About to lay 65mm of screed, but I'm a little confused how to mix it, wettish or dryish? I've seen both done (after the event). If it's fairly wet I expect it will be easier to put down and level. Should I put a retarder in it to help out, how about a plasticiser?
I discovered that 'almost everybody' in my area lays Kingspan or similar insulation under UFH. Apparently this is not ideal, extruded polystyrene is better, such as Dow Floormate or Knauf Standard Floorboard. The proprietor of the biggest UFH specialist in my area advised me and showed me samples. Kingspan etc is not so resistant to compression and can crumble, the two I've mentioned don't, he said. Probably better if there's a lot of big footed contractors going to walk all over it before it's screeded.. I found Sheffield Insulation were cheapest where I live, also Warrens in Chesterfield were not bad.

I cannot see why screed cannot be mixed on site ready mix is relatively expensive though nice. I have even seen it mixed by hand for a whole house!

For sure lay it semi dry and with fibres in it for ufh 75mm minimum sounds good.

Thanks for comments so far. My problem is I need to lay the house floors (90 sq.m total) in three stages due to having the new roof trusses in the building - they can't come out until the present roof is removed. It's a long story... Anyway, I have to lay a third first, let it go off and then move the trusses onto it, and then do the rest. Not sure I can handle a readymix delivery on my own (or with the wife) and the quantity will be very small, about 2 cube to start with, so VERY expensive.. I was also planning to make it easier by doing one room at a time. I had the idea of delineating the partition walls using 4"x2"'s on the subfloor, acting as shuttering. The first third of the house to be screeded will in turn be split into three rooms and a hallway. I thought that if I left the wood in place it would be a good anchor and guide for the partition walls. I thought that using the wood as shuttering would enable me to spread the job (therefore easing the workload and my back) and make it easier to lay the screed. Is there a good reason not to do this?? The final finish does not have to be glass-like.. One area of the sub-floor which was laid very wet (not by me) looks good enough to tile, hence my wondering about wet screed. UFH supplier recommended 65mm screed.. Can I buy the fibres to put in the mix? I live in a very rural area in the Highlands and options available elsewhere don't always exist here.

Thanks Mike, have sent you an email..

Just to muddy the water an alternative to a screed over PUR/PIR board would be Lytag. I have PUR board then I have my underfloor pipes then Lytag which is light looks like cat litter and disperses the heat. The PUR board will not have to take any load or moisture and if you need to get to the subfloor you can hoover up the lytag bend the pipes etc. then bend back and spread the contents on the hoover back down.

It is also alot simpler than a screed.

I'm just about to replace a screed, to make room for insulation and electric UFH in a 3M x 3M bathroom. Screwfix have both the insulation boards and the heating wires, so will probably get that, unless somebody says not to. Is it better to put screed down first, then the insulation and wire then ceramic tiles? Or insulation first then the screed? Rest of house has rads.

Edf,

Warrens and Sheffield Insulations are part of the same company, although I have found that the people at Chesterfield (Warrens) can be more helpful, as try 'Seconds + co' (cheap but mixed quality).

Rachel

Limecrete every time? what about an earthen floor? Would be interested to hear of any experiences with Limecrete see:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=690&page=1#Item_9

J

So is that with the UFH in the Limecrete slab? How much cover do the pipes have, any trouble with slow heat up time?

Also did you lay the floor or a contractor?

J

I had 100mm leca first, then finer leca mixed with limecrete;100mm. Then fix the pipes down and pour in limecrete of about 50mm, covering pipes. Then on top 50mm of earthen floor in one area, slates in another, and limecrete, polished with pigment in it, in another area. I'm afraid I cannot comment on the warmth yet as I'm waiting to put up the wind turbine to pump the underfloor heating..

LECA is also known as Optiroc, made by Max-it.

100mm sounds very little to me, you need around 300mm just to comply with the building regs. More would be better...

LECA in the screed doesn't sound right as it has got insulating properties. Why would you want to try and stop the heat from escaping out of the slab?

And as Gervase suggested in my thread asking about LWA's the foamed glass is cheaper then LECA (and maybe a little bit greener)

OK, so I lay a sand/lime screed, an insulation board on top of that, bed the wire in tile adhesive then lay the tiles in more adhesive? Object of the heating is to create a warm floor to walk on with bare feet. I have put in some pipe to connect a rad to the rest of the system if needed.