thinnest plasterboard or other plaster substrate?

[Fred]

Hi,

I've been retrofitting celotex to a room. It's gone well except for one slight problem: the window reveal (I put 25mm celotex inside the reveal). I'm worried that even if I use 9.5mm, rather than 12.5mm, plasterboard on top of the celotex that it might overlap too much of the side of the window frame. I'm not sure how much of a problem this will be: any overlap will only be cosmetic, it's not going to block the window or prevent it opening or anything that drastic but will it look "odd"?

What would you do? Use 9.5mm PB? If there is an overlap (i) it will probably be hidden by blinds/curtains/whatever SWMBO puts there or (ii) if we do see it, we may grow used to it.

Alternatively is there something thinner than 9.5mm PB? I don't expect there is a plasterboard any thinner because it would be too fragile to transport but can you plaster on top of say hardboard? Does it need any treatment first (primer? undercoat?) or just PVA?

Or can you PVA celotex and plaster directly on to that?

TIA

[Jim K]

just buy a moulding to extend the winda frame?

Cheers Jim K

[andrew]

6mm plasterboard does exist, although probably a special order so may not be a solution. It's fibreglass reinforced I think, and designed for boarding curves.

I would have used an insulated plasterboard - something like the Knauf Thermal Laminate Plus at 27mm total thickness.

Alternatively, you could cover the insulation with metal lath, eg

then plaster. Not sure what thickness you'd end up with, but probably less than plasterboard.

A

[Fred]

That would have been ideal, what a shame I didn't know about it when I began!

I'll have to phone around to see who stocks 6mm plasterboard tomorrow. I think only Lafarge make it.

Thanks.

[fred]

In article , Fred writes

It hardly seems worth the effort to save 3mm.

You could face the insulation with 3mm ply and finish it as you had planned for the PB, either paint or paper.

If you decide to paint it you can leave it looking like wood or fill the grain with board filler or even polyfilla to leave a plaster smooth finish that can be primed then emulsioned. In the latter case it's not a skim coat you are applying, it is just enough to fill the grain so is nominally zero thickness.

You could put thinner insulation in the reveals, it would make little difference to the overall heat loss as the area involved is small but could help avoid local condensation. To avoid buying a full sheet of thinner stuff I have sawn small lengths of PIR foam in half depth wise but you wouldn't want to do a lot of it. You could use a layer or 2 of polystyrene tiles instead of PIR with some loss in efficacy but I don't think that's a big issue.

[js.b1]

AFAIK industry practice is...

- 12mm Celotex with 9mm PB over the top

- 20mm Polystyrene + PVA + plaster skim

You can use Marmox 6mm 10mm 12.5mm 20mm...

- Cement & glass-fibre mesh on extruded polystyrene + plaster skim

Marmox is actually better than expanded polystyrene re waterproof.

[Phil L]

How is he going to do that? - he'd have to replace the glass units.

[Phil L]

Have you took off the old plaster down to bare brick? - you'll gain up to

25mm there.

[Andrew Gabriel]

Yes.

You might want a scratch coat (use bonding coat plaster) as finish coat by itself might be a bit fragile on celotex.

You can plaster a sheet of glass if you PVA it first.

[The Natural Philosopher]

you could use rendering mesh and bonding plaster for a really thin finish.

[Andrew Gabriel]

It's called metal lath. You would want stainless steel, not galvanised, near a window, and stainless steel fixings. It's a complete bastard to handle. Comes in 8'x3' sheets, and every edge is like a giant razor blade, and even the surface is like a giant cheese grater. I used some a few months ago, and in spite of being careful, I still managed to cut myself several times. You don't even know you've done it, as mostly it's completely painless at the time. Then you notice things are getting blood on them, and you've got blood on your hands, and you start trying to work out where it's coming from. It's probably a bit thick for what you want, and it doesn't necessarily lay very flat against the surface, so I'm not sure you'd achieve significantly less than 9mm with it.

Another option I have used for a thin layer is fine welded mesh. That came from Wickes, but is galvanised rather than S/S. I don't recall if they or anyone else does a S/S version. It is much easier to handle.

[Fred]

Yes.

[Fred]

I thought that if I used 6mm plasterbaord rather than 12.5mm, that a saving of 6mm would be quite significant. I could only find 6mm plasterboard on lafarge's web site, not on British Gypsum or Knauff's. When I called Lafarge's freephone number to find a stockist, the lady told me they didn't make 6mm plasterboard. I tried to explain that they made a 6mm contourboard and that it was on their website but in the end I hung up in frustration because she wouldn't believe me!

I phoned around a lot of builder's merchants and no-one had any and no-one was interested in finding any. I did eventually find someone with just four sheets in stock but they wanted 19gbp plus VAT. So I decided that the 3mm difference between 6mm and 9mm plasterboard would not be noticeable and bought 9.5mm PB for a fifth of that cost and used that instead.

I had thought a lath may have allowed me to go thinner but like you said, the idea of cutting it and it cutting me put me off and being new to plaster, I wasn't use how easy it would be for a novice to skim?

BTW I have bought a couple of angle beads to use. Do you hold these in place with a couple of plasterboard screws, glue (no more tails type of thing), or just push them into the wet plaster?

Thanks.

[Andrew Gabriel]

Plaster board beads - I just run some plaster up the back, and then push on to the plasterboard, and plaster should ooze through the slots. If the beads or the edge are not flat, you might need a screw or nail. Again, should really use non-rusting ones, as if it ever gets damp, you can get rust marks coming through the paintwork.

[Fred]

That's what I did, thanks. Just wanted to make sure I hadn't done it wrong ;)

Thanks again.

[Mario Toro Paz]

replying to Fred, Mario Toro Paz wrote: you could use 6mm mdf

[tabbypurr]

that'll be useful, 6 years later.

try this

NT

[Andy Burns]

Home owners hub seems to filter out all messages mentioning its name, so Mario is unlikely to see your helpful link

[Fredxxx]

That's a shame. I've even tried to create an account there, but the site won't allow for a legal "_" in a name!

[Andy Burns]

Actually it seems to be playing catch-up, NT's and my message have shown there now, yours not yet, I guess an underscore or a space is enough to fool their filter.