I am having my cinder block party wall insulated with stick-on panels. The property is a two story terraced house, and it has been recommended to insulate the void from floorboard to ceiling between joists to a depth of 1m from the wall. An overviewshows the layout of the entire project if you wish to see the context.

The relevant part for this question is the following diagram showing the joists, and in order, images of the joist hanger and baton between joists obtained via an endoscope.

Notes:

• The floorboards extend continuously under the internal wall separating Study and Bedroom. It is preferable to insulate without lifting the boards.
• The top end of the hanger protrudes above the joist and appears to curl over towards the brick face. This suggests it is embedded in the mortar between rows. The image also shows the bottom edge of the skirting board which seemed to have the same metallic colour as the hanger, so at first it looked like a metal rail.
• There is a lot of rubble on the ceiling, making it not clearly visible. The ceiling appears to be nailed to a baton spanning two joists. A probe with the endoscope did not reveal any other batons in the void between two joists.

The internal wall separating study and bedroom has been built on top of the floorboards making them difficult to lift. The floorboards are sanded, varnished pine, and so I am exploring the idea of filling the void with spray foam.

The recommended density for a mineral wool insulation is 45 – 60 kg/m3. I have found one spray foam product (Icynene) with a density of 35 – 40 kg/m3.

Would this density be effective in reducing noise transmission from wall to joists and ceiling?