Plastering-slabs versus sand/cement

Mrs Doyle

I am building a new house which is due to be plastered shortly by a relation. It has a 200mm cavity with underfloor heating. The intention was to use the internal wall as a thermal store, and to do a sand/cement finish internally.
Now as the top cavity closer is inside the shell of the house and there is a step into it, the plasterer has suggested using a slab here (not insulated) as there is a chance a sand/cement finish would crack. This I get and I am willing to do on the top exterior walls.but he has suggested to do the whole house this way. Is there any reason I shouldn't? I guess I am concerned about any mould growing, loss of thermal store etc -he has tried to assure me it would make no difference, but I am unconvinced!

BryanF

Air-tightness

Stone Deaf 4evr

Are you more concerned about cracks or mould? It's two separate issues as far as I can tell.

Fir starters, your cavity closer should not be making a cold bridge into the house directly. And an uninsulated slab is not going to remedy this either.

Also, when you say sand and cement finish internally, are you talking about a finish smilar in texture to an external wall, or are you talking about one coat of sand and cement followed by skimcoat? If so, gypsum won't stand over the skim if there is any issues down the line. They reccommend using one of their own products called hardcoat. In my experience, it's a lot easier to apply and shrinkage / settlement cracks are greatly reduced.

Stone Deaf 4evr

BryanF wrote: »

Air-tightness

Exactly this as well, theres no substitute for a wet finish.

Stone Deaf 4evr

BryanF wrote: »

Air-tightness

Exactly this as well, theres no substitute for a wet finish.

Mrs Doyle

When u say 'air tightness'-are u recommending sand/cement?!
Yes cracks and mould are 2 separate issues-I am asking the question IF u can get mould behind a plasterboard-I don't know!
Yes the wall wud be skimmed after.
I am asking which is the best option if you had a choice-very green here!

DOCARCH

Sand/cement and plaster skim finish is the best (only) option...for airtightness.

Stone Deaf 4evr

Mrs Doyle wrote: »

When u say 'air tightness'-are u recommending sand/cement?!
Yes cracks and mould are 2 separate issues-I am asking the question IF u can get mould behind a plasterboard-I don't know!
Yes the wall wud be skimmed after.
I am asking which is the best option if you had a choice-very green here!

No, I'm recommending gypsum hardcoat finish which is then skimmed. For airtighness, this will help a lot and should be done in conjunction with taping of windows etc etc.

On mould, think of it as sitting in your car on a cold day. You breathe out and gradually your breath fogs up the glass. This happens because the glass is a direct path between the inside and outside.

Now back to our cavity wall. The cold should never have a direct path from inside to outside. Over windows, at doors and under window culls and even in your floor, there is a strip of insulation that breaks the path for cold to travel through.

If a path occurs, this is called a cold bridge. This point will tend to be colder than others and you will both lose heat through it and also potentially promote condensation / mould growth. An ordinary gypsum slab does not have any insulating properties, so will not prevent cold bridging.

You should consult your engineer on any technical matters you have doubts over.

fozz10

Stone Deaf 4evr wrote: »

No, I'm recommending gypsum hardcoat finish which is then skimmed. For airtighness, this will help a lot and should be done in conjunction with taping of windows etc etc.

On mould, think of it as sitting in your car on a cold day. You breathe out and gradually your breath fogs up the glass. This happens because the glass is a direct path between the inside and outside.

Now back to our cavity wall. The cold should never have a direct path from inside to outside. Over windows, at doors and under window culls and even in your floor, there is a strip of insulation that breaks the path for cold to travel through.

If a path occurs, this is called a cold bridge. This point will tend to be colder than others and you will both lose heat through it and also potentially promote condensation / mould growth. An ordinary gypsum slab does not have any insulating properties, so will not prevent cold bridging.

You should consult your engineer on any technical matters you have doubts over.

that stuff will be taken off the market just like their old stuff was this is just an improved version and its dirt just like the old base coat they sold. its all an effort to get a hold of the sand and cement market that they could never get into as sand and cement is a far superior product. believe me i know what im talking about i ve used the gypsum stuff briefly when they were down trying to show us it to push it on us. they often tried ringing to get us to us it on the next house. desperate to get people using it. not gonna happen here im afraid in this country

Rock12

Hi, I have a sand and cement plaster with a sponge finish on my internal walls and also have a 200mm cavity. The walls are not even but have great character and I have to say the thermal mass is working brilliantly at keeping the indoor temperature from fluctuating and holding heat late into the evening. I have considerable solar gains so this works with the thermal mass and store.

Hope this helps.

BarneyMc

Rock12 wrote: »

Hi, I have a sand and cement plaster with a sponge finish on my internal walls and also have a 200mm cavity. The walls are not even but have great character and I have to say the thermal mass is working brilliantly at keeping the indoor temperature from fluctuating and holding heat late into the evening. I have considerable solar gains so this works with the thermal mass and store.

Hope this helps.

Any chance of seeing a pic this sponge finish?