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PostPosted: Tue Jun 16, 2020 7:32 am 
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Hi. Welcome your thoughts on this. New single storey extension to be started along the back of our house. It will house a home cinema/theatre and additional kitchen space.

Three of the theatre walls will be exterior walls (Brick/concrete - two new walls and the existing back wall of the house.) The 4th wall is an interior concrete block wall - separating the theatre from the open plan kitchen/living space.

All theatre walls will be framed with decoupled 2x4 stud work, insulation. Stud top plates will be fixed to the new roof joists with isolating bracket so stud work will be decoupled completely.

There is nothing above the theatre. The main concern for soundproofing is flanking and noise for neighbours who are really close.

There will be a pitched roof above the flat ceiling of the theatre. The ceiling joists (horizontal) only touch on the pitched roof when they meet at the exterior wall, the rest is decoupled. The horizontal ceiling joists do also touch the wall plate where they are fixed on to the brick wall (left)
The ceiling space will be filled with 1ft of loose insulation as well. Ceiling joists covered in double drywall.

Attachment:
1_50.jpg


Just weighing up whether the extra cost and complexity of clips/channel is really worthwhile doing, given the roof joists don't meet for most of roof. It's really hard to judge how much or little difference those two points of contact will make.


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PostPosted: Tue Jun 16, 2020 7:35 pm 
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Hi,

I'm not completely following your description - is the image of the pre-existing building or does it depict the final construction?

Do you have a vented cold roof or a warm unvented roof?

Paul


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PostPosted: Tue Jun 16, 2020 8:11 pm 
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Hi Paul,

The sketchup model I added was my mock up of part of the completed new extension.

The wall on the left is the current outside wall of the house. The wall on the right is the new block/brick exterior wall of the extension.

I've attached a plan from the architect but unsure if vented cold roof or a warm unvented roof. The insulation is both between the joists and also then across the top of them running at 90 degrees.


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PostPosted: Tue Jun 16, 2020 9:05 pm 
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A couple of things...

This all depends on how much isolation you actually need. Do you know how much sound reduction you need? And down to what frequency?

I'm going to assume you'll want decent isolation all the way down to 20hz if you have subs...

How loud are you going to be watching movie?

On to the actual construction....

So, you'll build 4 new stud walls within what we see in your mock up?

And you want to attach the top plates to the ceiling joists in your mock up?

I'm going to assume you plan to drywall the bottom of the ceiling joists and that will be your ceiling for your theatre?

Your roof and ceiling joists will act as one since they are not decoupled. Therefore it will also "short out" your walls since they will also be directly attached to your ceiling/roof.

Now, all of this might not be a problem if it's not going to be super loud inside.

You have a nice lot of mass from the brick/concrete walls, that's good but it means the weak spot will be your ceiling/roof.

The roof must either be properly vented or properly sealed air tight and insulated on top of the roof deck, you cannot do anything in between these two options. This is purely from a construction point of view and not even regarding the sound proofing aspect yet.

I read the architects notes, they've specified a 20mm gap for airflow, I'm not sure where this gap is supposed to be but I don't see any mention of vents - maybe I missed it?

There needs to be venting at the eaves end and venting at the top, at the abutment with the rest of house. And there needs to be a passage of air flow below your roof deck between every rafter. 20mm is really small... I would leave at least 50mm under the roof deck.

Now, all of that venting can be done away with if you have a hot roof that is completely sealed air tight. Let me know if you want more info about this.

So back to the sound reduction...

As it stands, without the new inner stud walls you plan to build your isolation is dependant on the brick walls and the drywalled ceiling attached to the bottom of your ceiling joists. The mass of your roof does not come into play since we are assuming it's vented and therefore has lots of holes in it.

Your ceiling and your walls IS your outer leaf.

You have a mismatch of surface density between your walls and ceiling - the brick has way more surface density than your drywall. Therefore your ceiling will be your weakest link. Then if you add doors, windows, HVAC - they may become the weak link if not done properly.

Usually people make up for this by increasing the air gap between the outer and inner ceiling leaves - however, you can't really do this since you'd lose too much head height... the only way to do it by raising the ceiling joists or getting rid of them altogether and either 1. build a warm unventilated roof and use the roof as your outer leaf or 2. attach the drywall to the bottom of the rafters and have a half cathedral ceiling.

If you are convinced you need the isolation to warrant a two leaf system (which, by the sounds of it is your plan since you want to build 4 more stud walls inside) then you really need to do it properly, there is no point at all in building it unless it is decoupled from your outer leaf (your brick walls and ceiling). It would be a waste of effort and money and you'd end up with very compromised results, your flanking could even make things worse than if you hadn't built the new walls in the first place since it will amplify certain frequencies.

I am not a fan of using clips and resilient channel if it is avoidable, it's just a compromised solution.

What is the height to the top of the ceiling joists? And what is the depth of the joists?

The only way I can see you doing this without getting rid of the joists is attaching 3 or more layers of 5/8 drywall to the TOP of your ceiling joists, then building your inner leaf stud walls with a new ceiling joists below or in between your existing ceiling joists and then attaching layers of drywall to the bottom of them.

Your outer leaf ceiling would still be the weakest link, but you could build it heavy enough so that it still achieves what you need. The gap between the outer leaf drywall and inner leaf drywall needs to be as large as possible and fully insulated with fluffy attic insulation, in the walls and ceiling. And both leaves need to be completely sealed air tight.

You need to check that your joists can handle the weight.

If the ceiling joists must stay then I would build the room as in your plan first, without the new inner stud walls, then set up some speakers and blast some movies and go outside or to your neighbours houses and see if it's a problem. If it's not a problem then you don't need to do any further construction.

Paul


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PostPosted: Wed Jun 17, 2020 6:10 pm 
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Hi Paul,

Thank you for such a detailed reply.

I watch at about -10db or -20db on the AVR below reference. Plan is for an 11 channel speaker system and down to 20hz or just below due to subs. In my unisolated/soundproofed living room where I have things set up at the moment you can hear it outside.

I agree that at this point, the amount of isolation I require. I will test this with a pair of speakers and a sub before any work commences but I planning given my experience above.

So yes, 4 new 4" stud walls, attached with IB3 rubber isolation brackets at the top plate (which should go someway to isolating the walls from the ceiling). Ceiling joists would then have one layer of 18mm OSB, Green Glue and then 12.5mm plasterboard or soundblock PB. https://shop.soundproofingcompany.com/products/ib-3-decoupling-bracket?variant=33155431891079

Quote:
Your roof and ceiling joists will act as one since they are not decoupled.


They are, but only at one point. I guess I hoped that limited points of contact was better than being completely coupled.

Quote:
You have a nice lot of mass from the brick/concrete walls, that's good but it means the weak spot will be your ceiling/roof.


Agree. Although the roof is only transferring noise outside to neighbours, so I'll need to assess how loud that is. It would conduct through like it will the block walls if I don't add some isolation.

Quote:
The roof must either be properly vented or properly sealed air tight and insulated on top of the roof deck, you cannot do anything in between these two options. This is purely from a construction point of view and not even regarding the sound proofing aspect yet.


The roof design will be left to the architect, builder and be signed off by Building Control. I'll see what the builder says but it's a bit above me (no pun intended).



So back to the sound reduction...

Quote:
As it stands, without the new inner stud walls you plan to build your isolation is dependant on the brick walls and the drywalled ceiling attached to the bottom of your ceiling joists. The mass of your roof does not come into play since we are assuming it's vented and therefore has lots of holes in it.


There won't be any vents in the drywall ceiling. The air flow is below the insulated boards below the tiles on the vaulted roof.

Quote:
1. build a warm unventilated roof and use the roof as your outer leaf or 2. attach the drywall to the bottom of the rafters and have a half cathedral ceiling.


I've contacted Building Control regarding this but am yet to get a response. I wanted to know whether those ceiling joists are needed for the structural integrity of the ceiling. If they aren't then I could look at adapting plans and having a vaulted ceiling.

Quote:
The only way I can see you doing this without getting rid of the joists is attaching 3 or more layers of 5/8 drywall to the TOP of your ceiling joists, then building your inner leaf stud walls with a new ceiling joists below or in between your existing ceiling joists and then attaching layers of drywall to the bottom of them.


Have also looked at this but was unsure if possible due to the specification of roof binders. Thought they would get in the way. I thought about interweaving isolated joists and sitting them on top of the isolated walls.



Thanks again


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PostPosted: Thu Jun 18, 2020 9:11 am 
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I know there won’t be any vents in the ceiling, if you reread what I write you’ll see that I said your vented roof cannot be considered as part of your double leaf system.

Your new ceiling is going to be the limiting factor; which in reality is a single leaf and not a very massive one at that. Any decoupling and air gaps you include are completely compromised and may as well not be there.

To summarise, despite your massive brick walls and new inner stud walls, the amount of isolation you achieve will be a result of your ceiling only.

Paul


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