Benefits of 15mm plasterboard?

[Mathew Newton]

I am putting a new ceiling into our extended 'family room' and was planning on using 15mm plasterboard rather than 12.5mm. The reason being was that I figured it might provides benefits in terms of improved acoustic attenuation for the bedroom above and slightly better fire rating. Perhaps even some benefit regarding stiffness and strength? I've got a plasterboard lifter so the extra weight when installing isn't of concern and I probably only need around a dozen 2.4m x 1.2m sheets so the extra cost should be minimal.

However, I didn't realise that 15mm PB doesn't seem to be widely available, even aside from the current supply issues for all gypsum products at the moment, and so I am beginning to have second thoughts if it's not the 'done thing'. Also, where it is sold, it is usually all tapered edge and yet I was going to get the whole thing skimmed and so would be wanting square edge. Am I do something odd here?

Just wondered if anyone has any thoughts on whether 15mm would provide the assumed benefits, or should I just stick to 12.5mm?

[tabbypurr]

15 does provide those benefits, but how useful is it really on a ceiling? Not a lot. If you want better acoustics, dumping something on top of it after fitting also works. Whether that's pours of plaster, assorted construction debris, fibreglass batting, sand between plaster lines, whatever. I expect you could also pour plaster onto it just before lifting it into position on a lift to get more than 15mm total.

NT

[John Rumm]

It will give a small incremental improvement, but if fire protection was of particular relevance, you could get better improvement in fire protection by adding rockwool in the ceiling void / and or using two layers of 12mm[1].

Much better sound attenuation would be achieved by fixing the boards as a false ceiling just a little below the existing one such that its decoupled from the joists which are being walked on above etc.

As to rigidity, yup it will be a bit better, but you would have to question what is the actual benefit on a ceiling? (it makes more sense on wall likely to suffer impacts).

[1] Both of which would likely be building regs requirements in higher risk scenarios - such as a ceiling over an internal garage with a habitable room above it.

[Tim Lamb]

I chose to fit Rockwool in ceiling voids, partly for thermal isolation as we have individual roomstats but mainly to avoid the extra layer of plaster board required to meet the fire regs. Sound insulation was an expected side benefit. Building control wanted the Rockwool retained independently of the plasterboard so I stapled Chicken wire to the joists and fitted the insulation from above. Not as bad a job as it sounds but works best in new build. Difficult to say if thermal/sound insulation is significantly improved. Direct contact; dropped objects, is still readily heard.

[alan_m]

Does it matter if it's tapered if getting the ceiling skimmed? The joints will still need to be taped beforehand, The boards will only be tapered down the long sides so where you need more than one sheet to span the room you will also have butt joints.

However, you may not tapers where the board joins the wall. I used tapered boards for my suspended ceiling but I planned my installation on paper to make sure that I didn't end up with narrow strips at the walls. It worked out that I required full boards in the centre span and had to cut off approx 20cm (the tapered edge) from the boards that met the walls. None of the corners in my room were 90 degrees so I used one wall as the reference and place the supports square with that wall. The first board I put up was one in from a wall making it easier to measure the distance between this "reference" board and wall to allow the non-square cut to be made.

Tip: on an old house don't assume that a measurement half way up a wall is the same as the measurement at the top of the wall at the ceiling. In my rooms with a 9ft ceiling many of my wall are bow shaped with the old plasterwork being thicker at the skirting board and ceiling. Its not noticeable until a long straight edge is placed vertically on the wall.

[Andy Burns]

Got mine (15mm soundbloc) from SIG Insulation.

[harry]

Put two layers of the thin stuff up. Stagger the joints. You will need longer nails or screws.