Cellar conversion help sought and sound proofing


Dec 16, 2002
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I have been thinking about getting my cellar converted for use as a home cinema for sometime now and need some help and guidance on the matter of converting it. I can get a local company to tank and plasterboard the cellar and actually convert it into a dry, warm and useable room, it is relatively dry anyway.
It is the sound proofing and room dimensions i need help with.
Please see picture attached, the red line is in front of the wall that will need soundproofing and also the ceiling as this is just floorboards to the lounge. The three thick black walls go to the ground and are not attached to neighbours so i assume this will not need any form of sound proofing.
What material do i need to use for soundproofing the wall and how do i go about that. Also how do i soundproof the floor boards, can i just glue or staple rock wool (is it) to them. The gap between the joists varies between four and six inch, is this wide enough, there is plenty of depth in the joists.
Finally the room once complete will be 11ft x 11ft approx and just over 6ft high, will this be a problem with my pj etc.
Finally finally has anyone converted a cellar into a cinema and was it worth doing as this will cost me several thousand once done, Thanks Inza


  • cellar.jpg
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Does it need to be tanked ?
Flimber, according to the damp proof person then yes and to be fair although the 'main' room is quite dry there is a small puddle at the bottom of the steps into the cellar :(
There are some DIY guides online for tanking a cellar, that might keep costs down if it's a worry. Else, it might cost you a fair ol' sum ;)

Good luck with it and don't you dare do a thing without a full account in jpegs posted for all to admire :)

One cost effective method of soundproofing a wall is to build a wall one inch in front of the existing wall using 4" x 2"s. Fill it with Rockwool insulation (high density slabs may be more effective though more expensive - Wickes sell them), and then fit two layers of half inch plasterboard (or one half inch, and one 9/16ths).

Seal all edges with silicon. Where air can go, so can sound, so make sure there are no gaps.

For the dorway, you can probably make a second door in the new frame to act as a double system similar to those in real cinemas. Use a heavy duty door (there was a thread regarding this not too long ago) to help stop the transmission of sound, and replace the existing door with another hd one.

Try the search for some recent threads which covered the issue of romm soundproofing.


Gary, thanks for that i did see the post about the door which i thought a very good idea. Will also start to plan the wall with what you have said below and price some slabs from wickes, also do you have any recommendations for the ceiling as currently this is just floor boards to the lounge. Can i staple or glue rock wool to that and would it be sound absorbant enough?
Flimber, will do some jpegs once i get the project off the ground, i might get involved in some diy just yet but living in a victorian house for the last year and a half i wouldnt mind a rest from all the diy i have done to date.
What do you have for the ceiling - just joists with floorboards on top? If so, then you could use some Rockwool slabs and cut them so that they push fit between the joists (use the normal 65mm and the 30mm high density ones, or two/three layers of hd slabs), then fit a layer (or preferably two) of plasterboard so that you have a proper ceiling. Seal all edges again.

That would help a fair bit towards reducing the sound traveling upwards. Using silicon sealant between the floorboards would also be a good idea if you have the patience to do it. :)

I used no more nails when fitting the two layers of plasterboard together when I did my loft walls, as that helps stop any resonance, and makes the wall more rigid (a flexible wall can be too bass absorbant and reduce the bass in the room). You could do the same with the ceiling if you use two layers there too.

Gary thanks for that the joist are about 175mm so have planty of space to fill with rock wool, could fit 2 by 65mm and 1 by 30mm to help give sound proof. I have filled or covered a lot of the floor board gaps to stop the draft from the cellar as i sanded the floorboards back in Sept so that is that problem solved.
The only real problem then, i think, that i can forsee is the celing hight as with one layer of plaster board this would be just over 6ft, which is fine for me as i am only 5ft 7, its just whether the screen and pj would sit properly. I do have an old screen fram i made so i will put that in the cellar to get a feel of how the screen would sit
My loft is only 7ft, so my screen sits roughly in the middle of the wall, evenly (ish) spaced between floor and ceiling, and that works fine for me - even with the HT1000 which has a large offset. The screen is approx 18ins from the floor, and is 47.25ins high (84ins wide). Pics on my website if you're interested.

Hopefully you won't have any problems, even with a small ceiling. People will be seated when watching a movie, so you'll just have to have an illuminated 'Mind Your Head' sign to hand when the movie finishes. :)

inzaman - excuse the newbie question but what did you fill the floorboard gaps with?
I've sanded back most of our floors but I've still got wind whistling up from below and sound going back down :)

Not to mention the number of screws, nails and other fiddly bits I've dropped :blush:
Have a read on spectre stuff on this and his learning on his cellar, makes interesting reading
Amc do you have a cellar underneath the sanded floorboards?

Beekeeper, do you have a link, cheers Inza
I've got a basement with a plasterboard ceiling but the recessed downlighters 'leak' into the room above when they're on.
I have sanded floors through the house and it's stopping wind and sound travelling upstairs to the baby's room that I'm particularly interested in.
The builder we bought the house from sanded the lounge and dining room and filled the gaps between the boards with 'something'. I've sanded everything else but I'm left with gaps everywhere. I was hoping there was a suitable sealant to squirt into the gaps but I don't want to use the wrong stuff and end up with the floorboards welded together or something that peels and cracks after a few months.
Sorry I'll let you get back to your cellar now...:)
If the boards are clean and dust free, Silicon rubber will seal nicely,and it acts like a semi permanent glue. You should be able to remove it later with a sharp knife if you have to. Just make sure you don't go too deep with it because it can be quite tough sometimes when thick.

AMc as Gary has said really its just that if you can get to your floorboards from the cellar the sealant will not be as prominent in the room and you do not have to be as careful.

Gary on your website is that rockwool attached to your main wall. i.e once my cellar is tanked and battoned, the wall that i need soundproofing can i just stick rockwool between the battons then layer it with plasterboard, thanks inza
Hi Inza,

Yes - on one wall, I used the 30mm high density slabs which I fitted betwen the studding. The studding wasn't touching the brick wall and there was asmall gap between them. I also filled the gaps with silicon rubber - that's the brown stuff you can see - it was on special offer, so I bought two boxes. :)

I was concerned that the more flexible wall would reduce in room bass, and I think it may have a little to be honest. I was concerned that to much bass would affect the neighbours or ratle my roof tiles, so I can live with the reduction if it exists. I've augmented my bass with some bassshakers in the seats, so bass volume isn't a necesity as the tactile effect enhances it anyway.

I've another sub in my lounge, and I keep wondering what it'll sound like with that up there too, so I may try it sometime.

My first layer of plasterboard was horizontal for easyness. Both being vertical with the joins of one layer being in the middle of the next layer of board is better though, as it cuts down the chances of noise leakage through gaps which might line up. I have gaps crossing which isn't ideal, but I sealed them with SR and used no more nails to bond the two layers together for a more rigid wall. It reduces the risk of resonae too apparently.

I would have used 4 x 2 etc but being in a loft at the hall side of the house next door I felt that the extra wasn't needed. I used a foot of insulation at the other wall due to there being a chimney stack there, so I filled it all in with rockwool and made a flat wall straight across which hides the stack inside. There are bedrooms that side, so I felt the extra was needed there.

Your idea of filling the floorboards from underneath is a good one - I'll remember that for the next time a question like this crops up. :)

Would I be a terrible killjoy if I pointed out that a small square room with a low ceiling is going to boom and boom? :rolleyes:

Thought so. So I'll shut up <before> you spend several thousand on this project.

I also hope that it doesn't become a habitable room under the Building Regs. or you may end up digging downwards to get legal headroom. Not sure about this aspect of the Rules.

Don't start digging without expert advice or you'll probably undermine the foundations. :blush:


As an afterthought: You could drag a sub down there and give it some welly just to confirm my fears. Mind your head though.
Nimby i have the guy coming round again, well once i call him, who is going to convert it so was going to ask him all about the building regs, he never mentioned it on the first pass but we didnt want the cellar converting at the time just the puddle sorting at the bottom of the stairs and from that it somehow got into a cellar conversion conversation/quote. At the time we were not interested but i have recently been made redundant and got a semi generous? payout and have another job so i now have a bit of money burning a hole in my pocket hence why i am seriously looking into it.
The thing that worries me mainly is the ceiling height and i didnt think about the boom as i was more worried about the pj set up and soundproofing. Will try and get the sub down there over the coming week or so to see whether there is much boom, cheers inza.

If anybody else has any tips or dos and donts please post as i can still be swayed and save a few grand, inza. :smashin:
This is pretty much exactly what I want to do to my house (once I'm finally moved in!)

Sounds like exactly the same sort of situation, mines 10*10 and about 6ft high, underneath the front room, 1890s Victorian stripped floorboards which you can see up through.

Small amount of damp where the coal chute is which needs to be sealed, I wanted to tank the room as well to make it habitable.

If you get any info on quotes or procedures I'd be really interested in them, any DIY info would be superb if anyone has it, I don't mind getting stuck in.

It will just be stuff like moving gas meters and fitting a radiator etc that I'll steer clear of I reckon ;)

Believe me I am trying to be positive about your cellar rooms. I just don't want you to waste a small fortune on an unusuable HT room.

Remember: The smaller the room dimensions the higher the frequencies (tones) of the peaks and troughs. A square room doubles your problems.

I had one idea to soften the peaks in such a small room: Cover all the walls with the deepest corrugated roofing sheets you can find. The fibre reinforced cement boards would be perfect. Not thin steel sheets with small corrugations though. You will still need to sort out any damp first. You can paint the cement sheets. You might investigate making two slightly angled false walls using the cement roofing sheets to help reduce and spread the frequencies of standing waves.

If you are going to have a new ceiling anyway then slope it slightly downwards towards the TV end. Esily done with firring strips. Long tapered wedges fitted to the underside of the ceiling joists before the ceiling goes up. Two minutes extra work with a circular saw. This is to spread the peaks in the floor to ceiling response. If you make the ceiling lower at the seating end you'll only bang your heads twice as often.

Do investigate digging the floor deeper to get decent headroom. Even if it leaves a low raised concrete wall all round to support the foundations. You'll need expert advice for deepening the room without bringing down the house.

You are really interested in the second graph down on the calculator page. The programs are self calculating. Just enter the room dimensions in the boxes in metres.

Give a moment for the applet to download after you click on the link below and the calculator will show the likely peaks and troughs automatically in any room after you enter your room's dimensions.


I think maybe all the calculations are a bit over the top for what I want to acheive.

Yes, I would like the room to be as HT as possible, but ultimately I just want to enjoy films on a "nice" projector, with nice surround on a nice sofa with a nice beer ;)

I'm also going to be sitting around down there, using my PC, having a drink so I don't want it to look "too" over the top.

As long as the room is warm and dry and I get to see Star Wars in massive screen I'll be happy! :)
Right had the builder round again, no probs about regulations as i am not adding a room it is already there.

I think i will leave the headroom as it is as apparently my cellar floor has been bitumened? so is already water proof. It would apparently cost quite a lot more to have it dug out and redone and i dont fancy doing the digging myself as i wont know how far down to go due to the foundations etc.

The tanking that he will do is some type that you apparently splatter against the wall like cement or plaster so all the walls should remain solid and no plasterboard to worry about vibrating etc.

The ceiling can be soundproofed using rockwool as Gary as already stated, so is there anything else i need to cover before i start to go ahead, apart from the bass boom and the quote.
Good luck, and let us know how how it goes - progress with pictures etc will be a great help to others who may want to do something similar in the future.

If you get some bass boom, I'm sure there will be ways of remedying it. I've heard that soffits and colums can be used to help accoustics, so maybe a visit to the home theater builder part of the US forum (avsforum.com) would be useful.

Cheers Gary will take some pictures and post them of before, during and after. The builder is busy for the next few months so it will be about August time i think.

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