Distance between house walls and room-within-a-room walls

Hi, I've posted a description of my project in the thread called "Advice sought: should I build a new concrete floor?" I'm not sure what the protocol is for asking a new question: should I put the whole summary of the project in the new post , or refer to it by name, or what? New question: I noticed from the small_studio.skp and and the components.skp (which I found incredibly useful, but the way!) that the inner walls touch, or are very close to, the outside walls. I was under the impression that there needed to be a gap between the two, or that otherwise that would behave as a three-leaf system. I'm obviously missing something here, but what (if any) gap should there be, and should it be filled with anything? Many thanks! - martin

The "spring" needed between two layers, to create seperate leafs, also include the frame the layers are attached to. Minimum advised space is one inch. As long as your frame is freestanding (aka not touching the exterior wall) you'll be save.

Thanks for the reply. So, just to make sure I understand fully, can I have something like this?: - cavity wall (house wall) - 1 inch of air (should I fill it with rockwool or something?) - plasterboard - plasterboard - 100mm frame - plasterboard - plasterboard Cheers, - martin

- cavity wall (house wall) - 1 inch of air (should I fill it with rockwool or something?) - plasterboard - plasterboard - 100mm frame - plasterboard - plasterboard

No! That would make a four-leaf wall! Bad idea. For best isolation at low cost, you need to have two-leaves, and ONLY two leaves. Never three, never four, never one. You say you have an existing cavity wall: I guess that means a stud frame with drywall on both sides? If so, then you need to take the drywall off the side that faces your future studio, and use that drywall to "beef up" the other side of the cavity wall, from the inside, between the studs. Just cut the pieces carefully to the exact shape that fits perfectly, with a small gap around the edge for sealing. Do two layers of this "beef", and make sure to seal / caulk every possible gap on that wall: it needs to be totally airtight. When you complete that job, you will have one leaf. You will have a beefed-up wall with studs facing you. Now you need to build your second leaf. Leave an inch gap between the existing stud frame and the new stud frame. Then fill the entire space inside and between the frames with fiberglass or rock wool insulation. Yes, the entire space. Contrary to popular belief, you DO need to fill the entire airspace for maximum isolation. Just take care that you do NOT pack it in tightly, as that would bridge the walls, and couple them, which is bad: all you need is just enough insulation to just fill the space between the leaves, or maybe leave a slight gap. Better to have a little air gap, then to over-stuff it. At this stage you have the original wall with gypsum board on only one side, plus your new frame an inch away, plus the entire gaps filled with insulation. Now you attach your two (or more) layers of gypsum board to the new frame. That is your second leaf. Take care to seal the gaps between the edges of the sheets of gypsum board carefully, and stagger the panels between layers, so that no joints line up. Once again, you need to make this totally airtight. That is a two leaf wall. That's the best way to isolate at low cost. - Stuart -

Wow, thanks for the very elaborate answers. I'm very impressed. I wasn't as clear as I should have been; I'm sorry about that. In future I will try to be more thoroughly descriptive. My outside walls are the walls of the house; as far as I can tell they are two layers of brick separated by a gap filled with insulation (probably an inch or two); the walls themselves are 280mm thick in total. Those were built in the 1930s so there is no drywall (if I understand correctly that drywall is the same as plasterboard.) There is external plaster render, and inside plaster render on the inside too. So I'm a bit confused about the leaf business. Does my cavity wall count as a leaf? Or two? Cheers, - martin

(if I understand correctly that drywall is the same as plasterboard.)

Yup! also called gypsum board, sheetrock, and a few other things.

So I'm a bit confused about the leaf business. Does my cavity wall count as a leaf? Or two?

That's a really good question! It all depends on how it is built internally: If there are metal ties bridging the two sides of the cavity, or mortar that overflowed and dripped to form bridges, then it probably only counts as one leaf. But if there are two entirely separate walls with nothing bridging them at all, anywhere, then it probably counts as two leaves. Of course, the only way you can tell for sure, is to look inside the cavity between the two walls, which is probably pretty hard to do! :) But my guess would be that it is probably only one leaf, since bricklayers and builders are notoriously sloppy about what goes inside a cavity wall. Most likely there are bits of broken brick, bits of wood, waste mortar, sundry chunks of unrecognizable rubble, an odd bit of pipe, maybe even a kitchen sink or two... So if you can look inside the wall somehow (X-ray vision?) then you can figure it out for sure. If not, then I think I'd assume it is only one leaf. But either way, two layers of brick is a pretty substantial amount of mass, and that is GOOD! You already have a very decent amount of isolation, just from that, so even if it is already two leaves and your inner leaf makes a third leaf, I doubt it will be a big issue. The way I see it, the huge mass of two brick walls goes a long way to compensating for the decreased transmission loss caused by a third leaf. I reckon a stud frame with a couple of layers or drywall on one side of it will be all that you need for your inner leaf. That plus the brick should give you pretty good isolation, especially considering your isolated slab floor! Man, some guys have all the luck! Two layers of brick, plastered and rendered on BOTH sides, insulation in between, AND ALSO a decoupled isolated slab on grade! Sheesh! That's the stuff that studio-builder's dreams are made of! Take some advice, and go buy a few lottery tickets while your luck holds out... (If you win, send me half! :) )

I'm not sure what the protocol is for asking a new question: should I put the whole summary of the project in the new post , or refer to it by name, or what?

Better to keep it all in one thread, your design thread, so folks can just scan back and forth through one thread, rather than have to open half a dozen threads to keep track of what you are doing. - Stuart -

Thanks Stuart, That was, once again, a brilliant answer, which gives me lots to think about. I can actually see in between the walls in one place (actually where the wall is open between the CR and LR.) All I can say for sure is that there's some form of foam insulation in there. But who knows what else is there; I think your suggestion to assume it's one leaf is a good one. I've drawn a new plan of potential layout including the CR door and room leading into the WC. I've tentatively redrawn everything assuming a frame fastened to the outer wall and gypsum on the inside. Just for the plan I made that a total thickness of 150mm. I've ordered Rod Gervais' book. I can't wait, I'm sure that it will help me a lot (but I'm guessing that it will throw up even more questions!) I've calculated the dimensions, and ballpark I get h=2.6m, w=4.05m l= 5.04m Which gives me actual ratios of 1, 1.56, 1.94 which I suppose is not too bad. Anyway, once again, thanks a bunch Stuart. - martin

Hey Martin. Thanks for the kind words!

I've tentatively redrawn everything assuming a frame fastened to the outer wall...

For maximum isolation, the frame should be free-standing, a couple of inches away from the wall, and not attached to the wall, or even touching it. If you attach the frame to the wall, then you will create a flanking path for sound to get all the way through the all very easily. The idea is that there should be no mechanical connection at all between outer leaf and inner leaf. Anything solid will "bridge" the gap acoustically, and reduce isolation.

I've ordered Rod Gervais' book.

That's probably the smartest thing you did so far for the entire project! Seriously, it's a great book, very clearly written, and very to-the-point. I'm still curious about the angled walls at the rear of your room, next to the mini-bathroom. Wouldn't it be better to just take that wall straight across the rear of the room? That would give you more space in the bathroom, and would be a lot better (I think) and easier to build. It would also allow you to get those two doors on those rear walls combined into one, which can go in the middle of the rear wall (where it should be). Those two doors are taking up the area where you should build large bass traps. If you re-arrange things back there with a straight wall and the door in the middle, that would give solve a number of issues. Also, don't forget that each door needs to be very massive, therefore very heavy, therefore very expensive! If you can figure out a way to get rid of a door, that will save you hundreds of dollars, and many hours of painstaking construction. - Stuart -

Thanks Stuart for, once again, a brilliant reply. I had started the layout design with a straight wall, but then I thought the angled walls might help avoid parallel walls everywhere and lengthen slightly the CR. I'm perfectly happy to put it back to a single straight wall, but I'll have to do some hard thinking to reduce the number of doors. Unless we get in the CR through the WC (not a very nice idea) then it's either: 1) door between corridor and CR + door between CR and WC 2) door between corridor and CR + separate entrance elsewhere to the WC. I don't think number 2 is very good, not least because it means coming out of the studio and going into the house to go to the look. It's unfortunate that UK law forbids washing machines and dryers in bathrooms. I'm not quite sure why that is, and all the time I've lived in Canada and the US, I've never even heard or anyone who was electrocuted because they had a washing machine in their bathroom. But rules are rules, so not a lot of choice. I'll keep thinking and I'll try to post a new sketchup plan tomorrow. About the walls: I now understand better what you're saying. So conceptually speaking, the older plan (where I had a free-standing box) is better, right? So if I understand right, the layers would now be something like: (out to in) - brick wall (effectively my outer leaf) - gap - frame - drywall Then on the bathroom side: (in to out) - drywall - frame - gap - frame - drywall Am I getting the hand of this? Many thanks, - martin