deep void?  The stone is very soft and mechanically fixing is difficult.

 

I am considering two IWI options.

 

1.  Dot and dab, using 50mm PIR backed 12.5mm plasterboard.  This would give an air gap of around 10mm at the bottom and 60mm at the top.  That would give a U value of around 0.3.

 

2.  Build stud work, would 50mm timber be sufficient, infill with 50mm PIR - possibly more thickness of PIR towards the top of the wall, then use say 10mm PIR backed plasterboard screwed on top.

This would provide a vapour barrier and prevent any bridging?

 

I would be very grateful for any opinions or advice.

 

Thanks.

 

 

 

 

[Redbeard]

I think I would still err towards rigid wood-fibre, and getting rid of as much of the sand/cement as you can, and me being me, I'd lime parge-coat first and go with the shape of the wall. I think right-angles are over-rated, but of course you do not have to agree!

 

Quick comment on the VCL. I am, and others who feel the same are, regarded as pernickety for this, but I take the view that where the 2 boards butt there is no VCL, since they rarely go 100% tight to each other. In scenario 2 you can tape the 50mm as VCL and then the 10mm on top matters little if it is not 100% tight at the joints.

 

This (https://www.gov.uk/government/publications/retrofit-internal-wall-insulation-best-practice) is the Gov't's Best Practice advice,  introduced so 'quietly' a year or two ago  that many people I know have never heard of it! It states that vapour-closed insulants should *only* be used if there is a fully-ventilated min. 25mm cavity behind. This could mean 4 air-bricks per wall per room if there is not connection from one storey to the next. Imagine that on 450mm rubble-filled walls! It would appear that the insulation merchants have not heard of this, and I am not sure about the manuf'rs either. You can certainly find plenty of unventilated/hard-to-the-wall solutions online.

 

Edit: Just noted the bit re mechanical fixings being tricky. Yep, that makes rigid wood-fibre hard. Studwork, flexi WF and 20mm rigid WF as plaster carrier? Too thick perhaps. 

Edited December 19, 2023 by Redbeard
Tricky fixings