Please critique my new build

OK, here we go: I'm going to be building a new room at the end of the garden of my London terrace house. It will be primarily for rehearsing and mixing, but I'd like to be able to make decent demo recordings in there if possible too. Soundproofing, to avoid neighbour hassles when rehearsing, and maximising floor space are more important priorities than perfect acoustics. The planned wall construction is an outer skin of 8" thick dense concrete blocks, a 3" air gaps filled with rockwool, and then an inner skin of two layers of plasterboard, maybe with green glue between, on timber frame. The building will be sunk 1 foot into the ground. I have decided not to float the floor of the inner room. An alternative was suggested to me, of actually separating the foundations of the two skins, laying the outer one like a strip around the inner one, with a small gap between. My brother in law who is doing the building says this will be fairly easy, and we can't really get our heads around everything involved in floating the floor - the maths, load bearing etc. The inner room is actually a rectangle, but then there are also "false walls" along the sides, widening away from the mix position, creating two "cupboard-rooms" or walk-in-wardrobes, if you like, in the corners. One of these will be used for housing the four PCs which are the backbone of my setup. I don't use a mixing desk, but have a little desk for mic connections and various MIDI gear that need to go in a little "station" in front of the mix position. The ceiling will also slope upwards away from the wall where the speakers are, to a pitch about 2/3 of the way down the room. I believe that the angled walls and ceiling are good for acoustic when mixing, but they are also there for other reasons - creating the separate room for the PCs, and allowing for a pitched roof with quite a low edge (so that the building is not too imposing aesthetically, upon the garden) yet a higher middle. One question I have is how to work out the optimum angles of these. Their dimensions in the plans below could be changed - especially the tapered walls as they will be the last thing to go up. I don't really understand the technical details of early reflection paths to the mix position etc. There are various other questions too, but first of all how about a general impression? Here it is: (Heavy black is 8" concrete. Blue is plasterboard)

Whoops! Sorry about the picture size. I'm a bit of a novice in the whole graphics linking side of things. I just uploaded a jpeg to my band website, and it came out like this. What should I do stop it stretching the page: reduce the number of pixels or something?

Yes, trim the extra white space on the edges and keep the total image width <700 pixels.

Thanks . . . Better now.

Much better! 8)

. . . 3" air gaps filled with rockwool . . .

Are you referring to a 3" distance between the wooden frame and the concrete block wall? If so, then you'd add the width of that distance to the depth of your frame... And ideally you'd have that entire depth filled with insulation. The size of the air gap is just as important as the thickness and density of your leaves. I understand that about 12" gives you an impressive result relative to the lost square footage -- more is better, but the return on the space investment diminishes.

I have decided not to float the floor of the inner room. An alternative was suggested to me, of actually separating the foundations of the two skins, laying the outer one like a strip around the inner one, with a small gap between. My brother in law who is doing the building says this will be fairly easy, and we can't really get our heads around everything involved in floating the floor - the maths, load bearing etc.

Sounds like you're opting for the "isolated slab" concept. That's what I'm doing for my project (as you may already know). Tell us more about that plan. One thing we're not seeing in the illustration is whether this will be an addition or a standalone structure. Please clarify that. If at all possible, I think you should consider standalone, especially if you want to minimize sound leakage into the rest of your home. Got photos? Got a smaller drawing scale showing your entire property, including proximity to neighbors? Also, being in the UK, have you followed Paul Woodlock's mind-blowing studio bunker project? --Keith :mrgreen:

sharward wrote:

Much better! 8)

. . . 3" air gaps filled with rockwool . . .

Are you referring to a 3" distance between the wooden frame and the concrete block wall? If so, then you'd add the width of that distance to the depth of your frame... And ideally you'd have that entire depth filled with insulation.

I mean 3" between the concrete blocks and the plasterboard. So the timber frame would actually jut into that space.

The size of the air gap is just as important as the thickness and density of your leaves. I understand that about 12" gives you an impressive result relative to the lost square footage -- more is better, but the return on the space investment diminishes.

That sort of thing is simply not going to be practicable. The outside dimensions in the plan are the maximum I can stretch to, confined in one dimension by the width of the garden, and in the other by the amount of garden length that we are prepared to lose. As it stands, the walls will be a total of about 12" thick in total: 8" concrete, 3" air/rockwool, 1" plasterboard. Like I said, maximising floor space is crucial for rehearsal purposes, and a 12" air gap would make the walls almost 2 foot thick!

I have decided not to float the floor of the inner room. An alternative was suggested to me, of actually separating the foundations of the two skins, laying the outer one like a strip around the inner one, with a small gap between. My brother in law who is doing the building says this will be fairly easy, and we can't really get our heads around everything involved in floating the floor - the maths, load bearing etc.

Sounds like you're opting for the "isolated slab" concept. That's what I'm doing for my project (as you may already know). Tell us more about that plan.

A single rectangular slab on which the inner room is built. Then a very narrow gap all the way around, and another foundation forming a rectangle all the way around it, on which the outer room is built. One difficulty is knowing how wide the outer foundation has to be to be strong enough, given the weakening effect of not being attached to the inner one. As I'm constrained by the borders of the garden, I'm loathe to have to extend the foundations too far beyond the walls. My brother-in-law suggested that the outer foundation could actually extend higher than the inner one, to give it more strength. I've tried to show this in the plan. I'm interested that you're doing this too, as I'm not sure how common a way of doing things it is. Anything further you've learnt on the subject, that would be useful to know?

One thing we're not seeing in the illustration is whether this will be an addition or a standalone structure. Please clarify that. If at all possible, I think you should consider standalone, especially if you want to minimize sound leakage into the rest of your home.

It's completely standalone. There'll be some 25 foot or more between it and the house, so that's not really an issue. That will also help of course, in soundproofing it from the neighbours' houses, which are roughly parallel with mine. But I still want to soundproof it as best I can.

Also, being in the UK, have you followed Paul Woodlock's mind-blowing studio bunker project? --Keith :mrgreen:

I've had a look, and had some correspondence with Paul on the Sound On Sound forum. But that thread would take a year to read in its entirety!

Perhaps you don't need to isolate your inner slab if the entire building is standalone. Are you married to the idea of using 8" concrete blocks? I'm thinking that if you did double timber frame all around (no blocks), then you'd have the benefit of the air gap of the stud cavities on both frames. Using 2x6 framing (or thereabouts) would give you your 12" gap and would not cost you any additional square footage. Plus, much to my surprise, 8" concrete blocks, even if filled, do not offer the kick-ass isolation that I would have thought. I'm not the expert, but I would think that making the changes I detailed here would increase your isolation. Perhaps Steve can run some calculations of the various scenarios to determine how much better (or worse? :?) it would be. Be patient though -- he's insanely busy right now. --Keith :mrgreen:

sharward wrote:

Plus, much to my surprise, 8" concrete blocks, even if filled, do not offer the kick-ass isolation that I would have thought.

I often read concerns on here as to whether blocks are 'filled' or not, but I think solid concrete blocks are more common in the UK, they can be had very cheap from a builders yard. In any case, a brick/block based structure would offer better security. Out of interest Wurl, what type of blocks were you thinking of, how much and where from? I know Wickes do 4" thick medium density concrete blocks (not breeze nonsense) 50-odd quid for a pallet of 70. Rendered both sides I reckon these would be pretty decent isolation wise. I'm thinking of using these between my garage door (big metal thing!) and inner studio wall.

Deluks wrote:

sharward wrote:

Plus, much to my surprise, 8" concrete blocks, even if filled, do not offer the kick-ass isolation that I would have thought.

I often read concerns on here as to whether blocks are 'filled' or not, but I think solid concrete blocks are more common in the UK, they can be had very cheap from a builders yard. In any case, a brick/block based structure would offer better security.

Yes, I'll be going for solid concrete blocks, of the highest possible density. They are quite common in the 100mm thickness. The 190mm thickness that I'll be using is less common, but I have managed to track some down. I can't imagine that timber & plasterboard would have better sound isolation than these, would it?

Out of interest Wurl, what type of blocks were you thinking of, how much and where from? I know Wickes do 4" thick medium density concrete blocks (not breeze nonsense) 50-odd quid for a pallet of 70. Rendered both sides I reckon these would be pretty decent isolation wise. I'm thinking of using these between my garage door (big metal thing!) and inner studio wall.

If you go to the Concrete Block Association Website they have all sorts of information about the different types available, plus a list of suppliers. I emailed a few of them, and while some either said they only sell to builders' merchants, or didn't even reply (which I presume is for the same reason), I did get a couple of quotes. The best was from www.besblock.co.uk, at £14.40 + VAT per square metre for the solid 190mm blocks. That should come out at under a grand for my whole building.

Wurlitzer wrote:

I can't imagine that timber & plasterboard would have better sound isolation than these, would it?

Quite possibly. Don't assume anything.

Wurlitzer wrote:

I did get a couple of quotes. The best was from www.besblock.co.uk, at £14.40 + VAT per square metre for the solid 190mm blocks. That should come out at under a grand for my whole building.

Hmmm, those Wickes ones I mention work out at about £8 per sq. metre inc. vat, half the price. You may wanna do some research on the performance of 190mm blocks vs. 100mm ones otherwise you could end up paying twice as much for just a slight increase in performance. Personally I'd go for the 100mil ones, you'd probably get them cheaper than Wickes if you shop around. Not only would you save £500, but you'd also have a bigger airgap between leaves which is a good thing indeed, and you could go upto 3 or even 4 layers of plasterboard on the inner wall, at minimal extra expense. Not only that but unless your roof is also 8 inches of concrete then your wasting your money on such heavy walls. Also sinking the floor is a great idea but make sure the DPC is flawless, you will need guttering too, my garage seeps water in at one corner where a drainpipe is broken, the water runs straight down the wall and seeps in, and my floor is level with the outside. Thinking cap back on!

Just did some basic calcs on your options - keeping the total wall thickness constant, using 100mm concrete block with the extra 90mm added to the air gap, improves the low end TL by about 5 dB over the thicker block - in each case, the inner leaf was 2 layers 12.5mm gypsum. The added air gap helps the low end more than the extra thickness of concrete. So as long as your walls won't fall over being only 100mm thick, that would be your best bet. Cheaper AND better isolation... Steve (this is with 100mm block, 165mm air/insulation, 2 layers 12.5mm gyp)

What if he eliminated blocks altogether and did double-frame construction? Wouldn't the increased air gap in the stud bays offer additional benefit without loss of square footage? That's been my understanding... Though I'd welcome being proven wrong. (Wouldn't be the first time! :lol:)

knightfly wrote:

Just did some basic calcs on your options - keeping the total wall thickness constant, using 100mm concrete block with the extra 90mm added to the air gap, improves the low end TL by about 5 dB over the thicker block - in each case, the inner leaf was 2 layers 12.5mm gypsum. The added air gap helps the low end more than the extra thickness of concrete. So as long as your walls won't fall over being only 100mm thick, that would be your best bet. Cheaper AND better isolation... Steve (this is with 100mm block, 165mm air/insulation, 2 layers 12.5mm gyp)

Hey Steve, I really appreciate your doing this, though I haven't got my head round the graph yet. When you say it would lower the Low Frequency TL - what about other frequencies? Would they be much the same, or better or worse? This certainly is a major change to be contemplating (we are supposed to start work next week! Though the first week will be spent digging and I haven't ordered the blocks yet, so there's a little time). But then that's what I posted this for - if the thinner blocks would really be better, I suppose that's what I'd better do. I hate to be a pain, but any chance you could translate your graph into ordinary verbal terms to clarify for me what's going on? Thanks again. I could certainly use saving £500 or so, quite apart from the better soundproofing.

Whoa - I just had a thought. I'm pretty certain my brother-in-law said that for a building with only one heavy structural wall (ie this one, because the inner wall is just timber and plasterboard), UK building regs specify that the wall has to be at least 190mm thick. I seem to remember that was one reason why we went for the thicker blocks. You can only use 100mm blocks when building two skins of them with ties between, for stability. Now maybe we could try and get away with it, but it's probably not a good idea. My bro-in-law isn't all that experienced, and I don't really want to cut corners in accepted levels of structural strength without someone really knowledgable giving it the OK. If the Regs people have this rule, I figure there's probably a reason why. So unless anyone can see a way round this, I figure I'll just have to do without the extra 5db TL. Thanks anyway.

I realize that building techniques and products vary throughout the world... But I keep proposing dual frame construction (no blocks) because I really do believe that may be best for isolation. Would an outer frame, sheathed with OSB for stability, and then a thick layer of stucco applied to a styrofoam base be an option? Does it have to be concrete blocks? If so, then fine... But nobody has answered that question yet. --Keith :mrgreen:

Fortunately the type of construction you are thinking of has several similar designs tested by the NRC with the OITC data. OITC IS usefull for comparing music isolation. Sorry, I don't hve URL but do a google search for NRC an duse their search engine for OITC. That should get you useful results. NRC also has (literally) hundreds of test results with the TL data but only STC summary analysis. Teh needed info is still there but you have to work it out. Good luck! Andre

sharward wrote:

I realize that building techniques and products vary throughout the world... But I keep proposing dual frame construction (no blocks) because I really do believe that may be best for isolation. Would an outer frame, sheathed with OSB for stability, and then a thick layer of stucco applied to a styrofoam base be an option? Does it have to be concrete blocks? If so, then fine... But nobody has answered that question yet. --Keith :mrgreen:

Well I also live in London, and there's no way I'd build my studio with two wooden frame stud partitions, simply because some cheeky bugger will probably try and break in and steal stuff! :evil: Concrete blocks/bricks would offer much better security I would've thought. :?:

Deluks wrote:

Well I also live in London, and there's no way I'd build my studio with two wooden frame stud partitions, simply because some cheeky bugger will probably try and break in and steal stuff! :evil: Concrete blocks/bricks would offer much better security I would've thought.

Someone would, what, bust a hole in 1" of solid stucco attached to styrofoam attached to OSB? :shock: OSB is about the same strength as plywood, and stucco is a type of cement/concrete mix. Are we on the same page here? :?

Another reason for masonry outer, especially in foggy old London and thereabouts, is WATER retention - masonry can retain approximately 100 TIMES as much water as gypsum before having problems with mold, etc - review Building Science Corp's PDFs (links in the REFERENCE section) for more on this. Yet another reason - last year another UK member was looking for alternatives to gypsum - seems the stuff cost about 4 TIMES as much there as here, while masonry products were almost reasonable.... Steve pps - mass is mass; concrete is approx. 3 TIMES the density of gypsum so 4" of concrete would equal about 12" of gypsum, which would REALLY get expensive at $25 or so per sheet :cry:

I stand humbly corrected! 8) (Sitting, actually...)

Standing/sitting corrected is better than laying, rejected :wink: As to wall construction - if you build 4" outer masonry and "4x2" inner stud frame with 2 15mm gypsum, the best way to handle insulation (both for sound AND moisture) is to NOT put insulation against the masonry; instead, try to find some of that fiberglass strap material that's used to band heavy boxes and stuff together - build your inner frame on the floor, add a "4 x 1" diagonal brace (you can leave this in place across the inside edge of the frame), then fasten the fiberglass strapping (it's about 15mm wide, yellow usually) across the inside of the frame, perpendicular to the studs and parallel to the floor. I like to use roofing tacks, at least 1" long - neither Rod nor I trust staples for this. Place the straps across the studs at about 1 foot intervals, bottom to top - then stand the frame up (if you build the frame so it is 1/4" shorter than the total distance from floor to ceiling, it will just clear when tilting) - or, with new construction you may have these interior walls up before the interior ceiling frame, so no matter. (Keep in mind that you want TWO leaves of mass ALL AROUND, so the ceiling should be similar in construction to the walls in that respect.) Once your wall frames are standing, you can place fiberglass insulation batts in the stud cavities - if your space will be heated more often than cooled, any foil or paper backing goes to the inside of the space, against the inner gypsum. The fiberglass strapping across the backs of the studs keeps the fiberglass from pulling away from the inner layer of gypsum, thereby improving the damping of the wall for better isolation. If you intend to support an inner ceiling frame on the inner wall frame, be sure to double the top plates. IF your bottom plates will rest on concrete, be sure to get some building felt (bituminized paper) and place a layer or two under your bottom plate for a thermal break; keeps any sweating from the concrete away from the wood. Pressure treated lumber for the base plate wouldn't hurt, if it's available. Beyond that, most of what you need to know is in the first 2-3 threads in the REFERENCE section... Steve

Steve, I just spoke to my man who's here today to start digging. Turns out I'm wrong about the building regs. We can either use 190mm blocks, OR use 100mm block with piers every three metres at least for structural support. We had originally decided against the piers, because we're building right up to the edge of the property and didn't want to lose floor space by allowing the extra space around the building for piers. But now we've come up with the idea of putting the piers on the INSIDE of the outer leaf. This would mean we'd have a 165mm air gap, but where the piers are they'd jut into that for 100mm. We would only need one at each corner and probably one half-way along each side. My question is - how much would this affect the TL? I'm presuming that the important thing is the GENERAL distance between the two leaves, and that it's not the end of the world if that distance is shortened in just a few places (as it will be anyway, for example, by the struts for the inner leaf). Am I right? If this sounds OK, I think we'll definitely go with this design instead. Many thanks for any help with this. We need to start ordering blocks pretty soon.

You presume correctly; I doubt your TL would drop by even a dB because of the piers. Just make sure the inner wall frame doesn't actually TOUCH the outer masonry wall... Steve

knightfly wrote:

Just did some basic calcs on your options - keeping the total wall thickness constant, using 100mm concrete block with the extra 90mm added to the air gap, improves the low end TL by about 5 dB over the thicker block - in each case, the inner leaf was 2 layers 12.5mm gypsum. The added air gap helps the low end more than the extra thickness of concrete. So as long as your walls won't fall over being only 100mm thick, that would be your best bet. Cheaper AND better isolation... Steve (this is with 100mm block, 165mm air/insulation, 2 layers 12.5mm gyp)

It now looks like, due to engineering factors brought about by the split foundation we will be using, the total wall thickness will need to be 360mm instead of 300. So using this revised plan, we'd have 100mm block, 275mm air/insulation, 2 layers 12.5mm gyp. Question: The air gap is now pretty big. Does that affect the issue of what thickness blocks would be best? ie, for a total wall thickness of 360mm, would we now be better off with 190mm block, 185mm air/insulation, 2 layers 12.5 gyp? I'm not sure at what point increasing the air gap starts to give diminishing returns, and how it compares to increasing the thickness of concrete. Many thanks.