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Green Building Bible, Fourth Edition
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    • CommentAuthorLehobbit
    • CommentTimeNov 8th 2021 edited
     
    Does anyone have experience of using vermiculite insulating concrete? It was suggested to me by another forum member for insulating a raised hearth of am making. The hearth will be 1.4m from rear wall to front and nearly 3 metres wide, with the fireplace legs (sides) falling within the footprint of the hearth. It will be raised about 100mm higher than the surrounding pine boarded floor.

    For the depth of the vermiculite concrete it will range from 80mm depth to around 150mm depth. The current hearth is uneven granite slabs and then in front is an existing concrete slab with a DPC underneath. I plan to put a DPC plastic sheet under the rear part of the new insulated slab where the existing granite slabs are lying direcly on earth, albeit very well drained sandy soil.

    After the slab is laid I will top it immediately with about 30mm of a sharp sand and cement screed which will be bonded to the base layer of vermiculite concrete. This will provide a hard level surfe to tile onto.

    Having never used vermiculite concrete before I wanted to know of people's experiences?

    Would it be advisable to install a reinforcing grid of steel in the slab? How hard does it set? I plan to tile the top and sides of the hearth and then it will need to support a 130 kilo steel woodburner.

    Interested in people's thoughts.

    Also for the main floor the previous owners installed a poured slab of concrete which is over DPC and earth in part and also has a block and beam element with cellar underneath.

    The plan is to batten over this floor with treated pine battens of say 75mm x 60mm, insulate between and then 25mm pine floor boards on top.

    As the current slab is not entirely level I need to lay a screed or use spacers on the batterns to get a correct level across the whole 50m2

    Could I incorporate vermiculite into the screed instead of or in addition to sand and would I be able to lay battens over this and screw into it using rawlplugs to hold the battens in place?

    It would add an extra layer of insulation over a cold concrete floor and prevent the battens making direct contact with the concrete (cold bridge). Obviously I would then insulate between the battens to their full depth.

    Any thought folks?
    •  
      CommentAuthordjh
    • CommentTimeNov 8th 2021 edited
     
    I've never used the stuff but the first hit on google - https://www.vermiculite.org/wp-content/uploads/2014/10/Vermiculite-Concrete.pdf - seems to indicate you'll be OK for your hearth but not for your larger floor. It doesn't mention reinforcement so I'd check with a manufacturer unless you can find some other statement online.
    • CommentAuthortony
    • CommentTimeNov 8th 2021
     
    I have used it, not easy to fix into securely, can’t be reinforced, I would use it under the hearth although I would never use a wood burner, too much harmful particulates
  1.  
    It is worth reading the ink djh posted above.
    from tony's experience

    Posted By: tonyI have used it, not easy to fix into securely, can’t be reinforced,

    but if the floor under the hearth is stable and tamped well if it was loose then reinforcement won't add any value. For the screed over the link quoted above says
    In order to provide a suitably abrasion and wear resistant surface on which the floor finish can be laid, the screeds are normally covered with a denser topping layer comprising of a sharp sand and cement mix. This denser topping layer distributes floor loadings and prevents surface damage and abrasion. For most applications, the topping mix should consist of 65mm (2 ½ inches) of 1:4 sharp sand/cement by volume screed laid over the set vermiculite concrete. This will be self-supporting.

    The quoted link mentions 2" - 3" screed lid on floors as insulation so this could be used to level the existing floor with the battens on that, insulation between and then finish with the pine boards. perhaps use 80mm frame screws to fix the battens to the vermiculite concrete, no abrasion proof screed needed under the floorboards.
    •  
      CommentAuthordjh
    • CommentTimeNov 8th 2021
     
    With regard to the floor, what's driving your desire to use vermiculite concrete? Why not fit the battens with spacers underneath where needed for levelling and then fill in between the battens with something like mineral wool?
    • CommentAuthorLehobbit
    • CommentTimeNov 9th 2021 edited
     
    Hi Peter. I have already downloaded and printed off that link from Vermiculite.org

    It says it is easy to screw into in that document. Doesn't mention reinforcing with steel bar?
    • CommentAuthorLehobbit
    • CommentTimeNov 9th 2021 edited
     
    <blockquote><cite>Posted By: djh</cite>With regard to the floor, what's driving your desire to use vermiculite concrete? Why not fit the battens with spacers underneath where needed for levelling and then fill in between the battens with something like mineral wool?</blockquote>

    <blockquote><cite>Posted By: djh</cite>With regard to the floor, what's driving your desire to use vermiculite concrete? Why not fit the battens with spacers underneath where needed for levelling and then fill in between the battens with something like mineral wool?</blockquote>

    Just so that the wooden battens are not in contact with a cold concrete floor and transferring that cold into the pine flooring. The current concrete floor slab is quite uneven and also needs to be extended where we are removing a wall. I thought that a levelling screed containing vermiculite would help insulate and then extra insulation between the battens carrying the floor. I thought that adding spacers would be loads of extra faffing about and aesthetically pretty awful.

    I still cannot find any information about how hard and solid vermiculite concrete sets? On the link PDF I downloaded it says it can be easily cut and screwed?? If I am laying it at 150mm thick for parts of the hearth with a 30mm cement and sand topping will it be strong enough?
    • CommentAuthorLehobbit
    • CommentTimeNov 9th 2021 edited
     
    <blockquote><cite>Posted By: tony</cite>I have used it, not easy to fix into securely, can’t be reinforced, I would use it under the hearth although I would never use a wood burner, too much harmful particulates</blockquote>

    Can you give me any more informtion? How hard does it set? Difficult to screw into? Will it be okay even at 150mm depth for the hearth? Definately no reinforcing bar?
  2.  
    Posted By: LehobbitI still cannot find any information about how hard and solid vermiculite concrete sets? On the link PDF I downloaded it says it can be easily cut and screwed?? If I am laying it at 150mm thick for parts of the hearth with a 30mm cement and sand topping will it be strong enough?

    The pdf quotes the 28 day compressive strength and recommends a 50mm sand cement screed over. IMO if you follow the advice there should be no problems.
    If your Stove has small section feet then as a precaution you could put a 3mm steel spreader plate under each foot.

    Edit to say that perlite would also be an option as a substitute for vermiculate if price or availability is an issue as it will do much the same thing (internet quotes perlite / cement mix having a compressive strength of 125 - 200 lbs./sq inch)
    •  
      CommentAuthordjh
    • CommentTimeNov 9th 2021
     
    Posted By: LehobbitI thought that adding spacers would be loads of extra faffing about and aesthetically pretty awful.

    We used spacers under the battens under our floor. Didn't seem to be much trouble or take any significant time. We used the plastic packers that are available.

    Packers act as spacers so reduce the thermal bridging quite a lot.

    Aesthetics? Of packers under battens under your floor????? Are you being serious?
    • CommentAuthorLehobbit
    • CommentTimeNov 9th 2021 edited
     
    Regarding aesthetics I'm just a bit of a perfectionist!!:bigsmile:

    Currently the concrete floor is a bit rough and patched up. I would say the difference between the highest part and lowest is about 3 cm. Also we have a 400mm thick stone wall to remove from the back of the room and then to extend the slab under the area where this wall is removed. (it's a secondary inner wall). I just thought a screed would tidy it all up before laying the battens and pine flooring.

    I can see your point about plastic packers acting to stop cold bridging.To fix the floor battens in place I presume you can drill and screw (with rawlplugs) through the plastic packers? I might go for this as it would save a lot of work and cost of the screed and once the floor is laid you'll never see the underneath!!

    These are what they sell here:-

    https://www.leroymerlin.fr/produits/terrasse-jardin/terrasse-et-sol-exterieur/accessoires-de-pose-et-produits-entretien/pose-et-entretien-de-sol-composite/lot

    https://www.leroymerlin.fr/produits/terrasse-jardin/terrasse-et-sol-exterieur/accessoires-de-pose-et-produits-entretien/pose-et-entretien-de-sol-composite/lot-de-

    The concrete slab is really cold. What would be the best insulation between the battens? I am thinking PIR foam is the best performing, but our French eco building adviser suggested granulated cork as a more eco option and also much easier to run cables through. I think a loose fill material that is impervious to moisture (such as cork) would be a good option?
    •  
      CommentAuthordjh
    • CommentTimeNov 9th 2021
     
    Posted By: LehobbitWhere do you get these plastic packers from? I'm in rural France!!
    Dunno about rural France. I expect my carpenters got them at somewhere like Screwfix - they seem to have a fair range. I think the traditional would be bits of old slates etc, or wood packers.

    I jut thought a screed would tidy it all up before laying the battens and pine flooring.

    Your choice of course. I have the same sort of obsession about avoiding wet trades that you have about appearance!

    The concrete slab is really cold. What would be the best insulation between the battens? I am thinking PIR foam, but our French eco building adviser suggested granulated cork as a more eco option and also easier to run cables through.

    Again I have an obsession about avoiding PIR/PUR so I'd go for cork too. Don't forget to derate any cables that run through insulation.
    • CommentAuthorLehobbit
    • CommentTimeNov 9th 2021
     
    <blockquote><cite>Posted By: Peter_in_Hungary</cite><blockquote><cite>Posted By: Lehobbit</cite>I still cannot find any information about how hard and solid vermiculite concrete sets? On the link PDF I downloaded it says it can be easily cut and screwed?? If I am laying it at 150mm thick for parts of the hearth with a 30mm cement and sand topping will it be strong enough?</blockquote>
    The pdf quotes the 28 day compressive strength and recommends a 50mm sand cement screed over. IMO if you follow the advice there should be no problems.
    If your Stove has small section feet then as a precaution you could put a 3mm steel spreader plate under each foot.

    Edit to say that perlite would also be an option as a substitute for vermiculate if price or availability is an issue as it will do much the same thing (internet quotes perlite / cement mix having a compressive strength of 125 - 200 lbs./sq inch)</blockquote>

    Well I have the vermiculite now, 5 x 100 litre bags. I don't really understand the compressive strength figures? My concerns were that in places the vermiculite concrete will be 150mm thick and once I remove the shuttering from the front and front sides of the hearth slab that even with the 30mm sand/cement topping the sides will be weak? I plan to tile the sides and top of the slab so I just want to be sure the thing won't collaspe. Apparently they make pizza ovens out of vermiculite concrete!!
  3.  
    The PDF quoted above says the compressive strength at the weakest recommended mix is 0.7 N mm2 which is about 7 kg / cm2 and 12.5 kg / cm2 for the strongest mix so IMO no problem with the planned weight of your stove. The only caveat is that the stove could have a point load (because of the shape of the feet) higher than the rated figures so I would ensure a spreader plate or tiles (or both depending upon the tiles) under the feet. If you tile the top and front I don't think there will be problems with break out at the front. I would use a 50mm screed on top as recommended rather than the 30mm you are suggesting.

    If you use the Html option at the bottom of the comments box any quotes come out in blue
    • CommentAuthorLehobbit
    • CommentTimeNov 11th 2021
     
    Posted By: Peter_in_HungaryThe PDF quoted above says the compressive strength at the weakest recommended mix is 0.7 N mm2 which is about 7 kg / cm2 and 12.5 kg / cm2 for the strongest mix so IMO no problem with the planned weight of your stove. The only caveat is that the stove could have a point load (because of the shape of the feet) higher than the rated figures so I would ensure a spreader plate or tiles (or both depending upon the tiles) under the feet. If you tile the top and front I don't think there will be problems with break out at the front. I would use a 50mm screed on top as recommended rather than the 30mm you are suggesting.

    If you use the Html option at the bottom of the comments box any quotes come out in blue



    Okay Peter Thanks for your advice.
    • CommentAuthorGreenPaddy
    • CommentTimeNov 11th 2021
     
    If you're concerned about the face/edge of the slab, next to the shutter, fill the void next to the shutter with the non-vermiculite concrete, as you fill behind it with the vermiculite stuff. Will give you a good face to tile to, same as the top surface.
    • CommentAuthorbxman
    • CommentTimeNov 11th 2021 edited
     
    Hi
    Good luck any photos of your project you can show us ?


    Did you ever manage to locate any foamglass at a sensible price ?

    seem to remember you have a beautiful location

    Keep safe cheers Patrick
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