Standing rockwool batts vertically - which type?

[Spreadsheetman]

I’ve got a fiddly insulation job to do to a dwarf stud wall where I can only fit the insulation from the front. Wall is about 1.2m high and made of 75mm x 50mm timber with uprights spaced 400mm apart. There is a space vented to the outside behind the wall.

 

My current thinking is to use 75mm rockwool batts vertically behind the studs held in place with a run of plastering galvanised metal mesh laths (2ft wide) stood off with 90mm plasterboard screws and big washers. I can get in above and below the mesh to fit the screws since it doesn’t run full height of the studwork.

 

Once the rockwool is in place I will then fit 75mm PIR board between the wall uprights and plasterboard over the front. Should give u around 0.21 which is fine for this application.

 

Question is which one of the many types and makes of rockwool batts to use? It needs to be stiff enough to remain in contact with the back of the studs when the ends are not supported (2ft mesh leaves about 300mm top and bottom unsupported), but not so stiff that it can’t be stuffed in through the studwork frame.

 

 

[j_s]

Knauf Dritherm 32 (cavity batts) might do it but it comes in 100mm thick. Batts are 1200 x 460 I think. They did do a 75mm option but I've never seen it in stock. Dritherm 37 isn't quite as dense/rigid. 

Edited November 13, 2022 by j_s

[Buildtidy]

Try the new cavity product by

Rockwool.

https://www.encon.co.uk/rockwool-nyrock-cavity-slab-032!pd-71-7979

 

[Spreadsheetman]

Ok, so arrived at a solution. Bought some samples on eBay so I could try the ideas out. Had a false start with a Knauf product (rocksilk flexible slab - very floppy and crumbly, falls apart when I tried to pick a slab up - useless), but success with Rockwool RWA45 which is easy to get and relatively inexpensive.

 

The density of the product is key, RWA45 is 45kg/m3 so there is more meat to it and the slabs self-support, but are still flexible. Cut to fit between studs (2cm oversize on approx 400mm spacing) it stays in place and the next layer can be held behind the studs with plastic insulation support mesh stood off with big plastic washers (60mm) and long drywall screws. I’m doing 75mm between studs and a continuous layer of 100mm behind. No PIR board in this solution, so a lot quicker to fit and the batts cut up easily with an old handsaw.
 

Ubakus says u=0.206 which is great. (in context of the rest of the fabric)

 

I had a bit of a result; when the downstairs ceilings came out I could get at  the back of the dwarf walls from below so didn’t have to take them out. I can insulate the ceiling between the 175mm joists from below using the same stuff (with small rigid PIR sections to make sure the cavity between joists and floor-ceiling is completely closed even if the rockwool settles or shifts).

 

My late 70s chalet was quite advanced for the time as they had stuck a load of fibreglass behind the dwarf walls and in the corresponding floor-ceiling gaps below. That didn’t fall down with the ceilings as I expected, but I had to pull it all out anyway to get access for plumbing and electrics.