Insulating internal solid walls - remove plaster?

Discussion in 'Eco Talk' started by Flame, Aug 11, 2017.

  1. Flame

    Flame Member

    So my house has solid walls and is generally cold, damp patchs can occur if i leave things up against walls ect, although the windows are bad at the moment too, in the porch you can wipe the wall with a towel during the winter and it'll be soaked in no time, luckily we never go in there :p

    so my question is this, as i decorate and do up the house i would like to insulate with thermal backed plasterboard, to save space in smaller rooms like my bathroom, can i remove the current plaster on external walls, 20mm thick in places if not more & dot and dab the thermal stuff directly to the brick? It seems like the common sense thing to do however most illistrations online show Brick, Old plaster, Battons/dab, thermal. rather than brick, dab, thermal?

    I'm just wondering if there's a reason for more people not removing the old plaster, other than the mess?

    also, with regard to the vapour barrier, is this built into the thermal plasterboard, or would i need to stick this to the wall after, before dabing?

    I'd rather spend the money now than have to replace wallpaper & tear everything up over and over?

    any info regarding this would be great, have read through a few other posts and replys too, sigh. can't we have ever lasting sun?
  2. What I would do in your position is to remove the old plaster.

    Reasons for this are, yes, to gain space, but also to remove what could be failing bonding - but this depends on how old your house is (eg near-Victorian).

    There's another reason I'd do this too - I would seal the wall with tanking slurry (this is brushed on) to make certain no damp comes through from the outside, or from rising. That surface will then be suitable for D&Ding the insulated p'board on, and this should transform the insulation value of your house.

    One exception to sealing walls is if the house is really old and has solid walls with no DPC - these are 'designed' to breath out any rising damp.

    If space is at a premium, there's no need to go overboard on insulation thickness - even the thinnest stuff will make a HUGE difference.

    Others will be along to advise whether this is good advice... :)
  3. Flame

    Flame Member

    DA - I've seen your advice all over these forums and always see it as good advice :)

    The house is 1920s and that's another good point you've made regarding the bonding, a fair few places you can hear the plaster is no longer stuck to the wall by tapping, so will make sense to remove again.

    I've done a little googling on this tanking slurry and it is something i will use too, does this act as a vapour barrier? & also, when applying, would you go all the way down to the floor or leave an inch or so at the bottom? (i appreciate that may be a stupid question:confused:)

    Thanks again for the advice DA, much appreciated!
  4. Flattery will get you nowhere.

    But I'm a softie for it all the same... :oops:

    If there is any suggestion that your existing bonding is reaching the end of its life, then it doesn't make sense to adhere boards to it - it's gonna fail at some point. Also, if there is any penetrating or rising damp, even a small amount (there might not be, of course), then that bonding layer will just act as a sponge for it as it will no longer be able to 'breath' out that vapour when the insul-board is put over it, and it will decay even faster.

    It is a very messy job, especially if the bonding is poor and crumbly - horrible smelling dust - so take all precautions - mask, goggles, full overalls, and tape all door cracks to rooms you don't want dusty. Open windows in room you are working in.

    Yes, tanking slurry is specifically design to fully waterproof walls. There are different types of tanking, bitumen based etc., but I've read good things about the cementous 'slurry' types although I haven't used it myself. I'd imagine that it also would make an ideal surface for D&Bing on to, whereas you might have issues sticking to a bitumen surface.

    Yes, slurry right to the very bottom - and down to the DPC course under the floorboards too if you can. If you were to leave any part of the wall unslurried, then that should be the very top section. So, in case you run out, start at the bottom!

    I understand that insul-p'board does also act as a vapour barrier, but that might be some types, not all - so check. I personally would be tempted to use barrier type too, to be sure to be sure.

    As for your porch, that sounds like a lack of ventilation. Does it have opening windows? If so, try cracking them to 'vent' setting and I think you'll find most of the dampness will go. Or even try leaving the porch's front door open (as long as your house is secure) - that should also work.

    Yes, it'll still be freezing out there, but it should be dry :)

    NB - that is what I would do if it were my house, but I am not a pro, and someone might come along and advise differently for good reason...
    Last edited by a moderator: Aug 12, 2017
  5. "...and down to the DPC course under the floorboards too if you can..." but not over or beyond the DPC line.
  6. Bob Rathbone

    Bob Rathbone Well-Known Member

    Hi Flame. I have done this job on two houses, my brother's and my own. Bothe were 9" solid brick construction and suffered the problems you describe. I did not use insulation backed boards as it would have needed the removal of the old plaster. Instead I used Tiling lath, 2" x 1" and 1200 x 2400mm boards with insulation between and a vapour barrier over.
    This is how it's done.
    Assuming the plaster is not actually falling off,
    Fix the battens to the wall with 4 or 5 good screw fixings at 600mm centres. At this point install any additional switches and sockets using 25mm deep boxes fixed to the original surface.
    Cut the 25mm thick insulation to fit between the battens and put it in place.
    Fix a vapour barrier over the whole wall using a staple gun and a suitable PVC sheet.
    Overboard and plaster to suit.
    The old plaster will not fall off as it will be supported by the battens.
    This is a good DIY solution.
  7. dobbie

    dobbie Well-Known Member

    Why would you use 25mm boxes? 35mm boxes will be just under the face of the plasterboard and the correct depth if using 2x1 roofing batten and 1/2" platerboard.
  8. Bob Rathbone

    Bob Rathbone Well-Known Member

    Thanks Dobbie, yes, that would be OK, but you will have to be very accurate when you cut the hole to coincide with the box. With the 25mm deep box, you just make a small hole for reference and cut it to size when the board is in place, works every time. Don't forget the grommet and the earth on the box. Or you could use paramount boxes.
  9. dobbie

    dobbie Well-Known Member

    Not just OK but the correct way to do it.The boxes are not meant to be behind the plasterboard therefore you should fit an extension box on the 25mm box.

    What do you mean by paramount box?. A dry lining/fastfix box or an MK box with the little metal wing inserts.
    As far as I know Paramount partitions are now no longer used.
  10. Bob Rathbone

    Bob Rathbone Well-Known Member

    The regs require that no gap is left between the box and the plasterboard, my suggestion leaves no such gaps, the enclosure is completed by the plasterboard which will not burn, any fire is contained within the enclosure, regs satisfied. Either way is acceptable but I do accept that the deeper box will give certainty in fulfilling the requirements of the regs. It really depends who fits the plasterboards, if its a plasterer deep boxes are the safe way. Paramount Box = dry lining box.
  11. Flame

    Flame Member

    Although it has been a while i’m back with a few more questions, after more reading internal insulation seems to open up a can of worms.

    My bathroom is currently;

    Solid brick

    & is very cold causing plenty of water to accumulate on the tile.

    My plan is to;

    Remove plaster & tile (i’m quite confident they’ll both be coming off in big chunks together as i imagine the bonding to be poor)
    Polystyrene between battons (space saving)
    Aquapanel, sealed together
    Tile (floor & floor to ceiling)

    My worry is that moister will still settle on the cold brick between the polystyrene making that less effective, battens soak in the moisture and swell, popping off the tiles?

    Am i worrying to much or should this be ok? The alternative for me is to batten without polystyrene and accept a cold house/room but a lot if moisture will be settling on the tiles.

    I’ll be doing this job mid jan now, so i’ll be buying up the materials over the next few weeks - any extra advice or help would be much appreciated. Or just to put my mind at ease
  12. Mr Rusty

    Mr Rusty Well-Known Member

    Take all the old plaster off! It'll be 20-25mm thick. D&D insulated plasterboard direct to the brick. Unless your walls are perfectly square and flat battening is just a dogs dinner. Insulated PB with say 60mm insulation is megaflat. D&D it goes up like lego bricks in no time. Nice deep insulation and not so much loss of space. The insulation backed PB incorporates a vapour barrier.

    Just in the shower area you could batten/insulate and then board with tile backer/hardie backer which is waterproof. Everywhere else insulated PB is the way to go.

    Not only that, if its been damp, taking the old plaster off will give it a chance to dry thoroughly. In most cases damp will be condensation, not "rising". That old plaster will be all crumbly if it's been at all damp.

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