Plasterboarding a ceiling

I shall be plasterboarding the ceiling of a dining room extension to my property soon.
I recall reading somewhere that the best approach for a good ceiling that doesn't crack is to put up two layers of 3/8" plasterboard with staggered joints.
There is also a loft conversion above, and I recall someone in this ng saying it is good practice to have a DPM above the ceiling, but below the joists, to avoid condensation in the loft.
My proposal is to use one layer of 3/8" foil-faced plasterboard put up first (foil-face down); then to tape the joints with aluminium foil tape; then to put up a second layer of plain 3/8" plasterboard with staggered joints. This will/should give me a strong, 3/4" plasterboarded ceiling with an integral DPM.
Is this a good plan, or is there a downside (other than the cost).
When fixing the first layer of plasterboard, should I butt the edges, or leave a small gap?
Same question for the second layer of plasterboard.
I'll be having a smooth skimmed plaster finish (not Artex). Which side of the lower layer of plasterboard should face downwards - the white or the grey? The joints will be taped, probably with open weave fibreglass adhesive tape, rather than paper tape.
Any comments/advice/warnings appreciated.
TIA
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Mike
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mlv wrote:

you only need one layer of PB with staggered joints

There is a habitable room above the dining room? - in which case you don't need a DPC

Yes, the aluminium foil should be joist side up, that is, touching the underside of the joist, but as I've already said, it's not required.

Butt all PB's up and keep all gaps as small as possible.

The white side is for plastering

I you are getting a plasterer in for the ceiling, it may be wise to let him plate it for you as well
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Phil wrote:

That's what I have in the existing rooms, and they have all cracked along the joints. I read about the two x 3/8" layers of plasterboard method in a technical book some while back.
I agree the first layer of plasterboard has the joints staggered (like brick courses), but the second layer is put up with an opposing stagger, if you see what I mean. Consequently, no joints in the first layer coincide with any joints in the second layer.

The property is a bungalow with a loft conversion. Part of the ceiling is beneath a habitable room, whilst the rest is beneath the insulated roofspace (eaves).

I thought the important point was that the DPM had to be beneath the joists (as mine would be) and not above. The DPM doesn't actually have to touch the joists and 3/8" of plasterboard between the DPM and the joist should be irrelevant. Using my method (foil-face downwards) gives me the opportunity to tape the joints to give a continuous membrane. I couldn't tape the joints if the first plasterboard layer was foil-face upwards.

I wouldn't attempt the plastering myself, but I can recover the extra cost of two layers of plasterboard by fixing it myself ;-)
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mlv wrote:

Joists not substantial enough maybe, and flopping up and down under weight from above? Is there mesh across the joints?
Whatever; use screws rather than nails to fix the new board up, they are far less likely to loosen off and cause cracks.
(If you want to put up two layers for sound-proofing then that's probably sensible enough, but for any other reason I think you'd be wasting your time.)
David
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Lobster wrote:

Good soundproofing, thats all, and fire barrier is increased.

Not really. Depends on how artful you are in inserting them.

Agreed 100%.

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Lobster wrote:

Joists are correct size for application. Brown paper tape was used on joints.

Yes, I do intend to use screws.
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mlv wrote:

Whilst *air* tightness improves insulation, a continuos DPM is not required. It's merely a VAPOUR BARRIER to slow down moisture bleed through, so that loft ventilation is enough to cope, not hold a couple of gallons of water ;-)
Foil taping is for airtightness, not actual water penetration.
It's also a neat way to hold celotex sheets in place

No bloody point. Use a single layer, foil backjed with scrim taping to hold edges together. Thats MORE than good enough.
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mlv wrote:

It's up to you but it doesn't make any difference how many layers of P-board you put up, if it's going to crack on the joints, it's going to crack on the joints, the fact that there's another board above that is irrelevant. The only advantages are soundproofing and fireproofing, but these weren't mentioned in your OP.

Still not required - it's only of any use in flat roof spaces, where the insulation is bridging the roof and ceiling.

But you are defeating the object. The foil is a barrier against moisture coming from *above*! - if you have it foil side down, the opposite side (the roof facing side) of the foilboard could be soaked and will perish.

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Phil L wrote:

It most certainly IS NOT!
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The Natural Philosopher wrote:

It most certainly IS.
I don't recall ever seeing anyone plastering the foil side of a foilbacked PB ceiling, YMMV
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Well, you wouldn't. The foil goes on top (NP never suggested otherwise) and is NOT to form a barrier from moisture from above (as NP said) it's to prevent moisture going up into the cold space above and condensing. Putting polythene sheeting up first and then ordinary plasterboard just as good of course!
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Bob Mannix wrote:

It's all academic anyway as most builders don't do this, both small companies and large housebuilders.
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On 13 Feb,

The decorative finish on the ceiling will often produce a vapour barrier. Loft ventilation also reduces the need for it.
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On Wed, 13 Feb 2008 14:04:22 -0000, "Bob Mannix"
message:
.. xx .. xx> The Natural Philosopher wrote: .. xx>> Phil L wrote: .. xx>>> .. xx>>> .. xx>>> But you are defeating the object. .. xx>>> The foil is a barrier against moisture coming from *above*! .. xx>> .. xx>> It most certainly IS NOT! .. xx> .. xx> It most certainly IS. .. xx> .. xx> I don't recall ever seeing anyone plastering the foil side of a foilbacked .. xx> PB ceiling, YMMV .. xx .. xxWell, you wouldn't. The foil goes on top (NP never suggested otherwise) and .. xxis NOT to form a barrier from moisture from above (as NP said) it's to .. xxprevent moisture going up into the cold space above and condensing. Putting .. xxpolythene sheeting up first and then ordinary plasterboard just as good of .. xxcourse!
I have just decorated a bathroom where foil backed plaster board was used above, Vinyl matt had been painted on the ceiling which rolled off as soon as rhe roller touched it. The plaster was damp all over the room .... very naughty :-)
Mike P
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Phil L argued:

No, the foil is a barrier to prevent moisture rising up, through the ceiling and into the cold roofspace where it could condense and cause problems. That's why the foil (DPM) has to be below the joists. If the DPM is above the joists, the condensing moisture will be trapped and rot the joists.
My proposed 2-layer plasterboard installation had the first (uppermost) plasterboard foil-side down, but then covered by the second plasterboard layer. Therefore the foil was sandwiched between the two layers of plasterboard (and below the joists).

Me neither ;-)
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mlv wrote:

You've misunderstood my post. My idea was to place the foilside uppermost against the joist - just like you would have to do if you were skimming the foil boards opposite side.
Either way it's not required.
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Phil L wrote:

No, I understood what you meant - install the plasterboard foil-side up against the joists, as I would have to do with a single-layer, foil-sided plasterboard installation.
The orientation of the foil isn't really an issue, if it's on the uppermost layer of a double-layer plasterboard installation.

Well, that's debatable. I've installed a loft conversion and the roof ventilation will have been compromised (although I have added the appropriate ridge vents and additional eaves vents). For a small extra outlay I can use foil-sided plasterboard to reduce the amount of moisture passing through the downstairs ceilings, thereby minimising the possibility of condensation.
Whilst the foil is not necessary directly beneath the loft conversion area, it isn't worth messing about just using foil-faced plasterboard around the ceiling perimeters. Might as well use foil-faced plasterboard throughout and done with it.
I also intend to install an additional layer of 3/8" foil-faced plasterboard onto the existing ceilings (foil-face up this time). This is primarily to recover the old ceilings that aren't in very good condition.
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On 12 Feb,

The foil is a barrier for vapour from the hot side (in this case lower) getting into the colder side and dropping out its water content. So there shouldn't be a problem.
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The foil was never intended as a moisture barrier at all. It's a low emissivity coating to reduce radiative heat loss. At the time it was introduced, it was sufficient thermal insulation by itself. Of course, it falls way short nowadays and is pretty redundant.
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Andrew Gabriel
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Never underestimate the effort involved in attaching plasterboard to a ceiling. I would recommend either using small boards (B&Q do some called Handiboards 900x1500 I think) or getting one of those extendable poles to help you. Working over your head is painful, especially if you're not used to it.
Jon.
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