Wall-Build up

Slates

External wall with the following build-up

Dash
Scratch coat
100mm Conc. block
Cavity
80mm PIR insulation
100mm Conc. block
12.5x38mm insulated slab, dot and dab on internal wall

Does the internal wall need to be scratched before the insulated slab is fixed to it ?????

archtech

depending on how you propose to achieve and what level of air tightness you want

Quazzie

Slates wrote: »

External wall with the following build-up

Dash
Scratch coat
100mm Conc. block
Cavity
80mm PIR insulation
100mm Conc. block
12.5x38mm insulated slab, dot and dab on internal wall

Does the internal wall need to be scratched before the insulated slab is fixed to it ?????

Why not increase the wall thickness by 20mm and put in 100mm PIR in the cavity while your at it. Its much easier, and cheaper to do it now that try retrofit insulation at a later stage. You have already decided to increase the cavity, so why not go to the maximum, 150mm, while you have the opportunity.

Slates

archtech wrote: »

depending on how you propose to achieve and what level of air tightness you want

What is the difference in air tightness between scratched and unscratched ??

Carlow52

Slates wrote: »

What is the difference in air tightness between scratched and unscratched ??

at the risk of being wrong here, I think what u mean is the difference between skimming the interior block before u insulate it and not.
The skim makes the block more airtight as it seals the 'pores'

All external cement based renders will have a scud coat first, then a scratch coat and the the final coat, be it dash or normal plaster.

Slates

Carlow52 wrote: »

at the risk of being wrong here, I think what u mean is the difference between skimming the interior block before u insulate it and not.
The skim makes the block more airtight as it seals the 'pores'

All external cement based renders will have a scud coat first, then a scratch coat and the the final coat, be it dash or normal plaster.

I want to find out the difference in air tightness between scratching the internal wall before putting up the insulated plasterboard and the skimming, or not scratching the internal walls ??

Slates

archtech wrote: »

depending on how you propose to achieve and what level of air tightness you want

So should the wall be scratched out with sand and cement before the insulated slabs are dabbed onto the walls ??

Carlow52

Slates wrote: »

I want to find out the difference in air tightness between scratching the internal wall before putting up the insulated plasterboard and the skimming, or not scratching the internal walls ??

AFAIK the internals are skimmed before the insulation and not scratch coated

I cant help u with the delta airtightness question

see http://www.boards.ie/vbulletin/showthread.php?t=2055620468

sinnerboy

Scratching beforehand should take you from a tendency to acheive Q50 7/6 down to Q50 4/3

BUT BUT BUT

You will trap moisture behind the cosy boards . Don't do it .

MicktheMan

sinnerboy wrote: »

Scratching beforehand should take you from a tendency to acheive Q50 7/6 down to Q50 4/3

BUT BUT BUT

You will trap moisture behind the cosy boards . Don't do it .

Hi SB,
can you elaborate on this. I would have thought that the 80mm cavity insulation would keep the dew point away from the inner leaf!

sinnerboy

The construction moisture won't dry out - unless of course give time
1mm per day

Slates

I was talking to an airtightness expert and he recommends that the wall be sealed with a sand and cement coat before applying the insulated slabs

Carlow52

Slates wrote: »

I was talking to an airtightness expert and he recommends that the wall be sealed with a sand and cement coat before applying the insulated slabs

What exact thickness did this expert specify?
What exact proportions of cement and sand did he have in mind?
What admixtures, if any, did he prescribe?
What finish did he prescribe on the 'coat'?


The reason I ask these questions is that a thin layer of sand /cement will not have much less porosity than the blocks, if the blocks are good quality.

Because of the nature of the particle size in plaster skim, it provides a much higher level of airtightness.

sas

Carlow52 wrote: »

The reason I ask these questions is that a thin layer of sand /cement will not have much less porosity than the blocks, if the blocks are good quality.

Because of the nature of the particle size in plaster skim, it provides a much higher level of airtightness.

Great info. here.

Quick additional question. I've been told that there are gypsum base coat products which are an alternative to the sand a cement layer. Would using this also provide any additional benefit in terms of airtightness in your opinion?

Also, have you any thoughts on how to minimise the effect of sockets\switches on your airtightness in your external walls? Obviously not putting them there is the short answer. There are going to be external corners where you don't have a choice though.

Slates

Carlow52 wrote: »

What exact thickness did this expert specify?
What exact proportions of cement and sand did he have in mind?
What admixtures, if any, did he prescribe?
What finish did he prescribe on the 'coat'?


The reason I ask these questions is that a thin layer of sand /cement will not have much less porosity than the blocks, if the blocks are good quality.

Because of the nature of the particle size in plaster skim, it provides a much higher level of airtightness.

PM me and I will give you the company details and you can ask all these questions yourself