Minimum Reasonable wall width for good U values?

[joe90]

20 minutes ago, Declan52 said:

just @joe90 has seen more summers than me

very polite Declan ???,  most just say “old git” ?

[puntloos]

1 hour ago, Declan52 said:

Both saying the same thing just @joe90 has seen more summers than me so goes by the old school terms.

First coat is just sand and cement. You wet the wall and put this mix on and it's job is to get the wall perfect. Or you use a sheet of plasterboard and dot dab. Both methods are just used to give you a flat surface to put the skim on. You can't put skim on block wall. Just won't work.

2nd coat is the skim. This is just a powdered mix that gets mixed with water in a large bucket and ends up like cream. This goes on top of your sand cement or plasterboard.

 

Thanks @Declan52 for explaining it to a newbie.

 

Last very dumb question then, what's the purpose of skim (since it's such a generic word it's hard to google). I was expecting 'skim' to be 'paint'? But assuming that "sand+cement" already gives a nice flat surface, why apply another layer of "cream" (assuming it's not color?)

Edited March 21, 2021 by puntloos

[Declan52]

Sand cement is rough looking. You would have seen it used as a finish on the outside of many a building where it's just rubbed up to give it a sort of smooth finish. In reality it's not but for outside it's fine as it's mixed with sand grains so will never be smooth. 

Skim is like glass when it goes on as it's just a powdered mix so no fine aggregate. Its this that you paint to give you your final appearance.

[Iceverge]

5 hours ago, puntloos said:

18 hours ago, Iceverge said:

ditch the plasterboard. rubbish for airtightness, 

 

Is it the 'plaster' or the 'board' part that's rubbish?

 It's the plasterboard, sheetrock drywall whatever you want to call the sheets of gypsum that attach to the wall. It does not seal the porous blocks and leaves draughts blow through any openings for sockets and behind skirting etc. Plaster typically is a type of paste or slurry that is wet applied with trowels and dries on the wall. 

 

5 hours ago, puntloos said:

Greenhouse effect is very high though - strange? I seem to remember concrete/cement isn't great?

 

Not at all. Lots of energy required to manafacturer cement and concrete off gasses CO2 when curing. If you want to have a low CO2 house you should build it out of plant material like wood and hemp. 

 

 

[puntloos]

On 21/03/2021 at 16:06, Declan52 said:

Sand cement is rough looking. You would have seen it used as a finish on the outside of many a building where it's just rubbed up to give it a sort of smooth finish. In reality it's not but for outside it's fine as it's mixed with sand grains so will never be smooth. 

Skim is like glass when it goes on as it's just a powdered mix so no fine aggregate. Its this that you paint to give you your final appearance.

 

Coming back to this, so as a reminder, wall buildup:

 

<internal>
skim 5mm
sandcement plaster 15mm
medium block 100mm
EPS blown beads (with stainless steel wall ties perhaps) 200mm
medium block 100mm
sand cement render 15mm

<outside>

 

My quesion:

Would one put e.g. K-rend on top of the sandcement? From the k-rend site I should *replace* the sandcement with k-rend.. 

 

 

 

 

 

[Iceverge]

You might have to look into getting ties. I got mine from http://www.vartryengineering.com/products/extra-long-wall-tie-large-cavities.html via a builders merchant here in Ireland. I don't know if they'll supply the UK. 

 

Yup. Sounds good. K rend is normally either/or as i understand it.
One option I didn't consider was getting coloured sand and cement. 

A friend did their's in white and it looks painted. It cost only about €7-800 extra in materials over normal sand/cement. 

It did require a nap finish though to waterproof it. 

[Declan52]

2 hours ago, Iceverge said:

You might have to look into getting ties. I got mine from http://www.vartryengineering.com/products/extra-long-wall-tie-large-cavities.html via a builders merchant here in Ireland. I don't know if they'll supply the UK. 

 

Yup. Sounds good. K rend is normally either/or as i understand it.
One option I didn't consider was getting coloured sand and cement. 

A friend did their's in white and it looks painted. It cost only about €7-800 extra in materials over normal sand/cement. 

It did require a nap finish though to waterproof it. 

You can use all different colours of sand plus white cement to give you a different coloured finish.