New build design stage, a few questions :)

[tycho]

Hi all, hoping to get a few answers here as I've been reading quite a lot of contradictory information on the net over the last while!

The house:

• New build, single story
• Masonry (block-on-flat) + EWI
• no insulation or stud inside
• 150mm floor insulation on top of slab
• 70mm floating screed with UFH

Questions:

1. What would be the normal thickness for internal plastering? Ive read 15mm scratch coat + 4mm top coat?
2. I was advised to put in a scratch coat before the floor insulation+screed goes in (to make the walls airtight right down to the slab), is this advisable?
3. Is 70mm screed thick enough for UFH? Possible with sand and cement of liquid screed more suitable?

Any help would be much appriciated,

Cheers

 

[Deleted member 23452]

What did your architect specify on the building control submission?

 

[Dansouthcoast84]

internal plastering is 13mm nominal for hardwall and skim

if dot and dab allow for 25mm.

usually plaster after screed. the perimeter floor insulation will help with air tightness.

you cant plaster below dpc .anyway or youll risk.rising damp

if sans cement 70mm is plentys.
liquid is 50mm

 

[Nicm]

Architect should have all this covered.

 

[tycho]

internal plastering is 13mm nominal for hardwall and skim

if dot and dab allow for 25mm.

usually plaster after screed. the perimeter floor insulation will help with air tightness.

you cant plaster below dpc .anyway or youll risk.rising damp

if sans cement 70mm is plentys.
liquid is 50mm

Thanks very much 13mm sounds good, just needed a ballpark really for my final wall thicknesses, Cheers!

The first two course of (thermal) blocks are wrapped in DPC and the DPM is below slab (which comes up and joins to DPC outside), so i'd be fairly confident there would be no dampness at slab level inside, but you made me think about this further:
The scratch coat extending down below floor level to the cold slab will definitely act as a thermal bridge! This is something I really want to avoid.

I will prob just use some airtightness membrane around the perimeter instead.

thanks for your input

 

[Dansouthcoast84]

Thanks very much 13mm sounds good, just needed a ballpark really for my final wall thicknesses, Cheers!

The first two course of (thermal) blocks are wrapped in DPC and the DPM is below slab (which comes up and joins to DPC outside), so i'd be fairly confident there would be no dampness at slab level inside, but you made me think about this further:
The scratch coat extending down below floor level to the cold slab will definitely act as a thermal bridge! This is something I really want to avoid.

I will prob just use some airtightness membrane around the perimeter instead.

thanks for your input

your finish floor level should be at internal dpc height? when you screed there will be an upstamd of 25mm celotex against the wall this will help thermally and for expansion.
you can not plaster below dpc nor will it stick to any dpc membranes

 

[tycho]

your finish floor level should be at internal dpc height? when you screed there will be an upstamd of 25mm celotex against the wall this will help thermally and for expansion.
you can not plaster below dpc nor will it stick to any dpc membranes

Yep I agree with you, no plaster below floor level. I will just run 150mm of airtightness memebrane instead stuck to wall.

I think I will have to choose narrower upstand insulation though if the plaster is 13mm total thickness....