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Housing - Renovation: A question of DPC with an insulated slab

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  1.  
    I am putting in an extension (3m x 4m) with an insulated slab. the soil is very light and sandy and free draining with a water table at 1.5m. The floor make up will be
    sub soil
    200mm EPS
    120mm concrete slab
    From another thread I see that putting the DPC under the EPS could / would create a bucket from which any water or damp from building would never escape so putting it under the slab is the better place. But given the properties of EPS is a separate DPC under slab needed at all and if it is protecting against damp - From where is the damp coming?
    (The main house to which the extension abuts is stone/rubble construction with no DPC)
    • CommentAuthorJeff B
    • CommentTimeAug 20th 2019
     
    I presume you are referring to the comment by Tony in the other thread where he says:

    [DPM under the EPS]..."Under it can act as a bucket that can collect water during construction, leaks and in extreme cold condensation".

    I guess this is a valid point for a new construction which is open to the elements. The job I did was inside an existing property (a 170 year old chapel) where these factors do not apply and I was adding insulation on top of an existing concrete slab where there is no DPM in the first place. In my case the dampness would be coming up through the slab, hence we used DPM under the EPS. I don't know when the concrete slab was put down, certainly not when the chapel was built but possibly early 1900's?
  3.  
    From tony comment I presumed this would also include water from the pouring of the concrete slab.

    My understanding of EPS is that it won't wick up or soak water so would the EPS itself act as a DPC?
    • CommentAuthorGreenPaddy
    • CommentTimeAug 20th 2019
     
    Peter, my thought would be that you ought to have a DPM at some level. I usually put one on the solem, under the insulation, but then a second which goes over the insulation, onto which the concrete is poured.

    This upper one is to stop the water from the concrete dropping in between the insul butted edges, which would mean the concrete had insufficient water content, and would not cure correctly.

    The lower DPM is to prevent ground water from below, or from the surface adjacent to the house encroaching under the slab, from getting to the insulation, some types of which may degrade or lose performance in the presence of water.

    I've seen various opinions on here, about EPS and water - check it with the manufacturers, and satisfy yourself. It may be with your very free draining site, that you could forego the lower layer. A slab is usually in a recess in the ground, so there could be a tendency for precipitation to move towards that.

    Assess the risk, and the consequences, and make your choice. I'm guessing Hungarian building control, may not be checking too closely, or maybe that's my Boris-onian prejudices coming to the fore!!
    • CommentAuthorKenny_M
    • CommentTimeAug 20th 2019
     
    In previous threads on here there have been a lot of people stating that EPS is vapour permeable. I remember Tom talking about how you can suck air through a piece of EPS, so if it is not air tight then it wont be vapour tight.

    I did a lot of research on this at the time (in the context of wall insulation on an old house) and I found that while it wasn't as vapour permeable as say lime plaster or wood fibre, it was more so than most other plastic insulation types.

    I wonder though if the transfer mechanism would be different in a floor anyway. Is there likely to be much water vapour below a floor, with no heat to convert from liquid? If vapour does exist it wont be going from warm to cold so condensation might not be an issue. If it is water as a liquid only, and EPS doesn't wick up or soak water then maybe it would be fine.

    Isn't water vapour heavier than dry air, so would a barrier above the insulation just be preventing the vapour from rising out of the floor?

    Concrete/cement mortar is always said to be impermeable to vapour, so would any significant amounts of water come through a concrete floor?

    I am confused about this too.
    •  
      CommentAuthordjh
    • CommentTimeAug 20th 2019
     
    It's not vapour permeability that matters as far as the need for a DPM/C goes. It's whether the material provides a capillary break. EPS does not provide a capillary break AFAIK, and therefore a DPM is a good idea.

    Water vapour is lighter than air. (H2O weighs less than O2 or N2).

    Concrete is not impermeable to water, in general. That's why there are special products that are waterproof.

    Given what PiH and GP have said, I would likely go for a DPM on top of the EPS.
    • CommentAuthorKenny_M
    • CommentTimeAug 20th 2019
     
    Posted By: djhIt's whether the material provides a capillary break.

    This is what I meant by wondering about whether the mechanism was different - capillary action was the phrase I was searching for.

    Posted By: djhConcrete is not impermeable to water, in general.

    Not disputing that. I just think about one of my rooms which has what appears to have had a concrete floor underneath wood flooring. From small exploratory excavations it appears to be straight on to soil with no DPM. There is no dampness underneath the wood flooring, but there is around the base of the walls and any areas such as under the stairs that the concrete doesn't cover. I understand that there are many different mixes and forms of concrete - I'm just thinking out loud!
    • CommentAuthorJeff B
    • CommentTimeAug 20th 2019
     
    <blockquote><cite>Posted By: Peter_in_Hungary</cite>From tony comment I presumed this would also include water from the pouring of the concrete slab.

    My understanding of EPS is that it won't wick up or soak water so would the EPS itself act as a DPC?</blockquote>

    Take a look at this:

    https://jablite.co.uk/wp-content/uploads/2016/12/Jabfloor-Insulation-Over-Slab-with-Screed-Finish.pdf

    This specifies that a DPM must be used, either above or below the slab. Maybe worth having a chat with technical people at Jablite as they are a major supplier of EPS.
    • CommentAuthorgyrogear
    • CommentTimeAug 20th 2019
     
    Posted By: GreenPaddyThis upper one is to stop the water from the concrete dropping in between the insul butted edges, which would mean the concrete had insufficient water content, and would not cure correctly


    +1

    gg
  10.  
    In a similar situation at the moment now with the barn conversion. Proposed floor build up is as follows:

    subsoil (heavy clay)
    100 - 150mm crushed recycled concrete hardcore whacked flat
    150mm EPS (in 2 staggered layers of 75mm)
    DPM going in here and lapped up the walls a bit
    100mm concrete slab (probably some insulation round the perimeter as well - either some EPS if any left over or 100mm Celotex)
    ufh pipes cable tied to A142 mesh
    screed
    top surface (blue brick or stone flags)
    •  
      CommentAuthordjh
    • CommentTimeAug 20th 2019
     
    Posted By: Dominic Cooneysubsoil (heavy clay)

    Have you checked expansivity etc to make sure 300 mm is sufficient dug out to prevent heave?
  12.  
    No I haven't.
    and it's more like 450 - 500mm dug out because there were concrete cattle standings put in probably 1960's
    on top of 200 year old barn floor inside, partial brick floor etc.
  13.  
    My experiences with EPS are that it does not take up water, this started some years ago when a dinghy sailor friend did an experiment to test EPS as permanent buoyancy. He submerged EPS in water for 6 months with no water uptake at all. I have never seen internally damp EPS.

    Whilst jablite recommend a DPC they only show their EPS above the slab whereas I will be putting the EPS below the slab. Also I feel that perhaps the specification of a DPC under EPS is non controversial, keeps BCOs happy and avoids the need for expensive test to prove the point. (And to some extent helps to guard against poor installation practice)

    I guess the real question is what purpose would a DPC serve between the EPS and the concrete slab where the EPS is on the ground with the slab above it. If it is to protect against damp then where is the damp coming from?
    If it is to stop the water draining out of the slab mix - well if the EPS is laid properly then there should be minimum drainage.

    For drainage prevention any old plastic would do, for a DPC then proper stuff is needed and care has to be taken to avoid pin holes and tearing when the builders run wheel barrows over it to lay the slab (not to mention the rebar mesh needed to spread the load of an internal wall). The trouble needed to ensure a good DPC is much more than the cost of the materials and if not justified makes life much easier.
    • CommentAuthortony
    • CommentTimeAug 20th 2019
     
    The way I see it is that contrary to popular belief if the house is warmer than the ground, which is the usual case, the partial vapour pressure if the moisture in the air in the house is higher than it is outside, in the concrete or in the ground (even if the ground is wet) moisture will move from the house toward the ground or the outside.
    •  
      CommentAuthordjh
    • CommentTimeAug 20th 2019
     
    Posted By: tonyThe way I see it is that contrary to popular belief if the house is warmer than the ground, which is the usual case, the partial vapour pressure if the moisture in the air in the house is higher than it is outside, in the concrete or in the ground (even if the ground is wet) moisture will move from the house toward the ground or the outside.

    This is true as far as it goes. But vapour is not the only form of water transport. It can also move as a liquid, and in that form it can move uphill by capillary action. DPCs are there to act as a capillary break.
    • CommentAuthortony
    • CommentTimeAug 20th 2019
     
    I think that we should tap that as a mechanism for producing energy
    • CommentAuthorgyrogear
    • CommentTimeAug 21st 2019
     
    Posted By: Peter_in_Hungaryif the EPS is laid properly then there should be minimum drainage


    well, that is somewhat missing the point: there should not be *any* drainage !
    (The amount of water in the concrete was decided by the engineer, for the application in question).
    (neither should there be any ingress of water (or water vapour) *into* the concrete...).

    gg
  18.  
    Posted By: gyrogearThe amount of water in the concrete was decided by the engineer, for the application in question

    Correct - but then what happens on site is a different matter, and how many builders take account of the rain that dumped on the all in ballast last night and add a bit more water 'cos its easier to lay a bit wetter etc.
    • CommentAuthorgyrogear
    • CommentTimeAug 21st 2019
     
    sorry, I'm not into un-green hypotheses !

    gg
    • CommentAuthorCWatters
    • CommentTimeAug 21st 2019 edited
     
    XPS or EPS...

    The makers of XPS like to claim it's better than EPS..

    https://www.kingspan.com/meati/en-in/product-groups/insulation/knowledge-base/articles/general/what-is-the-difference-between-xps-and-eps

    "Extruded polystyrene insulation or XPS Insulation is manufactured through an extrusion process. This manufacturing process involves melting together the plastic resin and other ingredients. The liquid formed is then continuously extruded through a die and expands during the cooling process. This produces a closed cell rigid insulation.

    Expanded polystyrene insulation or EPS insulation is manufactured using a mould to contain small foam beads. Heat or steam is then applied to the mould, which causes the small beads to expand and fuse together. This manufacturing process does not form a closed cell insulation as there can often be voids between each of the beads where they are not touching one another."
    • CommentAuthorCWatters
    • CommentTimeAug 21st 2019 edited
     
    Posted By: Peter_in_HungaryMy experiences with EPS are that it does not take up water, this started some years ago when a dinghy sailor friend did an experiment to test EPS as permanent buoyancy. He submerged EPS in water for 6 months with no water uptake at all. I have never seen internally damp EPS.


    I think that's correct but I have seen waterlogged EPS on a beach. It was too heavy to blow away in the wind. You could squeeze a lump and water came out. However I've no idea how old it was or what it had been through to get to that state.
    •  
      CommentAuthorfostertom
    • CommentTimeAug 22nd 2019
     
    Posted By: CWattersThe makers of XPS like to claim it's better than EPS..
    Kingspan are actually describing (one of) the virtues of EPS, compared to all the foamed plastic insulations. It's a tragedy that the trade has fallen completely for the profitable line that the foams are superior. As well as EPS gives best insulation per £, albeit at greater thickness.
  23.  
    Posted By: fostertomAs well as EPS gives best insulation per £, albeit at greater thickness.


    This is why we have dug out further and used EPS. It was cheaper than PIR even with a thicker layer.
    We would have gone thicker still, but were restricted by the depth of the footings.
    • CommentAuthorgyrogear
    • CommentTimeAug 22nd 2019
     
    "It was cheaper than PIR even with a thicker layer."

    Per my summary calcs, "price per R" (France) for Th 80mm :
    EPS = 3.99 E
    XPS = 5.27 E
    PUR = 5.29 E

    Perhaps one is paying over the odds for an (admittedly) higher water vapour resistance and fancy cutting-aid lines printed on an aluminium foil which performs no other function anyhow under a slab!

    gg
    •  
      CommentAuthorfostertom
    • CommentTimeAug 22nd 2019 edited
     
    What's white EPS @say 120mm thk and platinum EPS at say 100thk in France? That wd be comparable to 80 foamed. Similar price, I'd guess - wd be gd to re-check the price comparison for UK.
    • CommentAuthorgyrogear
    • CommentTimeAug 22nd 2019 edited
     
    Hi, Tom,

    the BD '"kingf***sher" catalogue stops at 100 mm,
    so adding 100mm and 20mm boards gives (11.81) + (2.15) = 13.96 Euros per sq.m.
    The respective R values being (2.7) + (0.5)
    prices include VAT at 20%)
    ================
    the grey stuff is here:
    https://www.leroymerlin.fr/v3/p/produits/panneau-en-polystyrene-isolant-pour-ite-1-20x0-60-ep-120mm-prb-e1501072210
    board price is 12.71 Euro for 0.72 m2, making it 17.65 Euros per m2 (I think!) for 120 mm
    or
    9.70 Euros per board = 13.47 E per m2 for Thk 100 mm.
    ==============
    White EPS, thk 120 mm seems not avl at same place
    but found it here
    https://www.leroymerlin.fr/v3/p/produits/panneau-en-polystyrene-isolant-pour-ite-1-20x0-60-ep-120mm-prb-e1501072210
    price : 14.20 + VAT @20% = 17.04 Euros per m2
    or 14.20 + 10% VAT = 15.62 Euros per m2.

    If U need any more look-ups, just call !

    Cheers,

    gg
    • CommentAuthortony
    • CommentTimeAug 22nd 2019
     
    Why I love EPS, nice cost effectiveness
    • CommentAuthorKenny_M
    • CommentTimeAug 23rd 2019
     
    I was curious about the suggestion that EPS does not take on water and found some comments about this in a place i didn't expect. Apparently EPS is used for surfboards and on a site about issues with boards sinking when the surface coating is damaged one site says:

    "Out of the currently utilized foams for blanks, EPS foam or Expanding Polystyrene foam absorbs water more readily."

    There are a raft (pardon the pun) of surfboard sites out there discussing the same issue.

    Granted these boards are semi submerged for a long period, but that won't be that much different to insulation sitting directly on wet ground, and it wouldn't sound advisable to pour wet cement directly on top of it either, although I can see that the latter might set before there is much penetration.
  29.  
    Posted By: Kenny_MI was curious about the suggestion that EPS does not take on water and found some comments about this in a place i didn't expect. Apparently EPS is used for surfboards and on a site about issues with boards sinking when the surface coating is damaged one site says:

    "Out of the currently utilized foams for blanks, EPS foam or Expanding Polystyrene foam absorbs water more readily."

    There are a raft (pardon the pun) of surfboard sites out there discussing the same issue.

    Granted these boards are semi submerged for a long period, but that won't be that much different to insulation sitting directly on wet ground, and it wouldn't sound advisable to pour wet cement directly on top of it either, although I can see that the latter might set before there is much penetration.

    I don't know what type of 'Expanding Polystyrene foam' they are using but that has not been my experience with EPS in boats. My son has a plastic EPS plastic sandwich sail dinghy (a real cheapo job) with a lot of de-lamination between the plastic and the EPS and a multitude of splits in the plastic and there has been no weight gain noticed.

    Also discussed here before a test detailing below ground performance of EPS showing minimal uptake of water after 15 years under ground

    see

    http://www.epsindustry.org/sites/default/files/EPS%20Below%20Grade103.pdf

    http://www.epsindustry.org/sites/default/files/EPS%20Below%20Grade101.pdf
    • CommentAuthorKenny_M
    • CommentTimeAug 27th 2019 edited
     
    Posted By: Peter_in_HungaryI don't know what type of 'Expanding Polystyrene foam' they are using

    I know they say foam, but a quick search of the internet shows loads of people discussing EPS being waterlogged.

    I'm not able to comment from experience, I'm just noting that there seems to be a wide variance in the experience of others. Someone on this thread has even mentioned seeing waterlogged EPS on a beach. Some of the descriptions that I read talk about EPS being waterlogged to the extent that it was very difficult to dry out. Its a little like the vapour permeability of EPS which is often lauded on this site, but many of those involved in heritage property insulation would push back on this, because while EPS is more vapour permeable than say XPS, its not nearly as vapour permeable as lime plaster or wood fibre.

    The sites quoted are from an EPS industry body, so not the most unbiased source, but the fact that they are researching graded EPS maybe highlights that comparing experiences of EPS is like comparing apples and oranges. Maybe the waterlogged EPS is cheap EPS, for packaging or other purposes, and that used in those reports is high quality, graded EPS. If EPS is being manufactured for packaging then they might be less concerned about the size of the air paths, and if those pathways are large enough they will also take in water. There must be a fine line between the size of pathways through EPS that will allow water vapour and that which will allow liquid to pass. If that's the case then maybe talking about EPS in general terms is misleading. Is it safe to say that EPS that is designed for insulation purposes is designed to have narrow enough pathways between the beads, that it will not allow the connected molecules of (edit: liquid) water to pass?

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