Screed and UFH

[Joh]

Hi again .. back from a mini break and raring to go (not) on my extension job .. a question related to tiling laying .. has anyone experience of UFH/screed laying prior to tiling .. ?

I'm not sure whether it would be OK to put down the heating pipes and screed before installing the windows and door .. there's a risk that the screed could get wet from the weather, but, on the other hand once the windows are in, the drying out time could be slower due to lack of air flow .. I want to move the job along as quickly as possible but without taking daft risks ..

any thoughts?

[whitebeam]

Windows and doors in then screed, you don't want your screed drying out to fast anyway.
Screed needs 1mm per day drying out

[Joh]

thanks, whitebeam .. have you done/seen much screeding done? .. if so, what is the best type for UFH and what's the best way of laying it (given the largest area is about 4 x 5 m)?

[whitebeam]

I've done loads of screeding but any water pipe heating has usualy been laid so not the best one to advise on that one, sure there will be someone along who knows better

[bighen]

You want to lay a 75mm - 100mm max fibre reinforced screed. (100mm takes slightly longer to warm up but retains the heat longer)
You need to work out your perimeter levels first. Laser level is ideal.
Then lay the screed 300mm wide around the perimeter, level and rub flat with polyurethane float.
Then lay a central band again 300mm wide and finish the same. This will then leave 2 bays.
You can then fill them and level using a aluminum straight edge preferably, working your way out and polishing with your float as you go along.
Don't walk on the screed for 48 hrs to allow to harden and allow drying time as per Whitebeams advice

[bighen]

You need to lay your DPM on top of the concrete sub base first, then lay 75 -100mm Kingspan or equivalent extruded polystyrene. Don't use Jablite or evivalent as this doesn't hold the pipe clips.
You also need to determine your room furniture layout so as to plan your loops so as to match the heat requirements.

The 2 main heating circuits are as follows: -

Serpentine Patterns
Serpentine patterns allow for the hottest water to border the exterior perimeter (highest heatloss areas). The water temperature is highest at the coldest walls and will decrease as it flows through the pipe towards the centre of the room.

Counter Flow
Counter flow patterns differ from serpentine in that the supply and return pipes are laid out next to each other, creating an average temperature between them.


You also meed to bear in mind the the maximum pipe length of 1 loop is 100m so your 4x5 room = 20m2. A circuit with 200mm pipe spacing will be borderline on this area.

You can go as close as 100mm pipe spacing to give a greater floor output but this would require 2 loops.


Hope this helps. It's not as daunting as it sounds. I've done loads and i'm the proverbial blond

[Joh]

thank you for the advice, bighen .. you sound really clued up on things .. you don't do work out in the peak district, by any chance?

[Joh]

Just reading your replies again .. now I'm wondering ...

1. I plan on using 100mm insulation, and this might be an idiot question, but is there a real difference between the stuff sold for walls and that sold for floors or is the difference just marketing?? .. for some reason my local merchant seems only to stock the cavity wall variety in 100mm
2. Must the concrete sub-floor be bang flat to take the insulation, as mine isn't .. puddles formed when it rained - i guess there might be a dip of 3 or 4 mm - and there are some ridges here and there. A bit concerned that the insulation might "settle" over time, and cause the screed to crack??
3. would you advise getting ready-mixed sand and cement screed with fibre - or what?
4. should there be expansion gaps round the edges as with tiling?
This all seems a complicated business but then I suppose things do when you've not tried them before.

[bighen]

Hi Joh.

With reference to your questions -

1. The main difference between floor and wall insulation is size. Wall insulation is mainly 450x1200mm to suit block gauge and handling. Floor insulation is mainly 1200x2400 although there are a few brands that use a smaller interlocking type. You are best to use the larger sheets ie. kingspan, cellotex rr or xtratherm as these do not lift as much when the pipes are cliped to them. If your merchant doesn't stock them, ring up a national such as CCF, Shefffield Insulations or Encom who specialise in insulation
2. A few minor humps and hollows shouldn't have any detrimental effect as the weight of the screed will hold it down. The ridged I would grind down.
3.I would recommend ready mix - If you look in your local yellow pages you'll find someone. If you give them your cubic area they will equate this into a tonnage for you. You'll need a fibre reinforced screed with 24hr cutback - so do not walk on for 48hrs after laying. It comes on a tipper lorry, so just sheet down, tip barrow away, level, done. Also you need to fill your pipes with water prior to screeding. I usually fill them to 3bar and leave them under pressure until the screed has fully cured.
4. With regards to the perimeter you will need this insulating. This complies with building regs and also makes sure that the heat remains in the screed, not the walls. You need to use 50mm kingspan although if building control aren't involved, you'll get away with 25mm.

Hope this helps. It isn't overly complicated but once you get set you'll breeze it.
Any Probs, PM me

Also a tip to remember when laying the pipe - always keep it vertical ie roll it when clipping. then the pipe will want to stay flat as opposed to lifting and twisting.

Andrew

[Gazzatile]

Very intresting should be usful

[Joh]

Thank you Andrew .. I imagine your info will help a lot of people ..

I assume from what you say, the readimix I want is the semi-dry stuff? .. rather than the wet/pourable?

[bighen]

Hi Joh.
Yes the screed is semi dry. When tipped off the lorry you should be able to squeeze a ball of it in your hand and it hold it's shape without crumbling.
When laying it you don't tamp it down as you would with concrete. You just tip it down, rough level, scrape flat and float buff. It's as easy as that