What plaster finish where?

[SuperPav]

Happy new year everyone. 

 

As we keep our fingers crossed that this never ending nightmare of a build will actually progress this year (builders showing up), starting to think about what plaster finish to put on various parts of the house. It's probably best to tackle bit by bit:

 

1) Downstairs - existing external walls (solid stone/brick, no cavity)

We plan on stripping this back to the brickwork everywhere, and then ideally the plan is to put a 25mm +12mm insulated plasterboard. There are some areas where we can get away with a slightly thicker layer of insulation, but running stuff through the calculator, it's diminishing returns after 30mm. + trying to avoid too much risk of condensation.

My question is around sticking this stuff to the wall. The last house we had done with dot and dabbed PB (on a cavity wall granted) made a mockery of any cavity insulation, as it was literally blowing a gale from any opening in the PB + some boards became slightly loose as I don't think they were stuck on properly. Would a slurry coat on everything first solve all of this problem? Or attaching with squirty foam with full perimeter cover on every board so there's no cross flow?

 

Something like marmox board (which incidentally can be rendered/plastered directly but don't think anyone uses it as insulated PB for some reason?) recommend using basically tile adhesive to attach the board to the wall, instead of Dot and Dab. I can see how this would reduce the air leaks if it's total coverage, but how practical is it? 

 

Maybe I'm overthinking it a bit, and having a wall which has no open cavity will naturally reduce air leaks, but I really don't want to end up with the same quality of PB as in the previous house.

 

2) Downstairs existing internal masonry walls

I don't have a particular preference on these to be honest - would probably prefer wet plaster, but have no rational explanation as to why.

 

3) Upstairs - new walls (inner leaf is 7.2N Celcon blocks thin joint)

I would really prefer a wet finish here, but have heard horror stories of plaster not sticking to celcon aerated blocks very well. Has anyone had any experience of this? 

The alternative would again be PB, but the same concerns as in the first point apply, although the joints are much smaller and the cavity will be full fill pumped Icynene so should seal most of the air leaks?

 

Unfortunately speaking with the current builders who have a fairly lax attitude towards airtight detailing/thermal bridges/moisture control hasn't given me much confidence so it's not just a case of asking them "what do you normally do"?

 

The bathroom upstairs would be marmox insulated cement boards directly fixed with adhesive to the blocks, and then tiled onto. Internal stud walls will be drylined and skimmed. 

 

 

 

Lastly, has anybody used marmox boards for ceiling instead of PB where fire resistance is not required (i.e. the roof)? The upstairs bedrooms are vaulted ceilings, with 200mm wool between rafters. I would either put 25mm PIR + 12mm PB on this, or the alternative would be to just whack on 30-40mm marmox boards, which will kill two birds with one stone + be much lighter than the PB saving my back a lot of pain

 

All dry boarding will be done by us to save money, but we'll be getting a plasterer in for all wet work/skimming. 

 

 

 

[nod]

Your not to far off with downstairs 

I would give all the original masonry a coat of 3-1 S&C Then dab your 37 mil insulated boards Pink foam only You can achieve excellent airtightness with dot and dab 

if done as pictured 

Thermal blocks are not really suited to wet plaster Unless you use HSB backing 

You will have to order that in 

As a Tiling business I use lots of Marmox 

But you can’t plaster it So I definitely wouldn’t use it on ceilings 

Your slopes are better with PIR insulation Leaving an air gap Then insulated plasterboard on the rafters 

[SuperPav]

Thanks @nod

 

When you say dab the insulated PB with pink foam only - do you mean adhesive foam (Soudal or similar) as opposed to the pictured d'n'd adhesive? 

 

Do the thermal blocks need any prep for d'n'd? In the past we've used the blue gritty prep coat before plastering on poor/old surfaces and that stuff was pretty impressive. 

 

Also, I'm interested in the comment about marmox board - they specifically state that it can be plastered, and we've done that in the past on window reveals to continue insulation into the existing openings (with 12mm Marmox) and it took a skim absolutely fine. I've never used it in larger sections, and wouldn't use it on walls as I have a (perhaps unfounded) concern that the impact resistance would be quite poor.

 

For a sloped ceiling that would never get touched by anything, I would've thought it'd be perfect?

[nod]

No sorry 

I mean the foam on the back of the boards needs to be pink

Tge others beads mechanical fixing 

No block work prep needed