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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorCiriboyc
    • CommentTimeJan 11th 2021 edited
    Hi all,

    I'm renovating a small (75m²) cottage. Caried out most of the work myself using Breathable Materials, lime mortar etc.

    However, when it came to the UFH I got a little scared and went with the more 'conventional' route...Concrete subfloor and slab.

    The concrete slab was poured over a year ago, so I'm guessing it's good and dry at this stage:)

    I'd like to finish it with a flagstone floor, but my plumber is a little worried about the heat penetrating through the thickness of the flags...while he's installed plenty of ufh systems, he has no experience in dealing with flagstones so I think he's being overly cautious...but I'd really like some more opinions on this.

    The first flagstones I'm looking at are Indian limestone calibrated to 20mm.

    I also however have the option of locally sourced slate flags, but these vary in thickness from about 20mm to 45mm. These would be irregular shaped, but I could work them a little and the cottage is extremely 'rustic' anyway so I would be more than happy with these.

    The second option would be my preferred choice...I would love to get them locally, plus I like the more natural look of them...and also the price is definitely right! (tight budget!) but there are 2 issues I'm worried about with these flags:

    1. If the plumber is worried about the 20mm...I'm afraid he'd freak completely if I mentioned the 40mm :cry:

    2. If the flags are of varying thickness, I'm assuming I would have to grade them to keep things level-ish? would this affect the heating?

    So, I guess my question are:

    Is there any limit on the thickness/depth of Flags that can be used? (I realise responsiveness becomes an issue the thicker you go)


    Is there any problem with flags of varying thickness?

    Any suggestions/advice is appreciated!
    • CommentAuthorrevor
    • CommentTimeJan 11th 2021
    I would think you will get away with the 20 mm flags. Once the floor is heated up you are then just ticking over Check with your u/f heating supplier. I am currently doing a bathroom and need to keep the weight down so could not use screed but the supplier of my system recommended laying on top of the pipes which are within EPS grooved boards up to 18 mm marine ply then 10 mm tiles on top. I would jib at the slate that is too much variation from a laying point of view there is a lot of adhesive to get any resemblance of level plus I would be concerned of a trip hazard. I would hazard a guess you may get uneven heating. You might be ok on your feet and balance etc but think of any visitors you may have in the future. It is unclear if you have already laid the pipes if so how much screed have you over them. Before you tile I would heat up the floor to whatever your ufh supplier / designer recommended do not depend on the fact that floor has had long enough to cure. If your operating temperature from your manifold mixing valve is going to be 40 deg C then run at this temperature for a couple of days. You can get away with a bit of variation in thickness there are special spacers that go under adjacent tiles and you put a wedge through to pull the faces together. Google tile levelling system, several type,s and you tube videos, to demonstrate.
    • CommentAuthorphiledge
    • CommentTimeJan 11th 2021
    If you can accept a long response time in heating up, then the only problem I can see is the ammount of adhesive youre going to need to bury the UFH pipework. Im assuming that the slab is sat on a good depth of insulation?

    Not sure what size pipework you're planning on using but Id check with the adhesive manufacturer that the thickness you need is acceptable. Use a flexible adhesive

    We've got wet UFH buried in a 4-5" thick slab with ceramic tiles over the top. Takes a couple of hours to start feeling heat but once its warmed up the tiles are comfy in bare feet the following day😊
    • CommentTimeJan 11th 2021
    Welcome to the forum :bigsmile:

    I presume the UFH pipes are already buried in the slab? As revor said, I'd operate it first to make sure things work and are properly dry. I don't think you'd have a problem with the limestone flags, and probably not with the slate ones either. They'll just add thermal mass, which means it will take longer for the floor to heat up when you turn the UFH on and longer to cool down after it's turned off. That's as long as you use something that's heat conductive to lay the flags. I think a lime & sand mix ought to work well and would allow you to build it up to level up the slate flags if you went that way. But I'm not an expert in this area, so please look for confirmation.
    • CommentAuthortychwarel
    • CommentTimeJan 11th 2021
    I have 19mm quarry tiles on mine and it works fine
    • CommentAuthorCiriboyc
    • CommentTimeJan 12th 2021
    Thanks a million for all the comments..they've been really helpful.
    Just to clarify a couple of points..

    The pipes are under a 2 inch fibre reinforced concrete slab and sit on top of 4 inches of insulation. The system was commissioned a couple of months ago and has been running constantly since, albeit mostly at a very low temperature.

    I do like the idea of a lime/sand mix to bed the tiles...but I'm more inclined to go with a flexible adhesive...I just think it's a 'safer' way to go unless anyone here has experience with the sand/lime method ?

    Also as regards the slate ( varying thicknesses)...my idea was to 'grade' them ...so begin at 1 end of the house with all the 30mm layed beside each other...moving up to the 35mm all beside each other and so on...

    ...a bit tricky, but I don't mind spending time at it.

    Also, there is already a slight run/fall in the floor which I don't mind.

    Thanks again
    I have used sand & lime (NHL 5) this way, to do a fully breathable floor system with UFH.
    In one room it was a mix of the original sandstone flags (4-6 inches thick) and some reclaimed sandstone flags (that were about 3 - 4 inches thick)
    The other room was the original Staffordshire Blue brick pavers about 2 -3 inches thick.
    All taken up, dug out to increase the depth, Terram membrane, LECA insulation, more terram, Limecrete slab, UFH pipes cable-tied to reinforcing mesh, then lime screed to set the stone or pavers on.

    Wood stove with back boiler provided the heat.

    No problems.

    The current project (barn conversion) was similarly dug out, then 150mm compacted crushed recycled hardcore, 150mm EPS (2 x 75mm), DPM, 100-125mm-ish concrete slab, UFH pipes cable-tied to reinforcing mesh, then sand & cement screed to set the brick pavers on (these ones are similar to Staffordshire Blues but cheaper, and about 2-3 inches thick). This will be a lot of thermal mass but what is we want.

    GSHP will provide the heat. Not up and running yet but not expecting any problems.

    The first place we did was a chapel conversion but the builder did the floor for us - that was Indian Stone flags set on sand & cement screed over the UFH pipes which were clipped into those egg boxes/trays. He did a really nice job of it, no problems.

    Gas combi boiler provided the heat.

    If you can lay your graded slabs the opposite way to the fall on the floor, you might end up with a level floor!
    As above, will add more thermal mass and slow response.

    More important would be the method of control and approach to the UFH. Most UK installers will treat it like a radiator system with short heating periods, relatively high temperatures and room thermostats.

    After 5 years in our house I am thoroughly convinced by the 'long and low' approach. Our heating runs with a single zone for the whole downstairs floor controlled entirely weather compensating controller and an external temperature input (with a heating period of c18 hours a day with the 'off period still being a 15C target temp). Flow temperatures today (it's 9C here today?!) is 23.5C rising to about 28C at 0C external I think
    • CommentAuthorjfb
    • CommentTimeJan 12th 2021
    What insulation are you using below the slab?
    Has the slab got metal mesh in it to strengthen?
    50mm seems really thin for a concrete slab - is there another slab below the insulation or is it just compacted hardcore below?

    Anyway I see no problem with thicker tiles especially if the slab is that thin - as others have said it just slows down the response time for heating and cooling the slab.
    Posted By: Simon Stillthe 'long and low' approach. Our heating runs with a single zone for the whole downstairs floor

    Agree totally with this.
    I'm not sure why we have to provide room-by-room heat loss calculations for the RHI when the building is an insulated rectangular box with rooms within that, MVHR and so gaps under all the room doors. I fully expect the house to all be the same temperature all the time?
    • CommentTimeJan 12th 2021
    Posted By: jfbWhat insulation are you using below the slab?
    Has the slab got metal mesh in it to strengthen?
    50mm seems really thin for a concrete slab - is there another slab below the insulation or is it just compacted hardcore below?

    Who are you talking about/to and where does the 50 mm number come from?
    • CommentAuthorjfb
    • CommentTimeJan 12th 2021 edited
    Posted By: CiriboycThe pipes are under a 2 inch fibre reinforced concrete slab and sit on top of 4 inches of insulation.

    I was replying to the original posters second post djh - assuming I have understood correctly!
    I wondered if he was actually referring to the screed layer, i.e. 50mm of fibre reinforced screed over the UFH pipes? If I recall they normally require 65 -75mm of screed or may be less with fibre reinforcement? This would make sense over the insulation (the more common way that UFH is put in) with a ground bearing (structural?) slab below the insulation.
    • CommentTimeJan 13th 2021
    Posted By: jfbI was replying to the original posters second post djh - assuming I have understood correctly!

    Thanks John. I didn't think to convert to imperial when checking :shamed: I suspect/hope that Dominic's guess is correct.
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