Screed Thoughts

[Ed_MK]

My thoughts are turning to screed for our new build, as with the roof scheduled in about 4 weeks,

it will be time to get it organised.

 

I keep hearing horror stories of 8 week drying times and cracking floors etc etc, and as i know little 
(very little) about screed ..I did a bit of reading up.

 

Now I have a good drainage site, with suspended B&B and it will have Kingspan insulation on top.
The only issue at present is whether we will go for UFH...as due to budgets...we will be tight...

...but we WANT to have it...So We will look at things from that perspective (optimistic!)

 

I suppose the Floor Area is around 100m2 when all told ...Still not sure if UFH goes everywhere or just in 
"rooms" ...I mean downstairs WC, Utility Room, Hallway ..is it common to UFH those ?

 

But either way ...Does anyone have any advice as to Cost and timings for various options on Screed for me?....
 

the builder seemed to think it would require a "standard 75mm" ...I am presuming depth ...I thought that sounded deep
but looking online it seems common ...especially as it has to "hide" UFH tubes.

 

I mean does having UFH in there make it "weaker" ? ...Will it be prone to cracking with heat? ...just how accurate is it when
people (anecdotal)  say it shouldn't be walked on for 4 weeks or worked on for 8?

[Onoff]

I would imagine you will want A142 mesh or similar in your slab to limit it cracking for a start. UFH pipe is comparatively cheap. A LOT easier to put it in now than later when tbh wet UFH will be impossible. Pipe gets cable tied to the mesh. 

 

I'm thinking 75mm would be better 100mm or more, more room to play and more mass to act as a heat store if your insulation levels are good.

 

I think there's some free UFH planning software available for you to work out your loops. Someone will comment on this shortly.

 

Although pumped concrete have a look at @Construction Channel's thread/blog on a mesh/pipe system above PIR. Think he's the latest to do this. Read also @oranjeboom's stuff on what can go wrong in terms of pipe springing out of the slab etc. Again that was wet concrete.

 

Cleverer people are reading this as we speak and will be along shortly to comment!

 

Edit: Worth a read:

 

 

 

Edited June 6, 2018 by Onoff

[Nickfromwales]

You can walk on most a few days after. 4 weeks is just utter nonsense. 

Anywhere between 1mm a day and 2mm a day is realistic for a slab to completely 'dry out', but its often not a requirement that it dries out before works commence eg you could be stud walling a couple of weeks later or maybe even less. 

You aren't slabbing, your screeding, so you need to look at that specifically. First question is how much Kingspan over the B&B and what is the depth you have left for screed?

[nod]

5 hours ago, Ed_MK said:

My thoughts are turning to screed for our new build, as with the roof scheduled in about 4 weeks,

it will be time to get it organised.

 

I keep hearing horror stories of 8 week drying times and cracking floors etc etc, and as i know little 
(very little) about screed ..I did a bit of reading up.

 

Now I have a good drainage site, with suspended B&B and it will have Kingspan insulation on top.
The only issue at present is whether we will go for UFH...as due to budgets...we will be tight...

...but we WANT to have it...So We will look at things from that perspective (optimistic!)

 

I suppose the Floor Area is around 100m2 when all told ...Still not sure if UFH goes everywhere or just in 
"rooms" ...I mean downstairs WC, Utility Room, Hallway ..is it common to UFH those ?

 

But either way ...Does anyone have any advice as to Cost and timings for various options on Screed for me?....
 

the builder seemed to think it would require a "standard 75mm" ...I am presuming depth ...I thought that sounded deep
but looking online it seems common ...especially as it has to "hide" UFH tubes.

 

I mean does having UFH in there make it "weaker" ? ...Will it be prone to cracking with heat? ...just how accurate is it when
people (anecdotal)  say it shouldn't be walked on for 4 weeks or worked on for 8?

You can normally walk on a screed after three days

As far as drying out allow a day per millimetre 

Another option is a concrete liquid screed Drying out time is around a week Add 30% to cost

As far as cracking screeds always crack exspecially with UFH But if you are intending to Tile or glue engineered flooring down The heating must be run for two weeks so as to completely dry out and to allow it to move

Its also worth installing an anti cracking matting prior to tiling 

[dpmiller]

if you want to go a little deeper on the insulation, the anhydrite and hemihydrate flowscreeds can go down to  50mm or less.

[newhome]

58 minutes ago, dpmiller said:

if you want to go a little deeper on the insulation, the anhydrite and hemihydrate flowscreeds can go down to  50mm or less.

 

We had anhydrite screed but we were told to leave it 60 days plus before tiling. Maybe that’s where the 8 weeks comes in? There were loads of other things to be getting on with so that wasn’t an issue TBH. 

 

[joe90]

My builder had an  anhydrate screed laid in my build, 75mm, no rebar required, I ( with help) put the UFH pipes down, clipped to Kingspan insulation ( not difficult). Used conduit through doorways, walked on it two days later, had no scum on it ( guy installing it said the “new” stuff produced no scum) scrapped the surface by hand to remove slight nibs, tiled 8 weeks later ( we needed two dehumidifiers fir a week as it was the wettest year on record, ‘‘twas February, ) just had house rendered/ plastered so lots of damp had to be removed from the build. No sign of cracking at all.

[Nickfromwales]

2 hours ago, dpmiller said:

if you want to go a little deeper on the insulation, the anhydrite and hemihydrate flowscreeds can go down to  50mm or less.

50mm thick screeds make for horrible UFH emitters imo. You need to have very accurate ( short hysteresis ) room thermostats and NOT use a regular TMV on the UFH manifold to achieve a smooth heating curve from such emitters. They ( typically ) heat up too quick, cool down too quick, and incorrect / unsuitable controls allow for uncomfortable overshoot / undershoot seeing the slab go from hot to cool, rather than staying 'warm'.

This can be of benefit if your not in all day, eg working family / kids in school etc, but for anything else id not go less than 75 - 80mm of screed. If building to passive / near passive standards then these problems become more apparent, especially the overshoot. 

With thinner screeds and the need to better regulate flow, a buffer tank is essential. 

[Onoff]

With no more access to the eBuild forum I can't be sure of the exact date my concrete slab was poured though it was definitely pre May 31 2016.

 

I think the slab should be cured enough for tiling by now!

 

[Nickfromwales]

3 hours ago, Onoff said:

I think the slab should be cured enough for tiling by now!

Yup. Any insects caught up in your slab will be fossilised by now ?

[epsilonGreedy]

8 hours ago, Nickfromwales said:

This can be of benefit if your not in all day, eg working family / kids in school etc, but for anything else id not go less than 75 - 80mm of screed. If building to passive / near passive standards then these problems become more apparent, especially the overshoot. 

 

 

I find myself between these two life styles. If I elected for 65mm of screed sitting on 150mm of insulation could you provide any guess how long the slab would take to warm up for the weekend? My new build will be insulated to just above basic building regs.

[Ian]

41 minutes ago, epsilonGreedy said:

 

I find myself between these two life styles. If I elected for 65mm of screed sitting on 150mm of insulation could you provide any guess how long the slab would take to warm up for the weekend? My new build will be insulated to just above basic building regs.

Mine takes approx 1 hour for every 1 degree C increase. It’s a holiday home mainly used at weekends and built to min Welsh building regs. Overall average area weighted U values of walls roof and floor are 0.15 with 0.8 3G windows but the floor is 0.1 because of the u’floor heating. The floor construction is 100mm concrete slab with rebar over 300mm of EPS.

I use a Hive controller to remotely control the heating.

Edited June 6, 2018 by Ian

[Declan52]

44 minutes ago, epsilonGreedy said:

 

I find myself between these two life styles. If I elected for 65mm of screed sitting on 150mm of insulation could you provide any guess how long the slab would take to warm up for the weekend? My new build will be insulated to just above basic building regs.

Mine is near enough that with 75mm sand cement screed. Takes roughly 2 hrs before you would feel the heat on your feet but similarly it will stay warm for the same when you turn it off.

How long it takes to heat your room up after that will depend on your heat losses. Doubt your room will ever be lower than 15-16 unless it's really cold outside and you have left a window open.

As above about 1 degree per HR wouldn't be far off the mark.

[epsilonGreedy]

4 hours ago, Declan52 said:

Mine is near enough that with 75mm sand cement screed. Takes roughly 2 hrs before you would feel the heat on your feet but similarly it will stay warm for the same when you turn it off.

How long it takes to heat your room up after that will depend on your heat losses. Doubt your room will ever be lower than 15-16 unless it's really cold outside and you have left a window open.

As above about 1 degree per HR wouldn't be far off the mark.

 

 

So much to learn...  given the two heat charge up time examples above and assuming the slab would drop to say 15 degrees by Friday AM following 4 days of no heat, then to get the slab up to a useful temperature of 25 degrees for friday evening the UFH should start its charge up at 8am.

 

I wonder if such a usage profile (mis-use of UFH) could lead to a stress fracture in the slab due to the expansion/contraction cycles? I guess I am only part-way mentally adjusted to the adoption of UFH so I am still thinking of it as a strange radiator.

[Declan52]

During the winter my stove comes on in the morning and if any stat calls for heat it will take what it needs. Same goes for the evening. Stove fires up and heats my thermal store and if needed it will go to what ever room where it is needed.

I will be in 4 years in September and have no cracks in my tiles or solid floors so heating up and cooling down regularly won't cause any damage.

What are you going to be using to heat your house???

You can use the blending valve to set the water temp going in to suit your needs. Can be anywhere from 25 -55 degrees. Depending on what your heat losses are well determine what yours will be set at. Mine is set for 35 but each system is different and it's easy tweaked once it's in and running.

So you can have it set at a lower temp like 25 so it comes on more often to keep the temp in the room at 20 degrees or run it higher so it takes 3-4 hrs to get to 18 and then you come home and enjoy it getting to 20.

My and my dearest have the usual stat wars every winter where she puts them up to 22 and I turn them down to 20. 

[PeterW]

A concrete slab heating by a delta 15c is not going to crack unless its stressed in a certain direction or has a weak point introduced. This is normally at  doorway or other narrow point so if you have a single monolithic slab its unlikely it will do anything other than sit there....

[Ed_MK]

Its time for me to get the Beams and Block cleaned and get the vapour barrier and Cellotex down (in prep for screed and some plumbing pipes first)

...but i am puzzled what to do at all the "holes" ?!?

 

Around the perimeter wherever there is one of those metal feet bolted to the slab to take the frame thee are Gaps ...between the kicker bricks and the foot itself,

some are inches ...but most are several inches like below and you can see daylight inside ...

 

How do i deal with these ...should they be filled as best i can, concrete ? or would that be bad against the metal ..puzzled (as usual)

 

 

[epsilonGreedy]

On 11/08/2018 at 04:42, Ed_MK said:

Its time for me to get the Beams and Block cleaned and get the vapour barrier and Cellotex down (in prep for screed and some plumbing pipes first)

...but i am puzzled what to do at all the "holes" ?!?

 

 

I watched 50mm of flowscreed being applied to my neighbour's plot this week. The installers spent time plugging such gaps in the perimeter as flow screed would escape round the frame brackets shown in your photo. No expert here but surely the solution is small insulation upstands around those frame foots if only to prevent cold bridges.

 

The cost of the job was £4000 for 90 sqm including UHF components and pipe installation.

[Ed_MK]

Thanks!

 

I have already cleared them out and filled with Expanding filler. so there is no path to outside now

Lots of the frame panels have DPC and polythene hanging off the bottom etc ....This will get int he way when I am trying to lay 

the Cellotax and the 1200 heavy membrane up the wall ...do you just cut it flush ? ...as the 1200 dpm is already going to be fixed

150mm up the panel 

[Ed_MK]

I discovered yesterday that some of the internal stud walls don't sit across the kick blocks evenly.

Which is not a surprise as the guys that put them in were less than accurate !

 

The guide for laying the cellotex is helpful ...but how do i cope with the edges on these walls (see sketch)

on one side i suppose i could just use 2 of the slim cellotex to pad it out to the correct frame level...
 

...but on the PROUD side ..If i put ANY insulation here ...it is going to push the skirting board out ....I am sure and I can see how the plasterer will be able to work up

to it 

 

what would you guys do ?

 

 

oh ..and some of these walls are supporting stuff ...I did THINK about taking a disc cutter to the blocks ...but it is probably not wise

 

 

 

 

[CC45]

you may have to bring the internal stud out a bit with battens and then board.  That's your easiest solution I imagine.

 

[Nickfromwales]

Won't you just trim the upstand insulation flush with the top of the screed ?

Then you apply the floor covering over the top of the exposed upstand. 

[CC45]

1 hour ago, Nickfromwales said:

Won't you just trim the upstand insulation flush with the top of the screed ?

Then you apply the floor covering over the top of the exposed upstand. 

that's assuming the final floor level is to the top of the block....  ours is but best check.

[Ed_MK]

Well thats another question ...

If i measure around the rooms at various points the bottom of the the Timber panel to floor

varies from around 150 to 175mm.

 

the floor Insulation is 90mm ..So I am presuming the screed will go to the top of the block:? ...does it always?

So that would mean that there would be between 60mm and 85mm depending on which wall ...
 

..to be fair most are OK ....but there is at least one side that is DEEPER.

 

what is the norm please guys ....I take it the screed should NEVER be above the TF bottom?

[CC45]

Ours goes to the top of the blocks.  The DPM laps with the DPC.