Effectiveness Rmax Thermasheath-3 aka closed cell poly iso | John Sayers’ Recording Studio Design Forum archive

Hi "atom", and welcome to the forum! : I fee your pain. Spending money on something that doesn't work is very frustrating, to say the least. And what I'm going to say is probably not going to help much in that respect, but hopefully it will at least help you understand WHY it isn't working, and what you need to do to make it work. One part at a time:

The booth shares two walls with the already existing garage.

There's your first problem. In order to stop sound from getting from one place to another, you have to break all the paths it could travel through. Shared walls are dead easy for sound to get through. If you truly want to block sound, there has to be a total mechanical break between the two sides of ALL of the walls, so that any vibration on one side has no way of getting through to the other side. It is possible to have walls in common with other rooms, of course, but they must be acoustically "decoupled". Yours aren't.

The only materials we have invested into the booth have been one layer of 1/2" drywall and 1/2" sound board from Home Depot.

There's your second problem: 1/2" drywall is too thin to be highly effective at stopping sound. In addition to cutting all possible paths from "A" to "B", you also need to put a barrier in the way, and that barrier must be both massive (very heavy) and rigid (stiff, not flexible). 1/2" drywall is neither. All over this forum you'll see recommendations for using only 5/8" fire-rated drywall, since that is much more rigid and much more massive.

The wood frame and some donated MLV that covers about one of the longer walls and one of the shorter walls.

There's your third problem. MLV is mass, yes, and it is even limp mass, which is good, but it's no use unless it is part of a properly designed system, totally sealed airtight, and surrounding the entire room. MLV is also very, very expensive mass! Pound for pound, it is one of the most expensive isolating materials you can buy. Sound waves can't read price tags, and they don't care how much you paid for your mass: all they see is the actual amount of mass in front of them, and that's what stops them. The price doesn't matter. So get the cheapest mass you can that will do the job. OK, your MLV was donated, so you didn't waste any money on it, but you probably don't have enough of it, and clearly it isn't doing what it should be doing.

My research has led me to believe there were a number of crucial construction steps missed in the building process

Exactly.

one of which being an installed "air gap".

True, but the air gap is only useful if it is designed correctly as part of a proper decoupled 2-leaf isolation system. An air gap on its own isn't much use unless the wall panels on each side are decoupled form each other, and sealed air-tight.

I have come to the conclusion that adding home insulation, green glue, and acoustic caulk are my best bets for sound blocking.

Of those three, sealing (caulking) is the most important. You need to entirely seal each of your two "leaves" to be totally air-tight, not even a tiny gap. If there is any place at all where air can get through, then so can sound. Green Glue is useful for one specific thing: as constrained layer damping in a properly designed two-leaf wall, with at least two layers of mass on each leaf. That does not seem to be the case with your booth, so it probably won't help much. It will help a bit, but not as much as you are expecting, or as much as it could in a proper wall. The insulation is also good, but once again, only if it is the correct type, and installed as part of a properly designed isolation system.

pannels of Rmax Thermasheath-3 @ R-9.5. And am planning to use it in the same manner as batted insulation, so i would be cutting these panels down to fit in between our studs. Does anyone know the effectiveness of these panels as a sound deadener?

No use at all, sad to say. It's a very common misconception that insulation stops sound: it does not. By itself, insulation does very little to "deaden" sound. But here too, if it is part of a properly designed system, it can have pretty amazing effects. In fact, it's one of the easiest ways of improving isolation on a properly designed two-leaf wall. It can make a big difference. But only when used correctly. And unfortunately, that stuff you got is no use at all, even if it was part of a proper 2-leaf isolation system. Closed-cell insulation has no uses, acoustically: it is great thermally, but acoustic insulation needs to be open cell: it has to have some type of "fibrous" structure that air can penetrate in order to have any acoustic use. The normal recommendation here is for either fiberglass insulation (the common "pink fluffy" type is good), or mineral wool insulation. If fiberglass, then it should have a density of around 2 pcf, and if mineral wool then it should be around 3 pcf.

The possible effectiveness of these panels mixed with green glue on the BS of the sheetrock with acoustic sealant to fill the corners and seams?

Except for the sealant, that would be ineffective. A waste of time and money. The insulation is the wrong type, and Green Glue is not meant for that purpose. Fortunately, there are solutions that will work for you, but in order to propose the right one, we'd first need to understand much better what you actually have, and what your goals are. So if you could post some photos and diagrams of they way things are right now, and provide more info on where you want to get to, in objective decibel terms, then we can probably help you figure out the most effective solution. - Stuart -