Sound/film room in the basement, insulation versus basement walls

[JFrisk]

I am working on acoustic treatment in my upcoming music and film room. I have had the room designed by an acoustic engineer, but not with regard to moisture. Even though his experience suggests there are no problems, I still want to double-check here so I don't make unnecessary mistakes where there might be a better solution.

The basement has been newly drained (due to the garage and patio being built outside the walls, not because of moisture problems) and shows no signs of moisture issues. The room has two exterior walls, one of which faces the garage with just a narrow passage between where hardly any rainwater or snow can reach. The house is also situated on a gravel/sand ridge, providing good drainage.

There was previously a wall with a wooden frame screwed directly into the concrete floor without any signs of moisture whatsoever.

Now I'm planning to build a podium 2m long and the entire width of the room (~3.5m). It will be insulated with 145mm insulation. I'm thinking of placing age-resistant plastic under the joists so they can't absorb anything. The insulation will be stone or glass wool, so it is inorganic.

I have an idea to lay a coiled copper pipe underneath, connected to a radiator, partly to heat the slab slightly underneath to push away and perhaps evaporate some water/moisture if any arises. Also, it would make an excellent air cooler during long sessions, and perhaps not least for cooling the electronics. Is this a completely crazy idea, or could it work?

The walls will also be covered with insulation, but there it's only 45mm and it's a completely open construction without covering boards or the like outside. Are there any obvious risks with this, and if so, is there a technique better than others?

The sections are calculated for the right absorption, so I can't add air gaps randomly, even though I plan on squeezing in a few wherever possible, is that right? Or is there just a risk that warm indoor air cools down and condenses?

Grateful for feedback and answers! As mentioned, there are currently no signs of moisture, but it's never optimal to enclose a basement too much (as far as basements are concerned, "at all" might be too much...) so I would appreciate some advice and tips before making any major mistakes.

 

[Björn Melander]

It's good that you are observant about these things, as a basement is often a risk construction. The basic rule is to never insulate directly against the floor or basement wall.

THE PLATFORM
I assume you plan to build a Helmholtz construction with a closed box volume and slat panel. Skip the plastic. Instead, use metal studs and apply with an EPDM strip (not the foam one). Stretch chicken wire to support the insulation, also against the wall. This way, the construction can be ventilated and breathe.

ACOUSTIC WALLS
Same thing here, metal studs and leave a gap against the outer wall (the air gap also makes the absorption more broadband).

 

[JFrisk]

It's good that you are observant about these things, as a basement is often a risky construction. The basic rule is to never insulate directly against the floor or basement wall.

THE PODIUM
I assume you plan to build a Helmholtz construction with a closed box volume and slat panel. Skip the plastic. Instead, use metal studs and lay with EPDM strip (not the one with foam). Stretch chicken wire that you let the insulation rest against, also against the wall. In this way, the construction can be ventilated and breathe.

ACOUSTIC WALLS
The same here, metal studs and leave a gap against the outer wall (the air gap also makes the absorption more broadband).

Thanks for the quick response!

Podium:
I don't think it will be a purely Helmholtz construction, it consists of a base of transverse 145-studs with insulation in between, then there will be longitudinal 145-studs on top and then a "roof" of tongue and groove lumber (or, the floor itself...) Open at the front, upper 145 and enclosed around. It resembles a Helmholtz but as I understood, there were no requirements for it to be absolutely airtight, it will probably be quite broadband. Here it should be possible to lay chicken wire without any problem! My first thought was to lay the conduit pipes in such a way that they created an air gap between the insulation and floor, I'll see which solution I go with, maybe combining them.

Acoustic walls:
The construction is drawn as it should be, adding an air gap before potentially creates more broadband, but the wall is drawn for the correct function, if I change parameters it will have a different function than intended. Additionally, the damping material becomes thinner if I add an air gap inside since I don't have much more than the 45mm to play with. But something small that contributes to ventilation, I will try to fit in any case. I'll try to figure out something suitable and discuss with the person who calculated and designed the room.

 

[Björn Melander]

It's good if you can build a ventilated construction. The air gap doesn't need to be large based on how you describe the conditions. I would probably leave the acoustic wall 5 mm from the outer wall and tension steel wire or masonry string to keep the insulation in place and ensure the air gap. These 5 mm will have a notable impact on room acoustics.

 

[JFrisk]

It sounds like a good idea, I should probably spend on a couple of wires or bricklayer’s strings! At least that way there is some chance for the moisture to be ventilated out behind. Putting covering things on the basement walls is probably wrong, so I’ll try to do it as little wrong as possible. A gap at the top and one at the bottom and some air spaces behind.