Which screed?

[WWilts]

New build in progress.
UFH on ground floor.
Concrete slab, PIR insulation.

 

Which to use, liquid screed (eg 40mm) or sand/cement (eg 65mm)?

 

Trying to understand the pros and cons for cost/heating performance/ease of build

[epsilonGreedy]

Safer thicknesses would be 50mm and 75mm respectively.

[nod]

3 minutes ago, epsilonGreedy said:

Safer thicknesses would be 50mm and 75mm respectively.

Plus one 40 is to thin

Liquid pros 
Flatter 

Drys quicker 

Conns 20-30% more expensive 

You need to make sure the LS can get under the insulation 

Ive seen a couple lift 

Worth putting a handful of knock ins in each sheet of insulation 

 

[epsilonGreedy]

3 minutes ago, nod said:

 

You need to make sure the LS can get under the insulation 

Ive seen a couple lift 

Worth putting a handful of knock ins in each sheet of insulation 

 

 

Did you mean can't get under the insulation?

 

If so I thought a second membrane over the insulation prevented this?

 

Does pouring screed with the UFH pipes filled help prevent lifting?

 

Are these "knock ins" a type of intersheet jointing fixture?

[WWilts]

21 minutes ago, nod said:

Worth putting a handful of knock ins in each sheet of insulation 

What are those?

[markc]

8 hours ago, WWilts said:

What are those?

Knock in insulation fixings. T shaped plastic plug (like a big nail).

[nod]

9 hours ago, WWilts said:

What are those?

They are just a plastic stem with a round plastic disk on the top Drill a 8 mil hole through the insulation and into the floor avoiding the beams Then tap in I normally put five per sheet when fixing 

Ready-fix sell them by the 200 box

[epsilonGreedy]

17 minutes ago, nod said:

They are just a plastic stem with a round plastic disk on the top Drill a 8 mil hole through the insulation and into the floor avoiding the beams Then tap in I normally put five per sheet when fixing 

Ready-fix sell them by the 200 box

 

That is a lot of holes in the first membrane.

[nod]

1 hour ago, epsilonGreedy said:

 

That is a lot of holes in the first membrane.

It is But necessary I’ve been to tile two flo screed floors that haven’t been flat 

The only explanation is the screed getting under the insulation 

But the plastic seals the hole tight and there’s a slip membrane between the screed and the insulation So no chance of damp 

It normally takes two of us about an hour to do a 150m2 floor 

If your screed guys are responsible for installing the insulation They will probably do this with asking 

 

 

Edited January 23, 2022 by nod

[epsilonGreedy]

3 minutes ago, nod said:

It is But necessary I’ve been to tile two flo screed floors that haven’t been flat 

The only explanation is the screed getting under the insulation 

But the plastic seals the hole tight and there’s a slip membrane between the screed and the insulation So no chance of damp 

 

 

Very interesting. A few years ago I remember a discussion here where I mentioned my concern that flowscreed might end up as a thin concrete crust lacking support from the insulation below. My concern at the time was that if PIR compresses it does not rebound back to its original shape unlike a cheaper spongy insulation sheet. I now realise that concern can be mitigated by installing a thin starter insulation sheet using a more elastic insulation sheet below the main layer of PIR and/or using sand to fill dips in the base floor.

 

Thinking about about a point I raised above I don't suppose leaving the UFH pipework filled with water creates enough weight to keep the insulation sheets pinned down?

 

Ho-Hmm I am starting to consider a traditional sand/cement screed again in the hope the extra weight keeps the insulation snugly weighted down to the base floor. 

 

Having watched many screed insulation videos I do not recall seeing use of these knock in pins. Is this an example of the industry solving a new problem that has come to light as thinner flow screeds are used?

[nod]

1 hour ago, epsilonGreedy said:

 

Very interesting. A few years ago I remember a discussion here where I mentioned my concern that flowscreed might end up as a thin concrete crust lacking support from the insulation below. My concern at the time was that if PIR compresses it does not rebound back to its original shape unlike a cheaper spongy insulation sheet. I now realise that concern can be mitigated by installing a thin starter insulation sheet using a more elastic insulation sheet below the main layer of PIR and/or using sand to fill dips in the base floor.

 

Thinking about about a point I raised above I don't suppose leaving the UFH pipework filled with water creates enough weight to keep the insulation sheets pinned down?

 

Ho-Hmm I am starting to consider a traditional sand/cement screed again in the hope the extra weight keeps the insulation snugly weighted down to the base floor. 

 

Having watched many screed insulation videos I do not recall seeing use of these knock in pins. Is this an example of the industry solving a new problem that has come to light as thinner flow screeds are used?

Absolutely no problem with traditional 

On 150m2 floor you would be looking at 20 ton load of screed No chance of anything moving 

 

As a business I probably only tile five flow screed per year and have seen two lift in the past  five years 

Flo is great for tiling But everything needs to be down tight Or as you say you will only get a fine covering on the pipes 

[Thorfun]

doesn't a liquid screed surround the UFH pipes better as it 'flows' around and under them to fill all available gaps? I didn't think a sand and cement screed did this so it would/could leave pockets of air resulting in slightly poorer heat-up times for UFH? I could be talking b******s though.

 

we're having a liquid screed in the basement for the ease of it all. 6m³ of Cemfloor is costing me £2200 all-in (based in the SE before @nod quotes his up'north prices ? ). seems very reasonable to me and I'm happy to pay for the quickness of install and drying. For the ground floor I'm still undecided as to which way to go and I guess I'm using the basement as a tester to see how the liquid screed works.

[nod]

30 minutes ago, Thorfun said:

doesn't a liquid screed surround the UFH pipes better as it 'flows' around and under them to fill all available gaps? I didn't think a sand and cement screed did this so it would/could leave pockets of air resulting in slightly poorer heat-up times for UFH? I could be talking b******s though.

 

we're having a liquid screed in the basement for the ease of it all. 6m³ of Cemfloor is costing me £2200 all-in (based in the SE before @nod quotes his up'north prices ? ). seems very reasonable to me and I'm happy to pay for the quickness of install and drying. For the ground floor I'm still undecided as to which way to go and I guess I'm using the basement as a tester to see how the liquid screed works.

I wouldn’t think flow screed price that much “Up Norf “ Or outside central London ?

[Thorfun]

1 hour ago, nod said:

I wouldn’t think flow screed price that much “Up Norf “ Or outside central London ?

sadly, SE England prices are very much similar to Central London.

[Adam2]

Just did ours, maybe we were lucky but no issue with liquid screed (Longfloor) over insulation without fixings. As mentioned above, we also used dry sand to fill any voids beneath insulation - laid on concrete planks so plenty of undulations at joints and of course a bit cambered. Taped across all insulation sheet joints and a thin membrane as per normal process. 

 

@Thorfun that price looks v similar to mine per m2 - mine wasn't the cheapest I could find but was recommended and only ~5% more than cheapest so happy to pay a bit more for a bit of reassurance