Caulking question / ceiling beefing layer

I need to beef up my ceiling, I plan to use 2 layers of plasterboard. How important is it that every layer is completely air tight? If the first layer has a gap in the middle ( completely covered by the second layer), will this be bad? Also, does it make sense to add a 3rd layer that doesn't cover the whole surface (covering roughly 95%) ? I know that in cars they often only add some mass in the middle of the surface (for example a door panel with a small patch of heavy material glued in the middle). Please see the attachment for examples. I'm asking because the beams that support my ceiling are not running entirely perpendicular (and despite their width and height, they are not completely straight either). So making the plasterboard fit exactly in between the beams is a bit of a struggle. Test fitting those plates is no fun. If I could place the first layer in two parts, that would speed up the process a lot. If this is bad for sound blocking (and I assume it is), then I will probably make some adjustable template that I can fit in between the beams to copy the dimensions accurately.

Short and sweet questions! :thu: So short and sweet answers... :) :

How important is it that every layer is completely air tight?

Very. Even tiny air gaps greatly reduce isolation.

If the first layer has a gap in the middle ( completely covered by the second layer), will this be bad?

Yes.

Also, does it make sense to add a 3rd layer that doesn't cover the whole surface (covering roughly 95%) ?

No: For the same reason as above. Effectively that's the same as putting in an excellent acoustic door on your room, but then leaving a large gap all the way around the edge.

I know that in cars they often only add some mass in the middle of the surface (for example a door panel with a small patch of heavy material glued in the middle).

Right, but that's for a different reason. It's not to improve isolation, but rather to change the resonant frequency of the panel, so it won't vibrate at some annoying frequency.

So making the plasterboard fit exactly in between the beams is a bit of a struggle.

You don't need to make it fit exactly. Cut it a bit smaller, so it can fit in easily, then fill the gaps around the edge with caulk. For example, you can easily have 1/8" (3mm) gaps on each side of the panel, and there's no problem with filling those with caulk.

If I could place the first layer in two parts, that would speed up the process a lot.

You could do that... as long as you also fill the gap with caulk before putting on the next layer. so in addition to the gaps at the edges, you could also leave a 1/8" gap done the middle, then fill that with caulk as well, just like the other gaps. Caulk is higher density than plasterboard, so you don't need to worry about discontinuity in density. - Stuart -

Thanks for your answers, very clear. I should have known there cannot be any shortcuts :) (like Rod Gervais says in his book "Home Recording Studio"). I was aiming for 3mm gaps. But when I make an error and 3mm becomes 8mm, it takes a surprising amount of caulking to fill those gaps. I'll try to make a tool for copying the distance in between the ceiling beams, so that I can cut the plasterboard to size with better accuracy. If I come up with something that works I will post some photos.

Photos! :thu: YES! Lots of them! :yahoo: That's a big part of the forum: members helping out future members by posting photos of what they did. As they say: "Photos, or it didn't happen!" :) Your experience can be very helpful for others, so please do post photos and descriptions. - Stuart -

I made the measuring tool for fitting plasterboard in between the beams of the ceiling, and it is working really well. Having plates that fit makes the work a lot more enjoyable, and not having to measure also makes it a bit quicker. The tool is simply two thin pieces of wood, connected by a sliding mechanism that I can lock with some knobs. I simply place slide the sides against the beams, tighten the knobs, put the tool on a piece of plasterboard and trace the sides. I attached some photos, it took me about half an hour to make (also courtesy of my local DIY store who sold some awesome threaded knobs). It doesn't work for curved beams (but it wouldn't be too hard to create something that would work for that).

What a great measuring tool ! Ingenious :wink: I wish I'd thought of something like this when I had to add 2 layers of 5/8" into the rafter bins. :roll:

Wow, what a great idea! Too late for me also.....

Very ingenious! Great idea. Simple, inexpensive, elegant... What more could you want? :thu: - Stuart -