30mm gap between plasterboard and ceiling - board up or fill?

[RookieTony]

Hello DIYNot board members!

I am putting up an extra layer of plasterboard over an existing plasterboard wall, and while leaving 1/2 inch gap between the floor and the new plasterboard arrangement, I'm left with 30mm gap at the top (between the new plasterboard and the ceiling):

The purpose of the new plasterboard is to provide better sound isolation, and it will rest on a layer of Green Glue. Should I cut out two extra pieces of 30mm plasterboard to cover this gap, or would it be acceptable to space out the boards equally, reducing the top gap to, say, 15 mm and fill it with plaster at the plastering stage?

Thank you!

 

[johnny2007]

Coving

 

[johnny2007]

Better still, why don't you use the plasterboard 8 foot side standing up?
Most ceilings are about that height.

 

[RookieTony]

Coving

Isn't coving going to leave a hole behind itself? The purpose of the wall is to soundproof, so I think the whole area should be covered at least with something.

The plasterboard is 2400mm, and the ceiling height is 2440mm, so the segment on the left won't cover the entire height. Is that what you meant?

 

[johnny2007]

Yes, you will leave 20mm up and down.

 

[23vc]

I thought 8x4 boards were 2440
Either way, just cut a 40 strip. Filling it with plaster might necessitate buying a whole bag of bonding, not worth the 8+ quid.

 

[RookieTony]

I thought 8x4 boards were 2440
Either way, just cut a 40 strip. Filling it with plaster might necessitate buying a whole bag of bonding, not worth the 8+ quid.

You are probably right! I've got the size from the website, but it may be rounded down..

 

[RookieTony]

Yes, you will leave 20mm up and down.

Thank you, that sounds reasonable. And getting back to my question: given the plasterer will come in afterwards to skim it, how can I fill the 20mm at the top/botom? Should I use an acoustic sealant of some sort?

 

[phatboy]

You are probably right! I've got the size from the website, but it may be rounded down.. View attachment 293887

Plasterboard is metric so 2400 x 1200mm, where as ply isn't so is 2440 x 1220mm

 

[Deluks]

This particular plasterboard is available in 15mm which will give better soundproofing.
You'll need big arms though as it's about 37kg a sheet!

You will need to cover the entire area with plasterboard, no gaps top or bottom, save for 3-5mm which should be filled with acoustic sealant. Then coving and skirting over that for good measure.
If you only require a small strip to complete the job, ask the merchant if they have any damaged board they can let you have for a discount.

Edit: I just see you are overboarding. In any case, the rules above also apply for the existing wall before you add the new stuff.

 

[RookieTony]

Big thanks to everyone for replies!

@Deluks , what do you make of this Green Glue compound? Is it helpful, or should I just screw the new layer of plasterboard through the old one, into the studs?

 

[If in doubt...]

Big thanks to everyone for replies!

@Deluks , what do you make of this Green Glue compound? Is it helpful, or should I just screw the new layer of plasterboard through the old one, into the studs?

I've been involved in design and build of very highly sound insulated recording studios. If you haven't bought the green glue yet then save your money. It's expensive and will only show a noticeable difference when part of a comprehensive fully planned design that involves many other aspects, even then its performance will vary. For general plugging of gaps or sealing joints of plasterboard with acoustics in mind the best / most cost effective product we have found is Everbuild AC50. Its available in 2 sizes of tubes with the larger (900ml tube) being best value but you do need a bigger / special size gun for it.

 

[RookieTony]

Thank you @If in doubt... ! Could you please recommend me what I should put between the existing painted plasterboard and the new layer of plasterboard that I am planning to put up, instead of the Green Glue?

Can I use the Everbuild AC50 that you suggested to create the 'air gaps' mentioned in Green Glue marketing?

I have to say that the existing performance of the party wall is quite good, and it's only the occasional bass on the edge of being audible coming through next door is what is annoying me. I know that with the bass only complete mechanical decoupling would do the trick, but perhaps an extra layer of heavy plasterboard could help the wall gain some mass and give me just a bit of extra attenuantion in lower frequency range that I seek?

 

[If in doubt...]

Thank you @If in doubt... ! Could you please recommend me what I should put between the existing painted plasterboard and the new layer of plasterboard that I am planning to put up, instead of the Green Glue?
View attachment 294294

Can I use the Everbuild AC50 that you suggested to create the 'air gaps' mentioned in Green Glue marketing?

I have to say that the existing performance of the party wall is quite good, and it's only the occasional bass on the edge of being audible coming through next door is what is annoying me. I know that with the bass only complete mechanical decoupling would do the trick, but perhaps an extra layer of heavy plasterboard could help the wall gain some mass and give me just a bit of extra attenuantion in lower frequency range that I seek?

Well for sound reduction the best thing to put between 2 layers of plasterboard is a big fat wedge of air as its the "2 leaf" design approach where the trapped air acts as a spring to damp the noise. But we are talking around 100mm minimum air gap and I'm sure you don't want to give up that much. On top of that the masses need to be fully decoupled from each other but efforts to do that are near pointless if just affecting one wall as the adjoining walls and ceilings / floor etc will happily keep everything coupled and transferring the noise.

The Green Glue marketting is referring to only a tiny air gap, the science is quite complex but it more along the lines of stopping the various combined layers "ringing" rather than the true air spring effect you get with large air gaps on 2 leaf systems.

If it were me I'd be using the AC50 instead of green glue, and using the soundlam or soundbloc (any blue plasterboard) or maybe 19mm "plank" board. AC50 has the grab affect. But I would also consider some mass loaded vinyl between the plasterboard sheets. Finally instead of leaving the gap at the bottom I would also rest the new plasterboard on acoustic foam rubber strip - but to be honest the results will vary widely depending on your exact building make up. Low frequencies are by far the hardest to subdue and there is also a risk with lower frequencies that putting certain mass structures in can, sometimes, make things worse through resonance and other factors. The way true sound reduction is approached is by identifying the actual specific frequencies needing attenuating, then defining by how many dB, and this will have to be a realistic amount as there's no such thing as total sound isolation. Once known then various designs or products can be researched / suggested based on the numbers you have given.

Sorry if that sounds a bit deflating but in general good sound reduction in buildings, especially low frequencies, is very expensive - usually measured in the £1000's.

 

[RookieTony]

That is brilliant, thank you @If in doubt...

I don't suppose you could help me figure out the following bit from the AC50 manual:

What application pattern do they mean? Where would the bead start and what are the centers the manual is referring to? I've googled it and couldn't find any drawings.

The manual goes on to say that "Highly porous substrates such as new plaster should first be primed with Everbuild PVA bond diluted 1:4 with wa-
ter."
But this woulnd't apply to an existing painted plasterboard, would it?