Hi there Phil, and Welcome!
The ceiling plasterboard will be taken down and 75mm PIR foam sheet placed up into the cavities between the 5inch joists
PIR is great thermally, but pretty useless acoustically, because it is closed-cell foam. To have usable acoustic properties, the insulation you use mist be open-cell, meaning that it consists of numerous tiny air spaces that are interconnected with each other, within some form of "web". Fibrous porous absorbers, such as mineral wool and fiberglass insulation, are the best acoustically, and also pretty good thermally. I'd suggest you switch to those if you want your room to be usable acoustically, in addition to being well insulated for heat.
The joists are under a bedroom with chipboard floor.
Do you need acoustic isolation up there? In other words, do you need to keep things quite by reducing the amount of sound that leaves your room and gets into the bedroom, up through that floor?
I would welcome advice on how to finish the ceiling surface, for the room to sound good for hifi listening and playing acoustic guitar.
Making the room sound good for critical listening and rehearsal has very little to do with how the surface of the ceiling is finished: it has everything to do with how the room is treated acoustically. With a room that size, here is the situation:
philhjobim-simulation-01.jpg
That's the predicted response for your room, untreated, in the low frequency part of the spectrum, which is the most critical. The final finish that you put on the wall and ceiling surfaces won't do much to fix that. What WILL do a lot to fix it, is the materials you use to treat the room before you put the finish on.
I guess I'm looking for a balanced result not dry.
In a small room like that with a very low ceiling, your options are rather limited. If you don't treat the low end sufficiently it will be boomy. Treating the low end enough, automatically implies that you are way over-treating the high end, which will make it muffled, muddy, thick, dead. You can deal with that to a certain extent with correct use of diffusive and reflective surfaces, but then you could mess up the mid range. Getting a balanced acoustic response out of a room like that is really hard to do.
The floor will be concrete>30 mm PIR foam sheet>22mm chip flooring>thick carpet
... which would automatically trash the high frequency response of the room, while also reducing the headroom even more, to give you an even lower ceiling. Carpet is pretty much the worst possible surface treatment that you can put into a small room. If you really want to make a good room sound bad, carpet the floor. And if you want to make a bad room sound terrible, carpet the floor with thick carpet.
Carpet is a selective acoustic absorber: it is really good at absorbing high frequencies, (which you do NOT need to absorb in a small room), it is measly and unpredictable at absorbing the mid range (which needs to be absorbed in a carefully controlled manner in a small room), and it does nothing at all for low frequencies, which need massive absorption in small rooms. In other words, carpet does the exact opposite of what is needed. Take a look at photos of professional studios from all around the world, or even good home studios as you see all over this forum: how many of them have carpeted floors? Answer: around zero. You pretty much never see carpeted floors in studios, for a good reason: it kills the acoustics. At best, there might be some carefully positioned carpet in a few key locations in the room, but you practically never see fully carpeted floors. It's a huge mistake. (The only place you see that is in the "YouTube" studios, built by people who don't have a clue about acoustics, and are the same ones who are likely to put egg-boxes on the ceiling and try to float their floor on tennis balls or rubber pads.)
Three walls are thick stone>25mm unfilled air gap>concrete block tanking wall>50mm PIR foam sheet> surface finish will be decided last to allow adjustment to the acoustic.
Once again, the PIR foam is useless acoustically.
What I would suggest is that you first run a simple acoustic test in that room, using the free REW software, and post the results here on the forum for us to take a look at. Based on that we can suggest specific treatment options, and specific locations where they would need to go, in order to give the room good acoustic response that would be somewhat usable for what you want. The room is too small and the ceiling is too low to be able to get really good response out of it, but it can still be pleasant in there, if it is treated correctly.
Basically, you would need to forget the PIR completely, and switch to something that is acoustically useful, such as fiberglass or mineral wool, for all of your insulation. That would do both things at once that you need: good thermal insulation, and good acoustics. Put that in your ceiling joist bays, frame up your walls with furring and do the same there, and use a semi-rigid fiberglass under your floor with proper underlay and laminated flooring on top. The wall framing might need angling in places, both to reduce flutter echo and also to provide a base for broad-band treatment devices, such as slot walls, and the ceiling pretty much has to be mostly fabric, perhaps with a few key reflective surfaces embedded in the insulation at angles that would help with diffusion, to a minor extent. That's about all you can do up there. With a normal height ceiling the correct approach would be to hang several small hard-backed clouds at angles, but you don't have enough height to do that.
So that would be my suggestion.
- Stuart -
