Dry lining - how? insulate with what?

[David in Normandy]

I want to dry-line the exterior walls upstairs. Can anyone give me any tips on how to fasten the sheets of plasterboard to the walls or what (if any) insulation to put behind it? I've never done this particular job before, so am fairly clueless. Googling hasn't been very helpful as all I can find are companies wanting to supply materials or their own services. Any good "how-to" web-sites?

There are two reasons for wanting to dry-line. First to provide a good, flat, clean surface, and second to help insulate the house, which gets extremely cold in Winter.

The walls are very old, stone / lime mortar / clay / rubble infil and about 2 feet thick. They are nice and dry, no damp to contend with, just cold. The existing lime plaster has become a bit grotty and crumbly in places since it was done on the 21st August 1892 (according to a date marked on it).

What size gap should there be between the plasterboard and wall? Should I put some insulation material there? What sort - there are lots to choose from? Preferably not too expensive as I'm on a tight budget. I'd guess that foil based perhaps would not be a good idea as it may prevent the walls from breathing and trap moisture?

As this is upstairs, can the plasterboard sit directly on top of the floorboards or does it need to be suspended a cm or something above the floor on wooden battens?

All feedback, suggestions, tips, URL's etc welcome please.

[Adrian]

There is plasterboard with poly foam bonded to the back of it - available in various thicknesses. Makes for a fairly quick job - depending on how 'sound' and 'true' the existing surface is.

I used to work for the people that made the stuff (Britich Gypsum) - their preferred technique for applying it was to trowel plaster dabs onto the wall, then press the cut sheets of board back onto the dabs.

If the existing plaster's not strong enough to support this then you could clean back to something solid (bolster / sds chisel) and sit your dabs on that.

The 'de-luxe' application method (particularly good if the underlying surface wasn't true / level) was to use thick dabs of plaster to stick small pads of plasterboard offcuts to the wall - and level / true these with a long spirit level. Once you've done this, and it's gone 'off', you can then proceed as above, but pushing the large sheets of plasterboard back against the pads, so the finished surface ends up true - and, better still, the edges of adjacent boards are at the same levels.

If possible, get the tapered-edge plasterboard, as it's easier to get an 'invisible' joint. If you're plastering the finished wall anyway - don't bother. Use the proper tape on all joints - otherwise they'll crack open (after you've decorated!)

Hope this helps Adrian

[Phil L]

Plaster isn't a good idea as an adhesive, dry wall adhesive is made for this purpose and most normal plaster has the words printed on the bag, 'not be used as a dry lining adhesive'

[Adrian]

You're right, of course..... sloppy language - should have said 'plaster' - that's what they always used to call it at BG....

The 'official' instructions (at the time - and we're talking a few years back) required that each sheet was also given a mechanical fixing with a 'frame fixing' - apparently in case there was a fire, and the poly foam melted, allowing the plasterboards to fall off the wall.....

I remember very well trying to get these darn fixings into the wall - in the days before I had an SDS !

Regards Adrian

[The Natural Philosopher]

If the walls are crumbly and uneven, I'd build a stud wall inside.. 2x2 studs, 400mm centers, off a base plate and ceiling plate.. levelled up with packing and fastened with long screws into the stone..using rawlplugs rammed WELL home.

Then infill with 50mm celotex, and plasteboard over.

[David in Normandy]

The walls are certainly uneven and crumbly in places. I don't think I'd trust adhesive / plaster to stick to the walls. Looks like the stud wall solution would be best with SDS drill at hand to put in some long screws - it is pot luck when drilling if I hit clay, sandstone or granite, small rocks through to large boulders.

I forgot to mention in the original posting that I'd already bought a big pile of plasterboard, so using one of the pre-foam backed alternatives isn't an option.

I just googled 'Celotex', it seems there are several different types and this is the name of the manufacturer. Is it some sort of rigid foam insulation? The manufacturers are a bit coy about the price implying it is expensive?

[dom]

Manufacturers advice:

the pdf advice sheets.

Knauf drywall also have good advice.

[robgraham]

I had to do this for a whole cottage - 1800's farm cottage in Scotland, refurbished in 1928, suspended floor with oodles of ventilation and lath and plaster on the walls with all the ventilation passing up behind the plasterwork into the roof space !!! COLD !

So we stripped off all the plasterwork, making sure none of it went down under the floor, new 2 x2 studding, rock wool bats, and very importantly a polythene membrane on top of the studs before the plasterboard. It is essential this membrane as any moist air passing through the insulation will condense if the stone wall is cold and saturate the insulation. This was done 20+ years ago and has been entirely successful.

[dom]

It's cheap, see some prices here

rigid foam with foil facings. For your application you could use sheets butted together and jointed with foil tape behind stud, and either vapour shield plasterboard or a separate membrane behind the plasterboard.

All the product numbers are misleading. There really isn't much variety, just thickness and sheet size.

[David in Normandy]

Excellent tip about the membrane. I hadn't thought about that, thanks.

[The Natural Philosopher]

Its polyisocyanurate foam boartd covered with aluminium foil.

Kingspan and Celotex are the two brands I know of. It is ****ing expensive..about a quid a square foot, but its the best insulant bar none in terms of heatloss per unit thickness.

Consider using epoxy mortar and screw fastening if you run into trouble. BUT a stud wall is more or less self supportng so it doesn't need to be as firmly attached as other things do.

[The Natural Philosopher]

Celotex has its own foil vapour barrier, and should be taped over with special aluminium foil tape as well to both hold it to the studs and provide a continuous barrier. Its about twice as good as rockwool at the same thickness.

[John Rumm]

Ecotherm is a cheaper (i.e. half the price) brand for much the same thing...

[mail]

Dear David I have just done precisely this in a cottage in N Wales with 600 mm rubble walls in lime mortar. I chose to ignore the technical advice to put insulation on inch battens to leave a ventilated gap at my own risk and very sucessfully adapted to the following method Apply dry wall adhesive to the plaster with a serrated trowel, say, vertically ditto to the 2" Kingspan TP10 or whatever PU equivlent is cheaper but horizontally Put the board on the wall and tap in in place with a plank and lump hammer drill 12 mm holes - about 5 of them in through the insulation into the stone of the wall (use the stone unless the mortar is not friable) such as to enable the insertion of the 2" diameter pink plastic washer/ plug fixings used for external attachment of boarding to walls and insert the stainless steel nail in with a lump hammer. Hit until it just indents the insulation Next ( and this is good for you as you have the plaster board) put on plaster board and staggered centres in exactly the same way but this time (having noted where the original five fixings were by photo or diagram) aplly another five fixings in different places. Next skim the whole lot next apply drywall top coat vapour check

Then any picture rails architraves etc have to be fixed with really long thin 5" or so screws or glued in place! Chris I have a series of photos if this would be of interest

[George (dicegeorge)]

am i right that one advantage of celotex/kingspan over rockwool is that if any water gets in there it will drain away but with rockwool it would dampen the wook making the insulation useless...

should a drainaway option be left, or is it best to try to make everything watertight?

what about having two vapour barriers as in one of the posts, wouldnt this mean any moisture then has nowhere to escape to and so will linger and rot wood?

is it cheaper to use glue or wooden battens?

there's a coldbridge issue with battens isnt there?

[The Natural Philosopher]

Any damp is a sign that tiou need to strip the while lot of anything and sort it.

Rockwool tends to sag in vertical cavities..

The latter.

No. Any vapour in the wall area can escape via the wall.

? Glue, but its harder on a undeven wall, to get things staright.

Some, but not major.

[David in Normandy]

Thank you for the replies everyone. Much appreciated.

[jk98]

ISTR some sorts of foam insulation plasterboard act as their own vapour barrier too - is it "closed cell" foam or somesuch (as opposed to the "open"ness of say, polystyrene? I used some on our solid brick walls a few years ago was pink foam sheets glued to 12.5mm pboard - dotted and dabbed with pboard adhesive, then when set, added a couple of suitably long frame anchor type things through into walls at abt 6ft to secure in case of fire - no obvious probs and a helluva lot warmer since! :>)) I could take up some unevenness in walls with thicker adhesive dabs as necessary but when I did it no more than a couple of inches over 2.4m (mixed units are my forte!)