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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorEdF
    • CommentTimeJul 5th 2007
    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.
    Hi EdF,

    Sand and Cement screed should be laid semi-dry, in order to get it level. It is very tricky to mix yourself in a small drum mixer and requires experience. I have laid miles of the stuff and I would not mix it myself - just not economical or practical anymore. Most ready mix concrete firms also do ready mix screed. Fibamix is better over UFH and insulation. Some would specify 75mm minimum - I would check with the UFH supplier.
    • CommentAuthortony
    • CommentTimeJul 5th 2007
    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.
    Tony. I agree that it can be mixed on site, though I think that if you have any quantity beyond a couple of ton, its just not practical. I have laid 20+ ton in a day and if you were to mix this quantity yourself you would be doing so from dawn to dusk and beyond, and it would still not be as good as readymix. I also do not think it is a DIY type of job.
    • CommentAuthorEdF
    • CommentTimeJul 5th 2007
    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.
    • CommentAuthorMike George
    • CommentTimeJul 5th 2007 edited
    Ah I see. In that case mix it yourself. Using very course sand is critical and a mix of around 4:1 should be fine. Here's a few tips:

    1. Do not overload the drum.
    2. Use small mixes and guage the material using buckets.
    3. Also guage the water so that the mixes are consistent
    3. No additives at all
    4. Jack up the back of the mixer. This will stop the mix sticking to the back of the drum.
    5. The consistency should be such that when you squeeze a handful it will just about hold together
    6 Do not overmix - as the sand/cement will turn to 'balls' ie marbles
    7. You can use battens to get temporary levels. Compact them in the screed so that they are at finish floor height and level. Do this in bays and use a straight piece of timber to drag the area between battens to a level finish. then float with a plastic float and trowel finish -easy if the consistency is right. Then remove the battens [while cement is still wet] fill and float/trowel over

    Good luck with it1
    • CommentAuthorEdF
    • CommentTimeJul 6th 2007
    Thanks Mike, have sent you an email..
    • CommentTimeJul 6th 2007
    Mike George, Thanks! very useful post
    • CommentAuthormoogaloo
    • CommentTimeJul 11th 2007
    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.
    • CommentAuthornkmanglam
    • CommentTimeSep 27th 2007
    Helo Moogaloo,

    I was also planning to use Lytag over UFH pipes but i was told that Lytag is not good as Sand and cement screed in terms of Therma mass. One builder also told me that Lytag act as in insulation.

    It was interesting to see your comments here. Can you give me some detail about using Lytag instead of sand and cement screed?

    Lytag is leightwieght as well.

    Thanking You

    • CommentAuthorBluemoon
    • CommentTimeSep 27th 2007
    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.
    • CommentAuthorRachel
    • CommentTimeSep 27th 2007
    Hello. Can anyone point out to me where the 'green' is on this topic and why the c (cement) word is even used? Limecrete rather every time for a screed....if you want to be green.

    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).


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


    • CommentAuthorRachel
    • CommentTimeSep 28th 2007
    I have used limecrete floor in my house with U'floor heating pipes in it. Then earthen floor on top. Leca insulation underneath it all. No concrete required...
    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?

    • CommentAuthorBluemoon
    • CommentTimeSep 28th 2007
    Would a mix of Leca and lime be suitable for a screed of about 90mm, with the UFH wires and tiles on top?
    • CommentAuthorRachel
    • CommentTimeSep 30th 2007
    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..
    I wouldn't screed with LECA - the screed needs to be made with a denser and tougher aggregate than LECA, unless you're going to lay some pretty substantial slabs over the top. Ideally you'd use sharp sand and pea gravel, or glaster, mixed 3:1 with NHL5 for the screed.
    • CommentAuthorJohan
    • CommentTimeOct 1st 2007
    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)
    • CommentAuthorRachel
    • CommentTimeOct 1st 2007
    I think you misunderstood. The leca is underneath the pipes. There is 200mm leca below, then a screed of fine leca/limecrete. Then the pipes are set in pure limecrete:NHL5 & course sand, 3;1 as above.Then earth floor/limecrete/ slabs on top. It works well and there is good insulation and breathability below.
    • CommentAuthorBluemoon
    • CommentTimeOct 1st 2007
    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.
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