Fixing celotex or similar to the ceiling

[Pete Verdon]

I have a plasterer coming round this weekend to do my kitchen and bathroom ceilings. The main job was the kitchen, which was damaged by a water leak, but while he's here I thought he might as well skim over the ugly old artex in the new bathroom.

It's occurred to me that the bathroom ceiling is really in need of some more insulation. Unfortunately the odd shape of the roof makes it very difficult to get at the loft above it. So I thought I might fix some insulating boards to it before the plasterer visits, and get him to skim onto those instead. This will also cover an ugly gap that exists at the top of the tiles.

Any comments on this plan? Screw or glue?

Cheers,

Pete

[Tim W]

Pete Verdon wibbled on Wednesday 18 November 2009 00:13

You could consider Marmox or equivalent - those are a layer of foam faced on both sides with glass mesh and cement. They can be glued or screwed up and available in a range of thicknesses. Not as efficient as celotex type materials but totally vapour proof and according to Mamrox you can apply skim plaster directly to them (tape joints with scrim as per plasterboard. I'd fill the joints with silicone first (as I did on a damp floor when using Marmox under wood) to complete the vapour barrier).

Very light and easy to cut - score n snap as PB, though that stuff will destroy one stanley blade every few sheets.

[Pete Verdon]

Hmm, I actually have a few sheets of 6mm Marmox in the shed, left over from the main bathroom build. Not sure that's enough insulation though.

Pete

[Tim W]

Pete Verdon wibbled on Wednesday 18 November 2009 01:09

I was thinking more like 20 or 30mm?

Are you actively trying to stop heat loss or just reduce the cold surface a bit to lessen condensation?

[Pete Verdon]

Yep. In fact I might go even thicker just in order to cover up the gap at the top of the tiles. (At one time there was a plan for a false ceiling on battens, so the tiler didn't bother filling the last inch or two at the top.) The alternative is some thin coving.

The latter.

Pete

[JimK]

get some extruded polystyrene sheets (B&Q apparently do some in packs) stick em to the celing and then screw you rmarmox underneath so that plasterer can skim? best of both worlds?

JimK

[Andrew Gabriel]

Even 25mm Cellotex will achieve that.

[Pete Verdon]

That's good to know.

I've never actually handled Celotex before. I had been kind of assuming I'd stick something up and let the plasterer do his thing, but it's dawned on me that plaster probably won't stick to plain Celotex, will it?

Pete

[John Rumm]

You could use a pir foam board and plaster board below. Use long screws through both layers.

[NT]

Sounds like you've got enough room there to use cheap polystyrene then a layer of plasterboard

NT

[Pete Verdon]

The more thickness the better, within reason. The ceiling is unusually high for a modern house, so no worries about lowering it.

Is polystyrene in the ceiling OK from a fire point of view?

Pete

[Tony Bryer]

IIRC the really bad thing about polystyrene tiles beloved of 1960s home 'improvers' was that in the event of a fire, the fire spread across them and burning droplets then spread the fire. Above a ceiling I would have expected it to have melted long before the plasterboard was penetrated and if you reach that stage you've got serious problems.

[Roger Chapman]

I thought you could get plaster board faced PIR. Would that be more expensive, or less expensive than getting two separate layers? Fitting a combined sheet should surely be easier.

[Tim W]

Pete Verdon wibbled on Wednesday 18 November 2009 23:52

No it won't It's (usually) faced in shiney foil.

30+mm Marmox will probably achieve what you want, though not strictly equivalent to 25mm celotex, but we're not working with critical numbers here.

If using celotex (or any equivalent product) then you'll need to screw it to the ceiling timbers with big washers (they make washers for this purpose). At this point you'll need to cover it with something that takes plaster. Your choices are now plasterboard (moisture resistant - pink - would be a good idea), or Marmox or something else with a suitable surface. This would be screwed through the celotex into the same timbers.

Cheers

Tim

[Tim W]

Tony Bryer wibbled on Thursday 19 November 2009 00:26

The same might be true of Marmox. Celotex (PIR foams in general) does have the advantage it doesn't do this.

[JimK]

yup abt =A318+vat / 8' X4' sheet for small quantities recently...

Bloody heavy though - 2 man job for any ceiling tho - quick if awkward

JimK

[Bruce]

There is one Celotex product (PL3000) that has a sheet of Celotex insulation material bonded to a sheet of plasterboard.

Thickness starts at 37mm (25mm Celotex + 12mm plasterboard) and goes up from there. The plasterboard has a taper edge.

you now have to register with your name, address and telephone number to obtain the data sheet.

[Andrew Gabriel]

Yep - I stopped planning on using their products at that point.

[Andrew Gabriel]

PIR foams natively are very flammable. The ones used in building insulation have varying degrees of fire retardants added (varying from nothing, so be careful). In any case, even when there's enough retardant to prevent spread of fire, they still burn in a fire, releasing toxic gasses.

[geraldthehamster]

I would locate the joists, first attach 25mm foiled Celotex or Kingspan with plasterboard screws, then screw 9mm plasterboard over that, through to the joists.

IME, Kingspan foiled board is covered with a strong reinforced foil, and you could screw straight to the ceiling and the heads won't pull through. Celotex has a crappy thin foil so you'd need to use bits of card or similar as washers, to hold it up while you fix the plasterboard.

Or you can get insulation-backed plasterboard to do it in one.

All are semi-rigid materials in large sheets, that would be quick and easy to cut and fix, preferably with an assistant to help hold it up while you screw. So to speak. Don't forget that you have a head to hold things up with, as well as two hands ;-)

Cheers Richard