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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorjemhayward
    • CommentTimeOct 23rd 2018
    To our surprise the listed building people are happy for us to excavate our cellar floor, and put in foam glass insulation and limecrete - they have also approved using a drainage membrane which will hopefully channel any rising water to our flood pumps, and act as a radon barrier at the same time. However, it will then make the floor impermeable - and non breathable, so is there any point in putting limecrete on top of a plastic layer - and we will then possibly get back to the old problem of damp walls rather than a damp floor, as we have in our barn due to the concrete floor installed there some decades ago.

    Anyone got any relevant experience to share?
    • CommentAuthorjfb
    • CommentTimeOct 24th 2018
    i can't see the point of using limecrete floor if you are going to have an impermeable membrane below.
    What are you planning in using the cellar for? Is there any ventilation to the outside?
    I imagine the general principles of dealing with moisture/water in cellars is either seal it up/tank it so that no moisture gets in or accept moisture from outside can get in. But in an older property if you seal it from the inside it will probably cause longer term damage to the structure hence the need for some breathability.
    Either way you are going to need ventilation and I would think in your situation through ventilation is going to be key.
    French drains on the outside might also be worth considering.
    • CommentAuthorjemhayward
    • CommentTimeOct 24th 2018
    That is what I'm thinking. We cannot tank the whole thing as it's 500 yrs old, listed, and looks amazing. It has flooded once, and the ground water rises and falls in the test pits we have dug, so we will be installing a proper flood sump, with two pumps in it, which I feel will get use in the wetter parts of the year. We have also dug a French drain at the rear, which also has pumps in it, and that may be why it's not flooded since. We have a (minor) radon issue, so a radon barrier appeals, but I think that ventilation is going to be important, so I'm putting a vent pipe in the lightwell at the front, and I'm seriously considering adding a fan to that as well so there is a constant flow of air outwards taking excess moisture and radon with it. We will also dig a radon sump, but suspect it may fill with water, but whilst we are digging, we may as well do it. We're also going to have a radiator down there, mainly to avoid cold air being sucked up the stairs into the main house. I think I'm going to go for a geotextile, foamglass, geogrid and limecrete construction, so the floor is totally breathable, and then hope the pumps will keep the actual water at bay, and accept that it's going to be a bit damp down there, as it is a cellar after all.
    Agree with jfb and you, jemhayward.

    No apparent logic that I can see for limecrete on drainage membrane. If there is a logic for (impermeable) drainage membrane, then you can put concrete of anything you like on it - it's not going to 'breathe' to the sub-floor (though I accept lime may offer some benefits in terms of moisture buffering).

    I agree that the radon sump might flood! Advice is often that radon sumps be fitted with the provision for retrofitting a fan. What use that would be if the sump is full of water, who can tell?! I think your above-'slab' vent plan might actually work better in practice, and it is no worse (indeed somewhat better) than the strategy recommended for sub-floors in (IIRC) low-to-medium radon areas, which simply involves lots of air-bricks. (A bungalow extension approx 3 x 6.5m had, if I remember rightly, 14 '150 x 225' air-bricks to vent a floor void approx 400 deep).
    • CommentAuthorSilky
    • CommentTimeOct 28th 2018 edited
    if you search posts I added some info on this subject, I was in a similar situation.

    In brief, foamglass floats, so if you are at risk of groundwater they usually advise not to use it. I did, but you must ensure the weight on top of the insulation is enough to counteract buoyancy. i.e. I ended up with 300 mm foamglass + 120/150mm concrete on top

    Foamglass keeps the building dry by draining down to ground, if you put an impermeable barrier underneath it's a bit pointless

    French drain ++

    I copied someone else who did a concrete island, i.e. you pour a concrete slab and leave a border between the slab and walls ( mine was about 300mm wide ), you then make up the border with something more breathable, e.g. expanded clay aggregate ( Leca / Lyapor etc.. ) + Lime/Sand mix. The floating risk here is reduced due to the friction between the border piece and wall and slab.

    My own personal opinion after much reading, the whole limecrete is breathable, concrete is not is slightly controversial. I don't think it is as black and white as whether you use portland cement or HL5. HL5 has a similar mu to regular cement, although it certainly behaves better in harsh conditions than portland. The breathability of the floor is due to the microscopic pores, obviously Limecrete will be more breathable when mixed and laid by hand because it is not as compact. Concrete can be made more breathable by doing a bad quality mix, i.e. more water and no additives. I had to argue with the concrete companies to try and persuade them to bring me an old fashioned mix.. i.e. lime / portland cement + sand + stones, no additives and lots of water. The floor was bone dry within a few weeks and has remained so since, even after rising ground water last winter. And of course NO DPM, which would trap the moisture in the slab. Also I would make sure the concrete company do not substitute GGBS for cement and if you can get them to use a crushed limestone aggregate instead of pebbles that will also increase the permeability of the slab. I would love to say mine was perfect but I clearly p***edoff the 1st concrete company because they couldn't understand the logic, ignore me and delivered the concrete with GGBS in the mix, luckily we were doing a large area poured in bays and I managed to get what I wanted for the next couple of pours. So lots to consider, find a sympathetic concrete supplier.
    • CommentAuthorjemhayward
    • CommentTimeNov 17th 2018
    I'm still waiting to hear from my builder if and when he can do this. I'm slightly concerned that the lime pointing we did down there is still soft after a week - no cracks though! I think the humidity - well dampness, is slowing the drying well, but maybe too well as I suspect there isn't much calcification going on yet - so how long is a limecrete slab 100mm thick going to take to become walkable?
    • CommentTimeNov 17th 2018 edited
    What type of lime in your limecrete?
    edit: and in your pointing? Also, what's its temperature?
    • CommentAuthorgyrogear
    • CommentTimeNov 18th 2018 edited
    Posted By: jemhaywardso how long is a limecrete slab 100mm thick going to take to become walkable?

    I went on mine after 88 days, and it took 100 days for it to ring to a trowel-strike, if that's of any use...
    I rehydrated it weekly during all that time...

    I think the limecrete will have NHL3.5 in it. My 'coarse stuff' pointing has now hardened, but when I do the next phase I think I'm going to use NHL3.5 sharp sand mix for my mortar to speed things up a bit. Temperature is around 12c most of the time, going a bit lower at the moment, as it's not weather tight at the moment.
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