Acoustic effect of Rigid Foam in outer leaf

First of all my compliments on this forum, it's an awesome resource. I've been lurking for a while, there is just so much information and I love the build threads especially! I'm constructing a soundproof work space in an existing building (so I don't annoy the neighbours when I use power tools). My only concern is sound leaking to the outside, I don't care about the acoustics inside the room (it is not a recording studio). I plan to insulate the outer leaf (a brick wall) and make a room-in-room construction. I can insulate the outer leaf with Rockwool (433 Duo, normally used in cavity walls) or with PIR rigid foam plates. The inner leaf will be wood framing with a double layer of plasterboard. The airgap between outer and inner leaf will be completely filled with Rockwool. Please see the attachment for a schematic drawing. My question is: what would be the effect of the PIR insulation layer on the sound proofing? Does a layer of PIR foam disrupt the sound proofing for this double leaf construction, or is it acoustically 'transparent' (ie: the only effect is that it takes up space in the air gap)? The reason why I would like to use the PIR is because it is immune to moisture, and I have enough leftovers to cover the wall. The PIR plates weigh 30kg/m3 and have a multi-layer aluminium backing on both sides. Thanks in advance for any answers!

Hi there "mvijfwinkel", and welcome to the forum! :) PIR is closed cell, as far as I know, so it won't have any useful acoustic properties, but it also won't do any harm: It just won't help.

the only effect is that it takes up space in the air gap

Yep. That's the way I see it. The space taken up by that foam could have been used for insulation that does have useful acoustic properties, so from that point of view you lost a bit of space that could have been used otherwise, but if there are overriding reasons for using it, then I don't see any problem. As log as the remaining air gap is still deep enough to get your MSM frequency down to the point you need it, then I think you'd be fine. However, you did mention that you want to use it because it is immune to moisture: That implies that you are concerned about moisture in your wall cavity, and THAT certainly is a problem. There should never be any moisture in that cavity. The wall should be built with a proper water barrier on the outside, and a vapor barrier on the warmest side of the cavity. Together, the help ensure that the cavity stays dry. If your wall already has moisture problems, or you suspect it will, then you should fix that problem first. Moisture in the cavity can lead to many bad things happening later on... - Stuart -

I have the luxury of space, so I can keep the air gap the same size (the PIR plates are only 6cm thick anyway). I did have a moisture problem: during very heavy rain a water pool build up against the outer wall, and some of that water seeped between the concrete floor slab and the brick wall. Unfortunately I do not have access to the outer wall (a shed has been build with only a 10cm gap between the walls). But I used a special mortar to 'round' the corner between floor and wall, and then a flexible waterproof coating ("Weber.dy Cement" from Saint Gobain, it's like very thick paint). Even after a single layer of coating it was completely waterproof, and since then 2 more layers have been added. Waterproofing a wall on the inside is not ideal, but I think I got it covered. I'll use a damp proof water barrier on the inside of the walls. So moisture proof insulation is more a precaution/extra protection, because I don't want any problems in that respect. Before I demolished the old inner walls, the place stank of fungus and dampness, really disgusting and unhealthy. Thanks a lot for your answer!

It, the PIR, would have to be installed per local building codes. Aluminum foil is a Class 1 Vapor Barrier. And both sides having aluminum foil makes double barrier, and that is never a good thing. BUT....your neck of the woods might be able to support this feature. But I recommend finding out where this product is to placed in the assembly. I would have to think that it would be at the outer most part but might even require a drainage screen in order to modify the trapping properties of the foil.