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    My plastererer friend wants to use dot and dab, I want to use foam....

    I've hacked off all the blown plaster exposing the brickwork and stonework...

    With foam I assume you just brush off all the dust, wet the wall and then foam on?

    What is the preferred choice?

    And will 9mm PB be sufficient for walls to replace the original plaster?
    Two points
    1, if you have hacked off all the old plaster do you have an air tight layer somewhere - or to use Tonys expression will you create a plasterboard tent?
    2 If the plasterer friend is going to do the job and wants to use dot and dab then I would let him use what he is happy using!

    BTW 9mm PB can be a bit flexible, 12mm is much stiffer. What does the plasterer friend recommend?
    • CommentAuthorDarylP
    • CommentTimeApr 5th 2017
    9mm is too thin on a 'brick and stonework' substrate IMO, unless very flat/level/dubbed out?
    12.5mm every time, sometimes 15mm even.
    dot / dab is out-of-date, a continuous ribbon of foam adhesive plus breaks in the edges to avoid air-flow.
    Look on YouTube for the current best-practice?
    If I go any thicker than 9mm it will stand proud of the existing plaster and also the coving.....

    I'm happy to use foam, but will it stick to stonework?
    • CommentAuthorDarylP
    • CommentTimeApr 5th 2017
    Adhesive foam sticks like the proverbial to a Salvation Army blanket.
    ...look for DryFix or similar, and ensure you have a decent gun.... :bigsmile:
    • CommentAuthorowlman
    • CommentTimeApr 5th 2017
    I've found on uneven walls dri-wall adhesive is better than foam because its got more "substance" it allows better positioning and tamping straight with a straight edge. Foam can sometimes "collapse" on such walls This I would think would be better in your case especially using only 9mm PB. You can of course use both as described above.
    So let the plasterer use what he's happy with; as others have said.
    • CommentAuthorwookey
    • CommentTimeApr 5th 2017
    Foam can be used to segment the wall and largely prevent any airflow behind the panels. D&D doesn't do this leaving all airflow free. This is the ost important advatage IMHO. It's true that it's quite a lot harder to get the edges to line up perfectly than with plaster adhesive and you have to hold it in place much longer whils tthe foam sets. It sticks ridiculously well to everything so no need to worry about stickiness. You do need to brush loose material off, obviously. Adding water is good as it makes the foam cure faster so you don't have to hold the panel for as long.

    I'd generally avoid 9mm on unplastered walls as it's very flexible - it really needs to be going on to battens (or smooth plaster). If you've got plaster you are trying to match up with then use wet plaster and do the job properly! That's better airtightness and a more solid wall for fixing things to.

    Use the glue foam, as opposed to the filler foam - it expands much less so doesn't push your panel off the wall so it goes all wonky (well, it does, but rather less :-). I assume it's a better glue too, although the difference in that regard is not too obvious. Use a proper gun.
    ''So let the plasterer use what he's happy with; as others have said.''

    But not if it's really 'dabs'! Yes if it is a full perimeter bead and 'stripes'. Do take the advice re parging, too, for air-tightness.
    • CommentAuthorowlman
    • CommentTimeApr 5th 2017
    The term "dot and dab" shouldn't IMO, be taken too literally.
    If dri-wall adhesive is applied correctly it is applied in continuous perimeter ribbon and continuous vertical beads at intervals, likewise around socket outlets, so the question of airflow is drastically mitigated and is no more than with ribbons of foam. The suction, once the board is applied is very great so no slippage and holding in place while adhesive goes off.
    • CommentAuthorowlman
    • CommentTimeApr 5th 2017
    Drat Nick ya' beat me to it.
    Agree with you, owlman, that no-one should really dot and dab anymore, and that even when they *say* it, they should mean the full-perimeter bash, but they do still do it - again and again -' 'cos it's what we've always done'. Argghhhhh!
    • CommentAuthortony
    • CommentTimeApr 5th 2017
    Problems when not done right here and how to put them right
    • CommentAuthorowlman
    • CommentTimeApr 6th 2017
    To put things in perspective I guess we have to differentiate between interior and exterior walls, and between homes with solid and suspended floors.
    For an good interior wall on a concrete floor dot and dab,- literally, possibly wont have much consequence,- it's still the wrong methodology though.
    On an exterior wall with suspended, leaky floor, and possibly ceiling. Well, maybe you ought to fix that first before any form of dry lining.
    As a wall finishing technique I have nothing against dry lining, and depending on what type/design of interior you want to achieve it can have aesthetic benefits over wet plaster finishes.
    ''On an exterior wall with suspended, leaky floor, and possibly ceiling. Well, maybe you ought to fix that first before any form of dry lining.''

    And don't forget that this may apply to internal walls too, where they continue down to a footing. One of the remedial jobs I did a few years ago was to a 1990s extension on a 1980s bungalow. The original bungalow had had retrofit CWI, but not till after the extension was built, so the rear wall of the original house (now the internal wall between the original house and extension) was not filled.

    The suspended floor of the extension was *hugely* ventilated (it's a fairly high radon risk area) - IIRC 12 225 x 150 air-bricks in about 15m2. The original air-bricks for the ventilated cavity of the original rear wall were left open, and were now below the extension floor. *And* that wall was dot 'n' dabbed. *And* the cavity was not closed! The entire wall, in 2 layers, was a big passive stack.

    The purpose of the job in the first instance was to address the dot 'n' dab leakage issue on te 3 external walls, but it was only as we de-constructed that we realised it was equally necessary on the internal wall!
    I like foam but, as mentioned earlier, the thicker the gap it has to fill or the greater the irregularities, the harder it becomes to get the PB level/straight etc. In fact the only safe way is when the gap/wall is such that you can spray the PB let it go off a bit ie complete its expansion and then push it up against the wall - so say less than 10mm avg finished gap. When the wall isn't good enough for this I always screwed up but now I knock in EPS mushroom head fittings into the wall BEFORE the PB, then wet the wall and spray lots, and spray the PB and even mark on points on good side where the really deep gaps are, then put the PB up pretty much immediately and brace against the fittings (which are obviously level!) with wood and at the bottom weights, putting in halfway PB screws for the braces if needed). Then watch like a hawk until set and then drill a 6mm hole for the deep gaps and put a little more in if required. 9mm is too flexible for this period. In this way 'really' bad walls can be PB'd or IWI'd without batoning.
    I like it, GNL!

    However, back to the OP: ''I've hacked off all the blown plaster exposing the brickwork and stonework... ''

    I think he needs a parge coat (or rather his walls do!) for air-tightness. Get that level-ish (+/-6mm-ish) and you can foam onto the parge coat.
    Best foam to adhere to stone?
    Insta Stik seems good for most things, but 'No Nonsense' do one which seems to work OK. How 'bumpy' is your stone. If you are not using a parge coat, are you certain that you can break the air-space behind the board up into several small 'compartments' without 'rumples' in the stone surface connecting them?
    • CommentAuthorRick_M
    • CommentTimeApr 6th 2017
    How do you create ribbons of plasterboard adhesive? I wonder if a repointing gun would do the job or is troweling it on quicker?
    I'm not too bothered as it's all internal walls....
    • CommentAuthorowlman
    • CommentTimeApr 6th 2017
    Posted By: Rick_MHow do you create ribbons of plasterboard adhesive? I wonder if a repointing gun would do the job or is troweling it on quicker?

    If you're referring to dri-wall adhesive, then with a hawk and a trowel. Fill the trowel about 1/3 full and go along or up the wall until you have a continuous bead / fat sausage. It takes a little practice, but you soon get the hang.
    • CommentTimeApr 6th 2017
    Posted By: VictorianecoI'm not too bothered as it's all internal walls....
    Maybe you should have mentioned that at the beginning:devil:
    ''I'm not too bothered as it's all internal walls....''

    What's at the bottom of the wall? Suspended or solid floor? If suspended, see my post above. A/T is not always just for external walls.
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