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.

Floor void Upper end of hanger Bottomof hanger with ceiling baton


  • 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?

  • Hello, and welcome to Home Improvement. Your diagram page is great, but it's best if the critical info is here; would you add a few diagrams to your actual question? And, you should probably take our tour so you'll know how best to participate here. Sep 5, 2019 at 22:17

2 Answers 2


I believe that rock-wool would give better results.

That much spray foam is going to be very expensive...and messy, especially if this is a DIY job.

  • I used Roxul "Safe'n'Sound" in the joists between the basement and upstairs, and it worked wonderfully. Easy to install and cut- I just used an old bread knife. Wear eye protection and probably a long sleeve shirt- while not as bad as fiberglass, it is a little itchy and you don't want it in your eyes.
    – Jamie M
    Sep 6, 2019 at 13:36
  • I agree that acoustic mineral wool is a preferred solution. The recommended product has a density of 60kg/m3 (known as Rockwool RW3 and comes in slabs of 1200 X 600mm). I could expose the void by parting the floorboard adjacent to the party wall, removing just the first 60mm of floorboard (the combined thickness of all the panels). Following @Jamie M, I would then have a small gap through which to feed Rockwool briquettes cut from the slabs. Sounds arduous - but is this the only option left?
    – Geoff
    Sep 6, 2019 at 21:21

You don’t mention what the “sound” problem is.

Is it sound transmitted through the party wall? Or airborne sound transmitted from upper floor to lower floor? (Or vice-versa?) Or is it impact sound (walking, doors closing, etc.) transmitted from upper floor to lower floor?

There are two kinds of “sound control” problems: 1) airborne sounds, and 2) impact sounds.

1) Airborne sounds (talking, tv, etc.) is the easiest to control.

2) Impact sound (closing doors, heels clicking on floor from walking, etc.) is much harder to control. Impact sound travels through building materials (floor joists, wallboard, etc.) much faster and very efficiently AND is very difficult to stop, if sound control systems are not designed into the original construction.

In order to control impact sound, you need to isolate the surrounding material. Some of these problems are “built in” and cannot be resolved easily (or cheaply).

We try to 1) stop floor joists and underlayment from extending under a party wall, 2) provide separate party walls between rooms (apartments) and 3) stop roof joists from crossing from room to room.

Obviously it’s too late for those techniques, so now you need to consider techniques for each individual problem.

Adding insulation in the joist space is good, but I don’t understand your statement about “installing it 1m out from the wall”. If it’s noise coming through the party wall, then the noise is already in the rooms. If it’s noise being transmitted from floor to floor, then you should do the entire floor.

For doors, I’d try: 1) provide rubber bumpers on jambs, 2) provide weatherstripping to cushion the door when closing, and 3) add closer to control speed of door closing.

Resistance to sound traveling through walls or floors is measured in STC (Sound Transmission Class) ratings. Each material (including an air space) has a rating...often given in thickness.

So, rockwool insulation has an STC rating that is much better than foams. See here: https://www.noisehelp.com/soundproofing-material.html In fact, foams are not recommended.

Rather than insulation in the joist or wall cavity, I’d use a “resilient channels” (on the walls or ceilings). It has a much higher STC rating than insulation and is relatively cheaper. It can be installed over the existing wall board or ceiling board and works for airborne sounds or impact sounds. See here: http://www.soundproofing.org/infopages/channel.htm

  • There is airborne conversation as well as scraping of chairs penetrating the party wall. If I tap my side of the wall in the bedroom, the tap is transmitted to the floorboards and ceiling in the study. So there is impact noise in the joists, floorboards and ceiling. A microphone in the void picks up the tap. Assume the wall has been soundproofed according to these [fitting instructions][1]. Will adding rockwool in the void (as recommended) help to reduce the noise further? [1]: soundstop.co.uk/resources/instructions/…
    – Geoff
    Sep 7, 2019 at 15:59

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