Cellar soundproofing project

Greetings, everyone. My name is David and I come from Reims/France I hope you are well under the circumstances? I've come to ask your knowledge because I'd like to turn my cellar into a studio. However, there are a few things I would like to discuss with you: 1) the dimensions are 3.6*3.5 by 2.07m high. the walls are made of cinder blocks and the floor is made of dirt. Is the height not a problem? I would like to pour a "floating" slab with peripheral insulation, for that I wanted to extract some dirt but it seems to me that the foundations are not very far from the ground level (see the isometric view of the cellar and the cellar detail view). 2) Once this difficulty of the slab had passed, I had considered to cover my walls with soundproofing plasterboard, and between the blocks and this sheet, to put rock wool (for thermal and acoustic insulation). I suppose it is necessary to do the same thing with the ceiling? On the other hand, for the slab I had considered laying parquet and possibly putting my drumset on a wooden platform to limit contact with the slab as much as possible. (see Soundproofing cellar). 3) Finally I would cover the new walls with acoustic insulation panels. I am waiting for your feedback to see if this project seems feasible to you. Don't hesitate to ask me your questions, I can also share pictures.

Hello everyone, I'm here to tell you about the progress of my project. The material that was under the footings of the foundations was in fact "clean concrete", it is a very poor concrete that is deposited on the foundation trenches so that they can be poured on a clean surface. So I was able to remove rat dirt from the foundations, on 50cm and thus gain height under the ceiling. A plastic film was placed on the freshly poured floor, as well as 40mm of extruded polystyrene and the concrete slab was poured over it. Metal pins were placed on the periphery of the slab, then acroteria were cast on these metal pins. A strip of acoustic insulation was deposited on the entire perimeter of the acroteria. (cf photo) Now I am going to tackle the insulation: I wanted to create cleats made of mdf (medium density fiberboard) and acoustic foam, i.e. 10mm mdf - 40 mm acoustic foam - 10mm mdf, all glued together. I fix all this to the walls, put polyurethane foam plates in between, and screw my (blue) acoustic plasterboard plates on top. For the ceiling and the floor, I put horizontal cleats with also a strip of PU foam to uncouple my insulation from the floor and the ceiling. What do you think about this? Do you think it's a good idea or do you have some advice for me? Thank you in advance.

David51 wrote:

Hello everyone, I'm here to tell you about the progress of my project. The material that was under the footings of the foundations was in fact "clean concrete", it is a very poor concrete that is deposited on the foundation trenches so that they can be poured on a clean surface. So I was able to remove rat dirt from the foundations, on 50cm and thus gain height under the ceiling. A plastic film was placed on the freshly poured floor, as well as 40mm of extruded polystyrene and the concrete slab was poured over it. Metal pins were placed on the periphery of the slab, then acroteria were cast on these metal pins. A strip of acoustic insulation was deposited on the entire perimeter of the acroteria. (cf photo) Now I am going to tackle the insulation: I wanted to create cleats made of mdf (medium density fiberboard) and acoustic foam, i.e. 10mm mdf - 40 mm acoustic foam - 10mm mdf, all glued together. I fix all this to the walls, put polyurethane foam plates in between, and screw my (blue) acoustic plasterboard plates on top. For the ceiling and the floor, I put horizontal cleats with also a strip of PU foam to uncouple my insulation from the floor and the ceiling. What do you think about this? Do you think it's a good idea or do you have some advice for me? Thank you in advance.

Hi, I'm glad you were able to fix your floor! I am not sure what you would like to achieve with your proposed wall assembly - you already have cinder block walls, the MDF sandwich is not likely to provide much more sound reduction, especially if you attach it to the wall directly. Acoustic foam really has no place in the isolation assembly, or acoustics for that matter! I really cannot think of a situation where I would use acoustic foam for anything in a studio, despite all the marketing. In short - your proposal is an unknown, I have no idea what it will achieve and can see a lot of potential issues with it. Tell us what you need in terms of: - sound reduction - do you even need "sound proofing"? If so, by how many decibels? - what is the purpose of the room? Control room and or tracking room? Paul

Paulus87 wrote:

- 1) sound reduction - do you even need "sound proofing"? If so, by how many decibels? - 2) what is the purpose of the room? Control room and or tracking room? Paul

Hello Paul, First of all, thank you for taking the time to read and answer my topic. 1) This 12.5m² room is intended to become my rehearsal room for my drum practice. So I didn't push the study in order to determine the exact amount of decibels to reduce, but I assume that who can do the most can do the least, so the maximum will be the best (of course within a reasonable budget, I'm not going to oversize either). 2) my fears: for the "high frequencies" (especially cymbals and snare drum) I don't worry too much since I'm under the floor, for the "low frequencies", and especially the bass drum, I'm very afraid of solidary transmissions, especially since one of the walls is common with my neighbor's garage. To give an example, when I would pick or ground during the work, you would hear/"feel" them all over the house as if you were in the basement.

I see. Well unfortunately the only way to reach that level of isolation down to such low frequencies takes up considerable space. You could either build a new set of walls and ceiling inside your current space from concrete blocks/block and beam or a traditional timber frame with drywall/plywood/osb etc layers. Whichever method you choose you’ll need to make sure the new inner shell is not touching the existing space in anyway other than the floor. The bigger the gap between the existing shell and the new shell which you are yet to build the better isolation you’ll achieve, to be precise, as the gap gets bigger the lower down the frequency spectrum you’ll be able to Isolate down to. This gap must be filled with cheap low density insulation. You should expect to lose between 6-12” all the way round the perimeter and ceiling of your current space. If you are only going to have a drum kit in there then you may still have enough space, it will be quite a violent sounding room though due to the proximity of the boundaries to the drums, but it would be fairly quiet outside of the room. By applying mass and rigid insulation and acoustic foam etc directly to the walls without any decoupling will only get you so far - since there will still be a physical connection to the rest of the house. You can expect to deaden some high end and high mids, but the low end will just transfer straight through like a knife through butter, and low end is your main issue. Paul

I made a diagram like this no ambiguities I thought I would place my cleats against the wall, vertically spaced 600mm apart, then up and down with between floor and ceiling and the two horizontal cleats, a strip of PU foam (to uncouple). Between each slat: PU foam (by the way, I saw that some of them didn't cover all the space between the slats as on the left picture, is this a subtlety?). After soundproofing plasterboard (blue) and above all still foams. (and of course I repeat the operation above and below for ceiling and floor). What do you think about it? I see this kind of process on many sites specialized in sound insulation. (ps: I've zoomed in on my cleats: mdf-insulating foam-mdf)

Greetings, As Paul noted:

Acoustic foam really has no place in the isolation assembly, or acoustics for that matter! I really cannot think of a situation where I would use acoustic foam for anything in a studio, despite all the marketing.

Likewise, killing low frequencies (don't forget, your snare is actually a lower frequency than you realize; think about how you can "feel" a solid rimshot) is going to be your main objective (I'm a drummer too). Paul's suggestion of how to proceed is your best bet, although your space will become smaller. Methinks it will also be less expensive than your own proposal (it's also much less fussy construction). All the best, Paul (a different one)

David51 wrote:

I made a diagram like this no ambiguities I thought I would place my cleats against the wall, vertically spaced 600mm apart, then up and down with between floor and ceiling and the two horizontal cleats, a strip of PU foam (to uncouple). Between each slat: PU foam (by the way, I saw that some of them didn't cover all the space between the slats as on the left picture, is this a subtlety?). After soundproofing plasterboard (blue) and above all still foams. (and of course I repeat the operation above and below for ceiling and floor). What do you think about it? I see this kind of process on many sites specialized in sound insulation. (ps: I've zoomed in on my cleats: mdf-insulating foam-mdf)

I do not have experience with the assembly you are proposing, so I really cannot comment on how effective it will be. But since you asked for my thoughts: Despite the use of PU foam it will not completely decouple at all frequencies, it will probably only be effective at decoupling in a very narrow range. I just don't think it will work the way you hope it will, despite what you may have seen on the internet. The sound block plasterboard (blue) is really not necessary, and likely a lot more expensive than 2 layers of standard plasterboard. All that matters is surface density, whichever has the most surface density for the lowest price will be the superior product in terms of sound reduction. I think you will spend a lot of money and effort trying to do this and you may get average results. That might be enough for you, in which case that's great. But it is definitely not going to isolate low frequency as much as a fully decoupled double leaf assembly with suitable cavity. Paul

Decoupling the floor from Kick Drum transmission to the walls and underneath to next door would be difficult. Why not decouple the Kick drum from the floor. A floating drum platform. The principles of wall and ceiling and indeed any sound proofing are well known and published. There are many guides as to how to achieve them. I don't know of anybody using foam. The air gap of any Mass Air Mass sound stopping system is best filled with light fibre, touching all around but not compressed. Support it with wires or whatever to prevent it sagging downwards over the years.

You're talking about building new walls that at no time, except for the ground, would be in contact with the existing cinder block (or concrete) walls, I get it. But if you look carefully, that's what I intend to do, at no time will my walls come into contact with the cinder block walls, except on the fixing cleats which will be prepared to transmit the minimum vibrations. 1) What you would advise me to do is to put partitions as far away from my walls as possible, but in this case how do I fix them? with floor and ceiling brackets? 2) Also the polyurethane foam is not appropriate according to you, you talk about low density materials, what is the best material according to you (I've heard a lot about rock wool). 3) Another solution exists and less expensive than the phonic plasterboard (which is a high density placo) is to double the thickness of classic plasterboard and put it in quincunx, what do you think of that. I read everywhere that it had a better result for less money. Ps: DanDan, I've done a lot of research on the subject during my research, and almost all the cellars are prepared according to the process I've explained to you. If you are interested I can pass you the links of the websites with example?

The easiest way to explain this: Frame out new walls with a bottom plate (or sill, depending on your terminology, same thing), studs, and top plate, and joists for the inner ceiling. The bottom plate is affixed to the concrete exactly as if you were building a house, nothing fancy. Your new inner walls and inner ceiling will be completely decoupled from the existing outer structure. Plasterboard the inside of the new wall and put cheap fluffy insulation in the cavity between the existing (now, outer) wall and the new inner wall. This is by far the best and most cost effective method. As mentioned earlier, the internet is full of costly, ineffective methods for studio construction, especially "soundproofing." All the best, Paul

As Paul said, but if I may recommend a detail. I would insert a layer of Neoprene under the floor stud plate and above the ceiling stud plate. Glue them in place. No nails or screws to damage the isolation. The best fibre to fill the air gap is the cheapest! There is no acoustic benefit to dense expensive batts. Isover have tested and shown that a full fill of light fibre (Partition Roll/Blanket) works best. It should touch the sides to damp vibration in the plasterboard, but not be compressed. Make sure it cannot sag downwards over the years by weaving some light wire or string through it. There are some heavier plasterboards which have much higher Transmission Loss numbers. Typical plasterboard is 34Rw, somebody, Knauf? has a 40dB one. Two layers of regular plasterboard will also do 40dB so do the Arithmetic re cost.

DanDan wrote:

As Paul said, but if I may recommend a detail. I would insert a layer of Neoprene under the floor stud plate and above the ceiling stud plate. Glue them in place. No nails or screws to damage the isolation.

Indeed this is the question I was going to ask. Thank you for answering it spontaneously :). On the other hand, you talk about gluing the insulation layer between the battens that will fix the frame, and the floor and ceiling, but to fix this battens + insulation on the floor and the ceilling, you also plan to glue them? I'd rather put dowels in my cinder block walls and screw the assembly in place. I'm afraid that glue with time and successive vibrations will not be as effective :/ My second question concerns the floor, would it not be relevant to do the same principle? For example, I square my concrete slab with a wooden reinforcement on which I fix my floor (same for the ceiling)? Or is it oversized and I can directly put a parquet floor for example and the vibrations transmitted by the solid state will be attenuated in spite of everything?

David51 wrote:

DanDan wrote:

As Paul said, but if I may recommend a detail. I would insert a layer of Neoprene under the floor stud plate and above the ceiling stud plate. Glue them in place. No nails or screws to damage the isolation.

Indeed this is the question I was going to ask. Thank you for answering it spontaneously :). On the other hand, you talk about gluing the insulation layer between the battens that will fix the frame, and the floor and ceiling, but to fix this battens + insulation on the floor and the ceilling, you also plan to glue them? I'd rather put dowels in my cinder block walls and screw the assembly in place. I'm afraid that glue with time and successive vibrations will not be as effective :/ My second question concerns the floor, would it not be relevant to do the same principle? For example, I square my concrete slab with a wooden reinforcement on which I fix my floor (same for the ceiling)? Or is it oversized and I can directly put a parquet floor for example and the vibrations transmitted by the solid state will be attenuated in spite of everything?

The neoprene on the bottom and top plates is a good idea but not for decoupling purposes in my opinion, though that will help a little in that regard. The fact is, you should bolt the sole plates down into the concrete anyway which will negate any decoupling achieved via the neoprene. But, the main reasons why I would use it: It helps to ensure a good air tight seal between the sole plate and the floor by filling in any irregularities in the concrete and timber. The perimeter will still need to be caulked all the way round to completely seal it. I personally don't think it's necessary to use neoprene on the top plates, as again your new ceiling joists will be resting directly on top of those plates and nailed/screwed down into them. Dowels directly into the cinder block walls to hold your new frame? Well, there would be absolutely no point building a new decoupled frame at all if you were going to then destroy that decoupling by making a physical connection to the cinder blocks again. Here's what you need to remember: your current walls and ceiling are one leaf (outer leaf) and the new timber structure that we are advising you to build is also one leaf (inner leaf). They need to remain completely separate, free standing and unconnected. No physical connection between them at all. The only place they will be touching is the ground, but that's okay since the ground in your case is an infinitely damped and massive concrete slab on grade. For your floor, is the concrete slab insulated? was a damp proof membrane installed before the concrete was poured? If yes, then you can simply put an underlayment down and put your parquet floor directly on top. Before you build the new inner leaf (your timber frame and plasterboard) you should seal up any and all the gaps/cracks in your current cinder block walls/ceiling. You can just do this with a non hardening caulk. If the blocks are somewhat porous, then you can also paint them to seal up any tiny gaps. One thing we haven't spoken about yet is air conditioning and ventilation, which you will absolutely need. Which is one of the only times you will make penetrations in your inner and outer leaves, but if designed and executed correctly it will not be a problem. Paul

I took the liberty of making another drawing to see if we agree. Small precision: the structure, I didn't have fun making the floor cleats, then the vertical cleats, then the cleats on which the ceiling will be fixed and finally the plasterboard. I just made a box by uncoupling what needs to be uncoupled so that we can make a final decision on the concept. Next: 1) For your floor, is the concrete slab insulated? was a damp proof membrane installed before the concrete was poured? If yes, then you can simply put an underlayment down and put your parquet floor directly on top. Before pouring the concrete slab, we actually laid a plastic sheet to prevent rising damp, then a 40 mm layer of extruded polystyrene and finally 80/100 mm of fiber concrete. The acroters are attached to the slab, but insulated from the concrete footings by a layer of insulating material about 5mm thick. My slab is therefore insulated underneath and on the periphery. 2) One thing we haven't spoken about yet is air conditioning and ventilation, which you will absolutely need. Which is one of the only times you will make penetrations in your inner and outer leaves, but if designed and executed correctly it will not be a problem. In my basement, I have an opening in the cinder block wall overlooking my garden. Above the basement there is a brick column, when I opened it the other day and put my hand in it I feel a draught going upwards, I think this column was originally intended for the old boiler, a pipe that was supposed to go up to the roof level. And by drilling my concrete slab (the ceiling of the basement) I can give onto this "air column, empty". I had left to install a vmc, but in the end I think that a natural ventilation could be possible, so a pipe that would open in this wooden construction and that would go in this column by crossing the upper slab, and a pipe that comes from the cut in the cinder block wall and that opens in my wooden structure. and moreover these two "arrival and exit" are facing each other. What do you think about it? 3) Also, my access to the cellar is through a trap door (see drawing) and I am adapting the old staircase to reuse it. It is absolutely necessary that the staircase opens in the cellar, then I open a door to enter in my wooden structure, or I leave a cutout in the wooden structure where the staircase will open, and of course the trap door will be closed at the top. In the meantime, thank you all for the time you spend on my project, it's very nice.

Greetings,

The fact is, you should bolt the sole plates down into the concrete

This. You should run a bead of non-hardening caulk on the plates before affixing them to the concrete, then as Paul suggested, seal at the edges.

In my basement, I have an opening in the cinder block wall overlooking my garden. Above the basement there is a brick column, when I opened it the other day and put my hand in it I feel a draught going upwards, I think this column was originally intended for the old boiler, a pipe that was supposed to go up to the roof level. And by drilling my concrete slab (the ceiling of the basement) I can give onto this "air column, empty". I had left to install a vmc, but in the end I think that a natural ventilation could be possible, so a pipe that would open in this wooden construction and that would go in this column by crossing the upper slab, and a pipe that comes from the cut in the cinder block wall and that opens in my wooden structure. and moreover these two "arrival and exit" are facing each other. What do you think about it?

Along with the air, sound will travel in and out. You will defeat the isolation without silencers/baffles. Also, this does nothing to address humidity levels. Your gear lives best in spaces with constant humidity (40-45% ideally). Also, when you're working, that sort of humidity level is very comfortable to be in for long periods. Paul and DanDan have given you great advice. Follow it and you will have a good space. All the best, Paul

Yes, don't worry, I take your advice into consideration, so much so that I have abandoned my basic project to turn to the one recommended here. But before I start, given the little space I have, and the not unlimited means, I really want to be 100% sure. On this subject, there are still three last points on which I would like to have your opinion, and I think I'll get started: 1) The vertical uprights on which the walls are scoured, we agree that at the bottom they are fixed to cleats screwed in the floating concrete slab, but at the top, I also fix them to cleats fixed on my concrete ceiling, or I fix them to a kind of frame detached from the ceiling (I think it's more logical no, but the rigidity of the whole doesn't seem to be very good, hence my question)? (cf the two pictures 1 and 2) 2) For the cleats screwed to the floor on the slab, do I leave a space with the acroteria (5cm type), or can I glue them to the acroteria (let's say I screw them 1cm from the acroteria so that they are not in direct contact), knowing that they are insulated from my foundations by an insulating material? (see the two images 3 and 4). Because the more I move them away, the more I lose in surface.... 3) Finally, for my staircase, it doesn't matter if there is a cut-out in my stall for access, and I just insulate the access hatch? because I won't be able to have a door in my stall otherwise I lose a monstrous surface area. Thank you in advance for your answers.

A bit late now but anyway, what is a Cleat? Acroteria? I am not sold on the idea of bolting the floor plate to the concrete. Given modern glues why diminish the isolation? If there is a substantial risk of movement there ways other than penetration and contact surely?

David51 wrote:

Yes, don't worry, I take your advice into consideration, so much so that I have abandoned my basic project to turn to the one recommended here. But before I start, given the little space I have, and the not unlimited means, I really want to be 100% sure. On this subject, there are still three last points on which I would like to have your opinion, and I think I'll get started: 1) The vertical uprights on which the walls are scoured, we agree that at the bottom they are fixed to cleats screwed in the floating concrete slab, but at the top, I also fix them to cleats fixed on my concrete ceiling, or I fix them to a kind of frame detached from the ceiling (I think it's more logical no, but the rigidity of the whole doesn't seem to be very good, hence my question)? (cf the two pictures 1 and 2) 2) For the cleats screwed to the floor on the slab, do I leave a space with the acroteria (5cm type), or can I glue them to the acroteria (let's say I screw them 1cm from the acroteria so that they are not in direct contact), knowing that they are insulated from my foundations by an insulating material? (see the two images 3 and 4). Because the more I move them away, the more I lose in surface.... 3) Finally, for my staircase, it doesn't matter if there is a cut-out in my stall for access, and I just insulate the access hatch? because I won't be able to have a door in my stall otherwise I lose a monstrous surface area. Thank you in advance for your answers.

The new ceiling timbers (joists) will rest on your new walls top plates, ideally exactly above the upright timbers (studs). Screw/nail them or use joist hangers. Make sure there is a gap between the new ceiling and your concrete ceiling. As big a gap as you are willing to lose. Same for your new walls. You could also build the whole thing inside out, which would mean your new framing will have the plasterboard on the outside, rather than the inside. Then you can use the open spaces between the timbers for acoustic treatment. This will save space, but decrease your isolation... swings and roundabouts. I do not understand what you mean when you say "For the cleats screwed to the floor on the slab, do I leave a space with the acroteria (5cm type), or can I glue them to the acroteria (let's say I screw them 1cm from the acroteria so that they are not in direct contact), knowing that they are insulated from my foundations by an insulating material? (see the two images 3 and 4). Because the more I move them away, the more I lose in surface...." I don't think you mean an acroteria... that is an ornamental piece placed on a plinth of monuments or important buildings. Perhaps you mean a plinth? A concrete "lip" ? and why are cleats needed? Whatever it is you mean, just keep it simple: the bottom plates of the new walls just need to be bolted straight down into your concrete floor. Use caulk/rubber under them before you bolt them down and then seal along the edges with caulk. You may have used some insulation between the old concrete and the walls and the new concrete, but it is not decoupling the new concrete floor, just treat your existing concrete walls and the whole floor as one single connected shell. As I said, it does not matter that the new walls are built on top of your concrete slab because the concrete slab is very very dense and damped by the earth. Regarding the stair case and the hatch... what do you mean by stall? Je parles un petit peu français, mais je pense que ce sera plus difficile pour moi de comprendre le vocabulaire en français, qu'en anglais, desolé! Paul

DanDan wrote:

A bit late now but anyway, what is a Cleat? Acroteria? I am not sold on the idea of bolting the floor plate to the concrete. Given modern glues why diminish the isolation? If there is a substantial risk of movement there ways other than penetration and contact surely?

Dan, given that it's a fairly small space it could work... but I personally would rather not risk it and bolt it. I am not aware of the building regs on this but I would be surprised if it would pass regs by just being glued and not bolted. Also, really what difference is it going to make in terms of isolation? If he glues it, it will still be in contact with the slab. The slab is far from his weakest point in the isolation envelope... the concrete is thick and dense and damped by the earth... hard to beat. It is rarely ever needed to float the floor and if a floated floor is needed then it needs to be done properly (expensive, plus a mathematical nightmare) with proper springs/resilient devices to even be effective. Either do it properly or not at all. The two leaves connected at the concrete base is the least of his worries in my opinion. Paul

DOH, in my mind I still had the Neoprene between Floor plate and concrete. Also between ceiling plate and whatever is above. So plate (is that the right term) glued to neoprene glued to concrete. Given a Kick drum I would think any extra bit of isolation from from the floor would be worth availing of.

DanDan wrote:

DOH, in my mind I still had the Neoprene between Floor plate and concrete. Also between ceiling plate and whatever is above. So plate (is that the right term) glued to neoprene glued to concrete. Given a Kick drum I would think any extra bit of isolation from from the floor would be worth availing of.

I think in order for the neoprene to provide decoupling it would need to be calculated to provide damping at least an octave below whatever frequency occurs in the room when the kick drum is struck, as well as the force from the impact. A bit like trying to calculate sorbothane hemispheres for loudspeaker decoupling - too high and it does nothing, too low and it makes things worse. I don't know about you, but that's way out of my league in terms of mathematical knowledge. What the neoprene will do, though, is make a nice seal and help to fill in any imperfections in the slab and sole plates. I'm still not keen on the idea of only glueing all the walls to neoprene though. Paul

DanDan, the 4 acroteria constitute the concrete belt that rests on my new slab and which is joined to it via metal reinforcements. Their function is to consolidate the foundations since I had to dig under the underside of the foundations. Between these foundations and the acroteria there is a strip of insulating material. Paul, 1) for the new ceiling I understood perfectly thank you ;). 2) as explained above, the acroterias are the concrete belt that reinforce the foundations. This belt and my new slab are one and the slab is insulated underneath with polystyrene, and around the edges, at the level of the acroteria so that they are not in direct contact with the foundations, by an insulating material. I perfectly understood that my cinder block walls and my old floor (under the slab) must be considered as a whole, and my slab and my box as another whole. My only question is: so, since the acroteria and the slab are one, can I glue as much as possible my lower slabs to the acroteria, or even on top of them, which would allow me to gain a significant surface area (I think you will answer positively because as said above: acroteria + slab = 1). 3) currently I have in my concrete ceiling a cut-out for the passage of the staircase leading to my cellar. If I want to access my new stall, I would also have to make a cutout in the new ceiling which will therefore remain open at all times. Only the initial cellar hatch will allow me to close my cellar (and not a second door to the stall). Is the hole in the stall disturbing to you? David

David51 wrote:

DanDan, the 4 acroteria constitute the concrete belt that rests on my new slab and which is joined to it via metal reinforcements. Their function is to consolidate the foundations since I had to dig under the underside of the foundations. Between these foundations and the acroteria there is a strip of insulating material. Paul, 1) for the new ceiling I understood perfectly thank you ;). 2) as explained above, the acroterias are the concrete belt that reinforce the foundations. This belt and my new slab are one and the slab is insulated underneath with polystyrene, and around the edges, at the level of the acroteria so that they are not in direct contact with the foundations, by an insulating material. I perfectly understood that my cinder block walls and my old floor (under the slab) must be considered as a whole, and my slab and my box as another whole. My only question is: so, since the acroteria and the slab are one, can I glue as much as possible my lower slabs to the acroteria, or even on top of them, which would allow me to gain a significant surface area (I think you will answer positively because as said above: acroteria + slab = 1). 3) currently I have in my concrete ceiling a cut-out for the passage of the staircase leading to my cellar. If I want to access my new stall, I would also have to make a cutout in the new ceiling which will therefore remain open at all times. Only the initial cellar hatch will allow me to close my cellar (and not a second door to the stall). Is the hole in the stall disturbing to you? David

Okay understood, yes you can attach the new walls to whichever part of the slab you want. Just space them out from the existing walls at the maximum distance you are allowing. Regarding the hatch, the stair case folds down into the hatch and then you can fold it back up once you are inside? If so, is it not possible to build a second hatch attached to your new ceiling which remains open when you are not in the room, then once you are inside the room you can fold the stairs away and close the second hatch? Sorry if I have completely misunderstood Paul

Hello everyone, sorry for the late answer, unfortunately I didn't have the possibility to access my computer this weekend. Unfortunately no, the staircase is a wooden staircase, "fixed" (not retractable). And moreover you will see thanks to the drawing below, that it falls right into the famous acroteria (I can fix that by making a quarter turn on the old staircase). I think I'm going to be condemned to scrap a big space, and put a door in my new box... Unless you still have some good advice to share with me.