How to construct wall between garage and house for good soundproofing?

[andeol]

I am going to install geothermal heating in a seventies house and the heat pump will be located in the garage which is attached to the house. The garage walls are internally clad with asbestos cement boards, so I thought I'd take the opportunity to tear them out before the geothermal system is installed.

With this, I also thought I’d take the opportunity to improve the sound insulation in the wall that the garage shares with the house, and the question is how do I do this in the best possible way?

Counting from the house side, the wall currently consists of:

1. Gypsum board
2. Vapor barrier
3. Rough lumber cladding
4. Standing 95 mm studs + insulation
5. Horizontal 45 mm studs + insulation
6. Asphalt sheathing
7. Asbestos cement board

As a new surface in the garage, I plan to use 12 mm structural plywood which I will paint. I think it will be a more durable surface than gypsum, plus I avoid spackling, plus anyway I need a board that's good to attach things to.

I am currently choosing between two options. Option one is to screw one or two layers of gypsum onto the asphalt sheathing and then the plywood. Option two is to screw up one layer of gypsum on the asphalt sheathing, and then frame a new freestanding wall with 45 mm steel studs one centimeter inside the existing one. It is insulated, then one layer of gypsum and then the plywood. Joints between walls/ceiling and boards I will seal with flexible caulk.

Priority one is therefore good sound insulation so that, for example, I don't have to hear the ventilation fan and geothermal heat pump in the house. Option two has the advantage that it also provides better thermal insulation, but that is secondary. Another advantage is that it then becomes easy to route all the electrical inside the wall. The downside of option two, besides extra cost and effort, is that it adds more thickness to the wall; I don’t actually want to extend the wall more than necessary.

What would you do? The sound insulation should be better with option 2, but the question is how much better? How many layers of gypsum should I use in each option? Does 45 mm steel studs with 60 cm centers make for too weak a wall? Are there other smarter options? How about fire regulations, are they met by both options?

It strikes me now after writing all this that a steel-studded wall with boards only on one side might be weak regardless?

 

[huggan]

If you want good sound insulation, the wall in the garage must be separate from the wall in the house.

 

[brajt]

Option 3.
Keep the eternit as it offers some benefits plus fire protection. Build up a 45 wall outside. Consider moisture issues at the base. Use sealant paper and/or steel studs. If you want, you can cross-brace for increased stability. If it builds up too much, remove the eternit.

Just applying 2 layers of plasterboard with a flexible filler also makes a significant improvement.

Regarding plasterboard, it seems fine to skip the putty and just paint. My recommendation is to place plywood inside the plasterboard. If you're using a separate stud wall, it reinforces better with the plywood directly against the studs.

 

[andeol]

The Eternit will have to be removed either way, some of it I need to take off no matter what, and I also need to make some penetrations. I'm not familiar with fire regulations, but if I have double gypsum somewhere in the construction, maybe that would suffice?

Regarding gypsum, I didn't find gypsum without joint edges, but I thought that I could use floor gypsum instead, so I get perfectly flat boards. On the other hand, they are only 60 cm wide, which complicates things if I want to mount double overlapping layers. Then I have to reduce the spacing to 30 cm, which involves more work with framing and insulation. Installing plywood directly against the studs is probably smarter stability-wise, as you say, especially if I choose wood studs and glue at the same time.

I've got the idea that steel studs are better acoustically than wood studs, but maybe it doesn't matter if it's a freestanding separate wall?

Which is more stable, 45 mm steel studs or 45 mm wood studs?

The wall is about 7.5 meters long, and part of it is next to the house which rests on two concrete slabs, one of which is offset by half a floor level. So behind part of the wall, there's probably a lightweight concrete wall, which means that part of the wall may be exposed to moisture. It's a bit difficult to explain, but the garage is partially built into a slope, you could say. There it would be best with steel studs and inorganic material, but I'll see what it looks like behind the Eternit first.