Benefit of beefing up a single wall?
Posted: Thu Dec 22, 2005 6:13 am
My studio build is nearing completion (hurrah!). The outer building is finished and the studwork is going up for the inner room.
Due to the inevitable irregularities and contingencies of building, the air gap between the two has turned out quite inconsistent. Most of the way round it's wide - around 280mm - but in the front wall ("A" in the drawing below) it's considerably shorter at aroun 165-170 mm for most of its length:
Now it occurs to me that this wall will be by far the weakest: it has the door in, plus two openings for ventilation ducts (which will have channels of the same high density block leading to fans leading to rockwool silencers, but still...) and it has two flat internal vent ducts running up the wall, eating further into the air gap. I had no choice but to put all this on the one side, as that's the only side I'll really have access too: the other three sides all face onto neighbours' properties.
And there's the rub: Side A is the one where I most NEED the soundproofing, since that faces directly back to our house and our neighbours' houses. The other three sides have a much further distance to any houses, B and D are each 2/3 against neighbouring garden walls, and C is 2/3 buried into the ground (which slopes sharply up at that end of the garden).
We're planning to hang two sheets of standard drywall on the studs. The isolation of the outer building is pretty good already and I don't have space to burn, so I don't want to go over the top with the wall thickness. But it occurred to me whether it might be a good idea to increase the thickness of JUST wall A. I could order a little more drywall and we could put it up, and doing just the one wall would only lose an inch along one side.
If necessary later, I could even add another layer of block to the outer wall. There's no window and the roof goes well past the wall, so it would accomodate another layer. The only complication would be the door, which I'm sure could be rehung.
I read that doubling the mass of a wall increases TL by about 6db. How does this work with a double-leaf wall? Does doubling EITHER leaf add 6db, or would one need to double both to achieve that?
More importantly, how much of this improvement is "directional"? That is, if I have (to simplify) three walls with two layers of drywall, a 280mm air gap and one layer of block, and a fourth wall with four layers of drywall, a 165mm air gap and two layers of block, would the overall TL be in line with each of those designs? Or would most of the sound find its way out through the weakest route around the building: ie would I basically have the result of two layers of drywall, 165mm air gap and one layer of block?
Or would the result be defined so much by the door anyway that it's not worth worrying about? The outer door is a 90mm solid timber "sandwich" with MLV between, and compression seals all the way around. The inner door I haven't done yet - partly I'm waiting to decide how thick the wall will be, before deciding how to do it.
There is a split foundation separating the two walls, rather than a floating floor, if that makes a difference.
Due to the inevitable irregularities and contingencies of building, the air gap between the two has turned out quite inconsistent. Most of the way round it's wide - around 280mm - but in the front wall ("A" in the drawing below) it's considerably shorter at aroun 165-170 mm for most of its length:
Now it occurs to me that this wall will be by far the weakest: it has the door in, plus two openings for ventilation ducts (which will have channels of the same high density block leading to fans leading to rockwool silencers, but still...) and it has two flat internal vent ducts running up the wall, eating further into the air gap. I had no choice but to put all this on the one side, as that's the only side I'll really have access too: the other three sides all face onto neighbours' properties.
And there's the rub: Side A is the one where I most NEED the soundproofing, since that faces directly back to our house and our neighbours' houses. The other three sides have a much further distance to any houses, B and D are each 2/3 against neighbouring garden walls, and C is 2/3 buried into the ground (which slopes sharply up at that end of the garden).
We're planning to hang two sheets of standard drywall on the studs. The isolation of the outer building is pretty good already and I don't have space to burn, so I don't want to go over the top with the wall thickness. But it occurred to me whether it might be a good idea to increase the thickness of JUST wall A. I could order a little more drywall and we could put it up, and doing just the one wall would only lose an inch along one side.
If necessary later, I could even add another layer of block to the outer wall. There's no window and the roof goes well past the wall, so it would accomodate another layer. The only complication would be the door, which I'm sure could be rehung.
I read that doubling the mass of a wall increases TL by about 6db. How does this work with a double-leaf wall? Does doubling EITHER leaf add 6db, or would one need to double both to achieve that?
More importantly, how much of this improvement is "directional"? That is, if I have (to simplify) three walls with two layers of drywall, a 280mm air gap and one layer of block, and a fourth wall with four layers of drywall, a 165mm air gap and two layers of block, would the overall TL be in line with each of those designs? Or would most of the sound find its way out through the weakest route around the building: ie would I basically have the result of two layers of drywall, 165mm air gap and one layer of block?
Or would the result be defined so much by the door anyway that it's not worth worrying about? The outer door is a 90mm solid timber "sandwich" with MLV between, and compression seals all the way around. The inner door I haven't done yet - partly I'm waiting to decide how thick the wall will be, before deciding how to do it.
There is a split foundation separating the two walls, rather than a floating floor, if that makes a difference.
