Is insulating suspended floor void with spray foam effective in reducing noise?

Question

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?

Answer 1

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.

Answer 2

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