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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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    • CommentAuthorrevor
    • CommentTimeFeb 21st 2018
    Does anyone have a view on the pro's and cons of the different screeds onto UFH. Have heard nightmare stories about both locally and am having difficulty in deciding which way to go. I have built most of the house myself but 160 sq m of laying traditional screed is a job I would rather not do. I have 150 mm of insulation to go down and 75 mm of screed on top. To ensure a quality job would like to do it myself so has anyone laid something different to a screed e.g a timber floor. Upstairs I did a battened out floor with rebated/grooved insulation in between with pipes in the insulation but don't think this system would work that well on the G/F
    I have a Listed building and will be laying a Limecrete system using foamed glass for the insulation and a lime foam glass for the screed.
    • CommentAuthorTimSmall
    • CommentTimeFeb 21st 2018
    I'm not keen on the pipes-in-insulation-grooves systems - they cut the heat output of the pipes by over 50%.

    I laid T+G chipboard on insulation, then sealed with PVA, and laid a 30mm screed on top between battens.

    The screed was mixed on site with a polycarboxylate super plasticiser (reduces strinkage, drying time, and increases thermal conductivity), and 70% GGBS / 30% OPC. 15mm PEX pipe at either 100mm or 150mm centres depending on room. T+G oak flooring on top, or lino on top. Oak glued down with MS-Polymer flooring adhesive.

    A readymix concrete firm like Hanson should be able to knock you up a batch like that, and put it on a transit tipper, if you don't want to mix on site.
    • CommentAuthorjfb
    • CommentTimeFeb 22nd 2018
    i've only done a little bit of traditional screening but when doing that I did learn a little bit about liquid screening. Can be very quick and good (easier to level obviously). But it is all in the preparation needed to get it right. Has to be a bit like a swimming pool so no leaks/voids for liquid to run into. Would need a well installed 500 gauge polythene membrane on top of the insulation. There are some good guides out there on the web.
    Never looked into how feasible it is to do the actual liquid screeding oneself but must be possible.
    • CommentAuthorgyrogear
    • CommentTimeFeb 22nd 2018
    I do liquid limecrete screeds, but it is only in a crawlspace so would not go in for any competitions, but it works OK for my purposes !

    Takes ages to go off (about 100 days) but is far nicer to have around than a mud floor !

      After a trip to Trowel Services....jpg
    A screeder told me that flowable screed are less prone to cracking; he repaires cracked screeds ( not his own !) and they are usually the sand cement ones that crack, according to him.
    • CommentAuthordelprado
    • CommentTimeFeb 24th 2018
    Have you considered not using screed at all?

    I am using geocell, then woodfibre boards, then UFH Lithotherm clay tiles. No drying time and down in a day :)
    If youre doing trad screed , get a professional screeder to do it , its harder than it looks to get a good well compacted finish
    UFH pipe are best ontop of insulation , personally I prefer cable tying them to a light weight mess . A98 or A142 which raises them of the insulation slighty and surrounds the pipe in screed , better again for heat transfer
    use fibres in the mix to improve strength , remember by the book it drys at 1mm per day and you'd want 75mm down ( you can walk on it next day (cover with corex or hardboard to stop dusting surface , but offically it need to be fully dry before laying any finish

    limecrete screed glassrock works good but is expensive , perhaps only suited to listed jobs

    flow screed , not used it yet , put it looks good goes down quick , self levels and quick to dry , Id give that serious consideration
    • CommentAuthorCWatters
    • CommentTimeFeb 24th 2018
    Caution: Some types of screed are hard to tile onto..


    Before laying tiles or other finishes onto Calcium Sulfate screed one of the most important tasks is the removal of laitance.

    Laitance is a weak layer of fine particles deposited on the surface of the screed as the anhydrite cures. This layer is too weak to tile onto and can also inhibit drying of the screed. Many of the failures we see are as a result of contamination of the adhesive by laitance – highlighting the importance of carrying out this step.

    Laitance should be removed by light abrasion using a suitable sanding machine i.e. a rotary floor scarifier and a 60’s grit sandpaper....

    • CommentAuthorCWatters
    • CommentTimeFeb 24th 2018 edited
    <blockquote> I have 150 mm of insulation to go down and 75 mm of screed on top. To ensure a quality job would like to do it myself so has anyone laid something different to a screed e.g a timber floor.</blockquote>

    Is there concrete down already? If so then you might look at...

    Floating Wood flooring (eg minimum 21mm Engineered oak)
    UFH pipe in foiled insulation/plates (note below)
    More Insulation
    Sand blind
    Existing concrete.

    I suppose it might be possible to lay this over a correctly prepared hardcore base but not sure what Building Control would make of that. Might be necessary to raise the DPM up so there is some insulation under it to protect it from the hardcore?

    Note: Wavin have a panel system intended for use over Beam & Block/Joisted floors. I cant see why this wouldn't work over other bases such as the above? I suppose putting a layer of OSB under their panels might eliminate any objections?

    Scroll down to "Panel Systems for Joisted Floors"..
    • CommentAuthorCWatters
    • CommentTimeFeb 24th 2018
    PS If you want carpets I wouldn't use the above. eg I feel that 19-21mm chipboard boards _and_ carpet over the UFH would be a bit much. Especially for a low flow temperature ASHP system. A higher temperature oil fired system might work but you still advised to keep the TOG value of the carpets down.
    • CommentTimeFeb 24th 2018
    Posted By: CWattersFloating Wood flooring (eg minimum 21mm Engineered oak)
    UFH pipe in foiled insulation/plates (note below)
    More Insulation
    Sand blind
    Existing concrete.

    There's another thread where the wisdom of putting a DPM underneath floor insulation on the ground is discussed!
    • CommentAuthorrevor
    • CommentTimeFeb 25th 2018
    Thanks for the ideas. There is a slab in place 100mm mm on DPM on 150 hard-core. Was going to go for the liquid screed but heard of some horror stories. A local joiner friend was on a job on a new build and noticed that the floor the tiler was tiling on did not look level by eye. He put his laser on it and it was low at one end by 30 mm. They ripped up £4K of tiles and the underfloor heating pipes and re-laid the floor and u/f pipes. It was a very big room so costly. My friend has mentioned several times that the self levelling is not self levelling. Also if the slump is not spot on then get problems with adhesion of tiles. On my build have had some tradesmen that can't or won't do a good job and have to redo some of the work myself. Can't take a chance with the floor it has to be level and precise to meet the door thresholds so want something I could do myself. Self levelling needs specialist gear and currently am not fit enough to lay conventional screed. A neighbour had a professional screeder do his floors and was plagued by cracks. The screeder blamed the concrete works for not putting enough fibres in the mix and the screed supplier blamed the screeder. Typical building blame culture.The suggestions of a floating floor is appealing it is what I have done on the first floor but really want tiles on GF not sure how this would work without being on screed.
    • CommentAuthorCWatters
    • CommentTimeMar 2nd 2018
    Covering the screed with plastic sheeting to slow down the initial drying process can help prevent cracks.

    Also consider expansion joints where two areas of screed meet at a doorway. We had cracks here and they aren't straight, they curve and propagated through the stone floor on top.
    • CommentAuthorMarkyP
    • CommentTimeMar 2nd 2018 edited
    Revor - liquid anhydrite screeds display good self-leveling properties but it doesn't happen all by itself. The installers should use a laser to set small tripods every few metres to provide a consistent level across the installation. As the screed is pumped they will move the hose and push the screed around to ensure it consistently reaches the same level against the reference tripods. They then dapple the whole screed with a long flat bar to ensure a smooth finish. My screed was flat as a pancake, barely any deviation under a 2.5m straight edge and level within a couple of mm end to end across a 80m2 floor. I was careful to find a reputable installer, and asked for their installation method, and was on site when they did it. Equally, they were very clear with me about the expected preparation of the insulation, UFH pipework and so on. It went perfectly. Find a good installer and you'll get an installation tolerance (flatness and deviation from level/datum) in the contract, and they will work to a reference datum you provide.

    In my experience, when tradespeople talk about self-leveling they are usually referring to thin (few mm) liquid top coat screeds that are applied to smooth a rough floor prior to applying a finished flooring. These should be called smoothing compounds rather than self-leveling. Some do flow better than others so perhaps could be said to be have self-leveling properties, but really these are just about smoothing out a rough uneven surface ready for a floor finish, and they take a bit of skill to apply well. They are for flattening a floor surface rather than leveling it. FBall is the go to brand for me.

    whether liquid anhydrite or traditional sand and cement, this is skilled work and best done by an experienced installer. Dont be put off by horror stories, you just need to look for a specialist installer.
    • CommentAuthorRex
    • CommentTimeMar 2nd 2018
    Another vote for liquid screed installed by a reputable installer. I had the entire 90 sq m. g/f set up and poured in about half a day. You could see from the reflection when wet that is was mirror flat.

    Two days later, walkable but of course, takes longer to be fully dried and cured but interior work could continue.
    • CommentAuthorrevor
    • CommentTimeMar 2nd 2018
    Thanks for the encouraging comments. I am in a semi rural area and there are only 2 installers of the anhydrite which could be considered local and one is about 80 miles away. Got their names from the concrete plant that makes the stuff they only have the 2 that buy from them. Having asked around had good and bad comments about both. About 4 yrs ago I did get to see one installer at work and he did seem to do a decent job but he sold his gear to one of the 2 mentioned above. There is close to 200 sq M and only one chance of getting it right. Hence a room by room solution would be preferable from a control point of view although if I had Marky P experience as a local recommendation I would feel a lot more confident about the liquid screed I agree it is the best and quickest solution.
    • CommentTimeMar 2nd 2018
    Posted By: revorAbout 4 yrs ago I did get to see one installer at work and he did seem to do a decent job but he sold his gear to one of the 2 mentioned above

    I presume he's retired then? It might be worth asking him whether he can recommend anybody, or even if he'd be prepared to supervise an installation, for a fee?
    • CommentAuthorrevor
    • CommentTimeMar 3rd 2018
    No don't think he retired was a youngish chap think was not enough work for him to make a decent living. 2 close neighbours have done fairly sizeable extensions but not included u/f heating preferring to keep to the traditional timber joisted floor and radiators. Locals are mindful that the cost of improvements in an area where wages are lower than the national average is not reflected in the final value of the property so only do the minimum. BCO said our build was quite unique for the area it was the first MVHR he had come across only on a further visit he stated he had been to a further build where the builder had installed the system and included trickle vents in the windows,!! Because I have done most of the build myself and spent relatively little on external labour I have already reached a level of spend greater than the final value of the property, though as I intend it to be my final place the cost is not as relevant to me as may be to others. However the suggestion of having someone supervise the job has reminded me that I know of a project manager who has managed local authority projects so he may be able to make some suggestions.
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