Pumped specialist screed vs sand/cement

[Lin]

Hi

 

On our conversion the building already has a concrete floor. We do not want to remove the floor so have to build up over.

Head height will be an issue.

We intend to use 75mm PIR insulation and then a sand/cement slab of 75mm. The height of the finished floor to the boxing in over the steel in the ceiling will then be 2.2m and the rest of the ceiling height just over 2.4m

Someone recommended the pumped screeds ofr UFH;  said that they are very strong and flat and quick to install by specialist. They can also be comfortably done at only 50mm thick.

 

Can anybody comment on these screeds for being good and strong etc and better than sand/cement?

 

Any other comments welcome

thanks.

 

[PeterW]

@Lin Do you want UFH...?

 

If so, have a look at this build up that both myself and @JSHarris talked about 

You could go with 100mm of PIR, 20mm EPS trays and then the flooring and still be under your 150mm build up but with a much more insulated floor. The downside is you have very little to hold the heat or transfer it to, so using it as an overnight thermal buffer wouldn't be practical.

 

[Nickfromwales]

Hi @Lin. What's the current / intended heat source for the UFH?

[Oz07]

Was suprised at just how strong these screeds are. The flow is superb aswell. I usually use agilia which is good no nonsense stuff however susceptible to cracking if movement joints not in asap. Lafarge do one of these thin screeds when I worked on a job with it was tuff stuff. Only 50mm thick you'd think it would crack everywhere but very strong. 

Edited December 28, 2017 by Oz07

[Nickfromwales]

The downsides with the thinner screeds are the surface preparation prior to bonding anything down ( mechanical abrasion for laitence removal, a considerable amount of work in larger instances ), and then the lack of heat retention and regulation needs addressing and mitigating against, as the thinner screeds warm up quicker, but also cool down quicker too. That leads to over / under shoots in room temp versus what the room stats are being asked to maintain. That needs an Ufh flow temp nigh on linear to the heat loss of the dwelling, to maintain the offset, plus a degree or two on top for comfort. To achieve that need digital room stats with at most a 0.5oC hysteresis to control the heated slabs vs 2oC typical for rads, as rads change the air temp much, much quicker than Ufh does. I'm struggling with this at the moment with my current customer as one day I go there and it's roasting and another day it's chilly, due to the mechanical rotary room stats ( which are soon to be replaced to finally tame things ). 

If your stuck with the thin screed regardless, then it's down to floor coverings and cost. Liquid may be cheaper initially, but beware the prep work if you expect to tile over it afterwards as dry screed would be fit to lay on without any prep. @Lin, is there definitely no option of excavating the existing floors out and doing a much better job of insulating and using a thicker dry screed or concrete slab?

[dpmiller]

isn't it true that the hemihydrate screeds don't suffer from laitance issues?

[joe90]

Well I have just had a pumped screed and it didn’t appear to have a laitance ( scum) on the surface ( they told me it was a new type that didn’t suffer from this). I just need to know what I need to do with it before I start tiling etc. I have got two industrial dehumidifiers running 24/7 and they both pull about 20 gallons of water a day from the air ( floor).

[Nickfromwales]

4 hours ago, dpmiller said:

isn't it true that the hemihydrate screeds don't suffer from laitance issues?

Living and learning. ?

[Nickfromwales]

3 hours ago, joe90 said:

Well I have just had a pumped screed and it didn’t appear to have a laitance ( scum) on the surface ( they told me it was a new type that didn’t suffer from this). I just need to know what I need to do with it before I start tiling etc. I have got two industrial dehumidifiers running 24/7 and they both pull about 20 gallons of water a day from the air ( floor).

I'd recommend watering down some flexible primer say 75/25% water/primer and applying it with a mop and bucket, literally saturating it to the point it's got puddles standing on the surface. Leave that soak in and dry. 

Then the primer gets mixed 50/50% and you apply that AS your tiling. I've done that on my current customers floors and mine are the only ones stuck to the original liquid screed. Other original tiles just lifted like a magic carpet. 

[Lin]

On 27/12/2017 at 23:18, Nickfromwales said:

Hi @Lin. What's the current / intended heat source for the UFH?

 

Hi; gas or ASHP. Prob gas

[Lin]

On 27/12/2017 at 19:11, PeterW said:

@Lin Do you want UFH...?

@JSHarris

You could go with 100mm of PIR, 20mm EPS trays and then the flooring and still be under your 150mm build up but with a much more insulated floor. The downside is you have very little to hold the heat or transfer it to, so using it as an overnight thermal buffer wouldn't be practical.

 

The thought of not having some kind of  thermal buffer does irk me

[SteamyTea]

Have you done any figures to see how much difference slab thickness would actually make?  There is a lot of talk about thermal storage and buffering in concrete floors, but very little real data or accurate models.

Edited December 29, 2017 by SteamyTea

[Lin]

21 hours ago, Nickfromwales said:

. @Lin, is there definitely no option of excavating the existing floors out and doing a much better job of insulating and using a thicker dry screed or concrete slab?

Never say never, but it's a time and money thing

 

The UFH where we live is on 50mm rockwool board. So 75mm PIR is definately an improvement. !

[Lin]

22 minutes ago, SteamyTea said:

Have you done any figures to see how much difference slab thickness would actually make?  There is a lot of talk about thermal storage and buffering in concrete floors, but very little ral data or accurate models.

If you mean thermal capacity stuff I wouldn't know where to start !

 

I can work out height differences though, just about.

[Nickfromwales]

11 minutes ago, Lin said:

Never say never, but it's a time and money thing

 

The UFH where we live is on 50mm rockwool board. So 75mm PIR is definately an improvement. !

How long do you really expect to live in the property ?

[Lin]

11 minutes ago, Nickfromwales said:

How long do you really expect to live in the property ?

A while I guess. Hard to say.

[Lin]

On 28/12/2017 at 08:48, Oz07 said:

Was suprised at just how strong these screeds are. The flow is superb aswell. I usually use agilia which is good no nonsense stuff however susceptible to cracking if movement joints not in asap. Lafarge do one of these thin screeds when I worked on a job with it was tuff stuff. Only 50mm thick you'd think it would crack everywhere but very strong. 

Thanks for your experience of screeds.

It's prospect of cracking that concerns me. Want a rock solid finish.

If that means having 75mm sand cement then I would go for that if it's more reliable.  It sounds like you have confidence in screeds.

Edited December 29, 2017 by Lin

[Nickfromwales]

5 minutes ago, Lin said:

A while I guess. Hard to say.

Don't be TOO specific please. We hate it when that happens

 

It was with respects to estimating the increased running costs ( losses ) vs time and then for us to recommend whether it's worth the extra effort and £ input. 

[SteamyTea]

2 hours ago, Nickfromwales said:

increased running costs ( losses ) vs time

Should we not be thinking about the the long term, regardless of how long the current occupier is in the house.

2 hours ago, Lin said:

If you mean thermal capacity stuff I wouldn't know where to start !

Not that hard to get a rough picture.  Just some multiplication really.

So the first thing to work out is the mass of the floor i.e. the length, width and depth, then the mass i.e. volume times density.

Then you look up the specific heat capacity of the floor material and multiply that by the temperature difference  (how much hotter it is than the air) and the mass.

 

There are other things to look at such as the different losses and areas between the ground, edges and the actual room, but they can wait for now.

[Nickfromwales]

55 minutes ago, SteamyTea said:

Should we not be thinking about the the long term, regardless of how long the current occupier is in the house.

If it's your money to spend then yes. 

Im certainly not going to future proof the house I'm in for the next occupier. I'll do a cost vs return analysis and spend what will cover me for my time in it, and not a penny more. 

We have a government that's supposed to be investing in our planets future, rather than well minded folk blowing what spare cash they have in this lifetime, to keep their children, and their children in a healthy world.

I don't mind helping out, but it's not for my wallet to perpetuate it surely ?

[SteamyTea]

45 minutes ago, Nickfromwales said:

I don't mind helping out, but it's not for my wallet to perpetuate it surely ?

Rather going off topic (my fault), so feel free to start a new thread about it.

[Nickfromwales]

3 minutes ago, SteamyTea said:

Rather going off topic (my fault), so feel free to start a new thread about it.

Relevant here too TBH, as it's down to costs / returns and why we arrive at such decisions here    

My point being that if they're only staying for 10 years and are in their 30's then let's face facts, they'll not want spend their budget on the next occupants. If it increases the value then it's game on though. 

[SteamyTea]

Start a new thread about it.

[Oz07]

5 hours ago, Lin said:

Thanks for your experience of screeds.

It's prospect of cracking that concerns me. Want a rock solid finish.

If that means having 75mm sand cement then I would go for that if it's more reliable.  It sounds like you have confidence in screeds.

From my experience the thinner liquid screed performed better in terms of resisting cracking than the thicker concrete. I don't know the science behind it thought perhaps someone else does? Flexibility perhaps?

[Gone West]

1 hour ago, Nickfromwales said:

We have a government that's supposed to be investing in our planets future, rather than well minded folk blowing what spare cash they have in this lifetime, to keep their children, and their children in a healthy world.

I don't mind helping out, but it's not for my wallet to perpetuate it surely ?

Bugger, that's where I've gone wrong .