The mystical parge...and how do I do this bit?

[mvincentd]

I've thermalite blocks for an inner leaf of cavity construction.  I'd planned to dot and dab then skim.

Website wisdom says while 'correct' d&d procedure is supposed to achieve airtightness one shouldn't just make a plasterboard tent, ....parge your blockwork.

All tradesmen and merchants so far have said "uh, parge, wassat then?"  Knauff Parge Coat or Gyproc Soundcoat isn't stocked anywhere locally.

It's all adding up to suggest nobody bothers with this step....do I have a false impression or misinformation or what?

 

I'm now looking at achieving both the parge and my final surface by using Onecoat plaster, thereby deleting all dot dab plasterboard and skim. If I use a plasterer he'll be half the time/cost of skimming...but even I can Onecoat, though i'll never learn to skim.  The tradeoff with Onecoat is the quality of finish not being as smooth as skim, but for me this is actually a desirable, I want texture....and so far in tests we've got the onecoat so smooth that i've added back texture with a paint roller.

It's almost too good to be true!

So what am I missing?

 

Detail question- At my window reveals I think I need to introduce plasterboard.  As pictured below my Ali framed windows are packed off the blockwork reveal by a good 20mm (weird I know-Idealcombi Futura windows) and I have airtight tape bonded from frame to blockwork( or in the case of the head; frame to Catnic lintel). On top of wall and lintel I have 4x2 timber wallplate that the joists sit on.  For Onecoat should I mesh the timber as pictured and would it be wise to let that mesh run down and under the lintel....either to go behind pb that I then d&d on, or to go onto pb that I d&d on before the mesh?  I'm looking for the onecoat to have a 'soft' corner where walls turn into reveals, so don't want to introduce a hard edged bead.

 

 

[nod]

Parge coat is a waste of money 

Making surer that the blockwork is pointed up correctly 

If the dot and dab is done correctly there is no need 

99% of the Parge I use is specked for party walls For soundproofing

As most sites insist that the blocks are pointed and bar jointed 

 

One coat plaster is for DIY and best applied in two coats 

It’s basically projection plaster 

If your not to bothered over the finish 

It will do the job Though quite exspensive 

 

you can bullnose the hard angles 

by lapping a fine mesh from the plasterboard Over the corner and fixing it to the block 

Two coats of plaster Seperate sets 

 

[bassanclan]

Sound coat you can get from wickes I think.

 

You can use a sloppy cement and sand mix and brush it on with a sweeping brush

[Declan52]

Make sure you soak the blocks before you start.  Thermalite blocks are very dry and will suck the moisture out of your plaster and make it dry out to quick and it will crack really bad. 

[Miek]

Are you chasing in your electrics on the thermalite? If so you want to do this before your air tight layer.  You can use air tight paint here (blower proof for example). 

 

The parge coat as I understand it is nothing more than a cement (and sometimes sand) water slurry which you can brush on to the block surface. You can omit this step if you wet plaster the block work since wet plaster is considered air tight. 

 

Lightweight block is infamous for cracking, I would read up on best ways to mitigate this before you plaster. 

 

[scottishjohn]

1 hour ago, Miek said:

Are you chasing in your electrics on the thermalite? If so you want to do this before your air tight layer.  You can use air tight paint here (blower proof for example). 

 

The parge coat as I understand it is nothing more than a cement (and sometimes sand) water slurry which you can brush on to the block surface. You can omit this step if you wet plaster the block work since wet plaster is considered air tight. 

 

Lightweight block is infamous for cracking, I would read up on best ways to mitigate this before you plaster. 

 

yes if you don,t have enough expansion breaks in a wall

very expensive way to parge coat using (blower proof )--

sand and cement  mix  would be my choice if not hard plastering inside 

Edited February 4, 2019 by scottishjohn

[MarkyP]

is it too late to consider wet plastering? It's a parge and finish in one.

 

Also even well pointed blockwork is still going to have hairline gaps in places in the joints. If you have to dot and dab I would use a parge coat. Prime the blockwork with PVA solution first, it's a nightmare trying to soak thermalites as they will take an amazing amount of water. Sand and cement wet slurry with fibres or even a thin skim of bonding or hardwall would do as a parge I think.  But even with a parge, if (when) the blockwork cracks then you'll get a potential air leak behind the boards.

 

with wet plaster, any cracks that do appear can be seen and addressed. You'll never know what's going on behind the dabbed boards. 

 

 

 

[MikeGrahamT21]

Having had a room done with dot and dab on celcon blocks, I can only recommend not doing it!! If I knew what I know now, it would be wet plastered, yes its more work for the plasterer and will cost more and leave a bit more mess, but its worth it in the long run.

 

I now have infront of me much drilling and many cans of expanding foam to try and salvage it, as heat loss is immense, despite having enough insulation to achieve around 0.18U

[mvincentd]

Yes am applying one coat in 2 coats.....seems necessary to eradicate block lines.

Am using SBR on therms before.  Has been built a year and had heavy green roof sitting on them and lots of stomping about....hopefully pretty settled, very few cracks so far.

Blower proof had been considered but is a very expensive option.  Anyway am effectively wet plastering now, with onecoat.

Choosing onecoat as doesn’t seem to have any negatives apart from people’s perception of it as being amateur diy material....it works and gets me the texture I’m after.  Materially perhaps a bit more expensive than scratch and skim materials but I don’t need the plasterers skills.

[scottishjohn]

I saw a system on internet that was made to seal up leaky houses 

it was basically a spray system that you set going whilst adding positive pressure to inside of house and the very small particles  in the airstream migrate  to the gaps  and block them up  and as it was a nice red colour you could see where it was going 

a bit like a "radweld " for your car .

whilst I,m not saying it is the best solution ,it shows how many people have problems for someone to make such a system 

Edited February 4, 2019 by scottishjohn

[Miek]

37 minutes ago, scottishjohn said:

I saw a system on internet that was made to seal up leaky houses 

it was basically a spray system that you set going whilst adding positive pressure to inside of house and the very small particles  in the airstream migrate  to the gaps  and block them up  and as it was a nice red colour you could see where it was going 

a bit like a "radweld " for your car .

whilst I,m not saying it is the best solution ,it shows how many people have problems for someone to make such a system 

Aerobarrier

 

I like the radweld analogy better be a LOT better than radweld though!