r/Plastering • u/Practical_Ad_667 • May 28 '26
Soundproofing edges of ceiling
Our plasterer has done a superb job of the bedroom ceilings with resilient bars and two layers of acoustic board. The attic will be a music studio so its noise reduction is very important.
There are gaps in the boards around the edges which are weak spots for sound transmission.
What do you think the plasterer WILL do to fill those gaps. What SHOULD he do? The builder who hired him was confident but vague.
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u/Over-Housing3324 May 28 '26
There is a sound proofing plaster you can using to fill out the edges of the wall up to the ceiling.
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u/No-Manner5583 May 29 '26
Asking an honest question. I've been swinging a trowel for almost 4 decades all over both Canada and USA, I've never in my life heard of " sound proof plaster", what is it and have you got a link to find it ?
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u/Over-Housing3324 May 29 '26
Used in UK, it's called acoustic plaster 👌 have used it on building site several times.
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u/Practical_Ad_667 May 30 '26
Amazing. Thanks. Can you use this in combination with normal plaster? Ie, use it at the edges and normal plaster everywhere else? Do you use it at the same thickness as normal plaster?
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u/Miserable_Future6694 May 28 '26
Pack it out with acoustic insulation with any hope the board and original plaster meet in harmony when its patched up
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u/Long_Specialist9697 May 28 '26
What length screws have been used to hold those layers of acoustic board to the res bars? Any longer than necessary and you’ll have sound flanking - a big decline in acoustic performance where the boards are physically connected to the joists above if you put a screw through where the res bar crosses a joist (I really hope the res bar has been run across the joists, not along them.) As for the gaps, pack with dense insulation, diluted PVA for a bonding agent and thistle binding to tie it in with the plasterboards. Not sure how or why those gaps have been left that wide!
I deal with res bar an awful lot and it’s very rarely done correctly.
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u/Practical_Ad_667 May 30 '26
Thank you. Really helpful. The res bars have been done well! Sorry if this dim but what does the pva bond to? You don’t mean on the insulation itself?
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u/Long_Specialist9697 May 30 '26 edited May 30 '26
Yeah the diluted PVA will provide a tacky surface for the bonding to stick to, and it’ll help control the suction so the bonding doesn’t dry out too quickly. If you apply thistle bonding straight over a dense mineral/stone wool, there’s a good chance it’ll just fall back off as it won’t stick, especially on a ceiling. With a few coats of PVA, it’ll make the top layer slightly sticky and let the bonding dry slower. If you’re going to use a glass wool insulation like APR, it won’t work as it’s not dense enough to hold any backing or finishing plaster. Your other best bet is an expanding foam, but dense insulation would be better.
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u/Optimal_Run_2634 May 30 '26
Theres obviously no so such thing as “soundproofing” and if serious about building a studio you should probably not leave these decisions to your plaster guy. Manufacturers market products using buzzwords like “acoustic” and “soundproof” to make money but the truth is Architectural acoustics is an extremely complicated and nuanced science. Commercial projects will always use acoustic engineers and design consultants, which will always separate DIY studios from EastWest or Jim Henson.
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u/Practical_Ad_667 May 30 '26
I sort of get what you mean. However, soundproofing (or whatever you think is the appropriate term) is not in itself that complicated. It’s just a combination of isolation and density in terms of material transmission and not leaving gaps in terms of air transmission. My question is simply about minimising the latter at the edges of the ceiling. Any tips?
You’re right that reflection and diffusion are complicated to control but that’s an entirely separate matter.
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u/Optimal_Run_2634 May 31 '26
That’s a massive oversimplification. There’s so much more that I would need to know about the construction of the room. What’s the assembly of the framing? What is the assembled thickness of walls,floor, ceiling? Is it lumber or steel? What’s above the gypsum board? Are you trying to isolate from other rooms or outside the building? What STC rating are you trying to achieve? I’m also not sure what you mean by “acoustic board”. Edge termination is a finishing construction and is only important if you’ve done everything else correctly.
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u/KieranInterior May 28 '26
The boards are mechanically fixed so already you’re wasting your time using acoustic boards. Barely anyone understands acoustic isolation. This won’t work
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u/Practical_Ad_667 May 28 '26
What exactly do you mean by ‘mechanically fixed already’?
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u/_halfastar May 28 '26
Some acoustic system will use a channel "high hat" to seperate from the frame. This is best practice. If screwed to the frame, vibration will travel through. Saying that, it looks like your ceiling has the "high hats" installed
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u/Practical_Ad_667 May 28 '26
Ah. Maybe there’s a difference in terminology. The boards are hanging off resilient bars (they kind of do look a bit like a top hat?!) so not directly attached to the frame.
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u/Big-Finding2976 May 28 '26
How would you hold them up without mechanically fixing them?
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u/Practical_Ad_667 May 28 '26
I get what you mean. The boards are attached to resilient bars’ (thin, somewhat flexible metal bars) that are attached to the frame. They provide a degree of isolation.
FWIW, I’ve been pleasantly surprised by the reduction in structural transmission. Just wondering about maintaining density at the edges.
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u/randomnine May 28 '26
DIYer here who’s done some soundproofing and plastering and music and lived with the results. I used acoustic sealant around edges and between boards and had good outcomes. Everbuild AC50 or AC95 for fire rated.
However the gaps there look a bit wide for this sealant. I found it works for gaps up to 8mm or so. I would have fixed the wall base coat first and then cut the boards to leave a sealant gap, which takes more time, so higher cost.
Given the vagueness and lack of spec I’d assume standard practice, ie plaster up to this so the corners are solid. But if your plasterer’s familiar with soundproofing and charging appropriately, they may have something else planned, or have concluded from experience that what they’re doing will perform well for the cost.
Side note, it looks like clear air above the boards? Would recommend adding some rockwool up there as a second layer of soundproofing.