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Green Building Bible, Fourth Edition
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    • CommentAuthorbardo
    • CommentTimeJan 20th 2019 edited
     
    Hello, I am new to this forum, a first time builder and just about to start work on the foundations for a timber frame home with straw bale insulation. These have been worked through and added to by a structural engineer. Would welcome some feedback on our make-up and any potential issues. Diagram attached to support the questions below.

    We are on heavy clay soil and building a 400mm blockwork plinth wall around perimeter with stub wall though centre. The engineer has specified a depth of 750mm wide x 900 deep. I was going for a stone trench however have decided to use 300mm of PFA concrete in the base and then coming out of that with two layers of blockwork spreading with lean mix infill. Either side will be compacted gravel filling in the void. The blockwork will continue up another 400mm with airbricks placed with a DPM on top and straps to fix to the main timber ringbeam/sole plate.

    Few questions. (1) Any potential issues with rendered finish direct onto the revealed concrete blockwork? I'm presuming cement or lime is ok? (2) Will I need drainage pipe within the gravel on the outer side of the blockwork as suggested by engineer? I can't see how drainage at this level will help as it will only drain if and when water rises from below meaining water will remain in trench (3) The plasticity of the soil is low to medium however the engineer suggested using 50mm strip of vertical polystyrene heave board. This is specified to sit directly above the concrete base against the inside soil wall with gravel infill separating it from the blockwork. Is this necessary? Conventional builders round here don't use it. (4) Building regs require that we put a weed barrier underneath the building. Was thinking a blinding using sub-base.Do I need a woven plastic membrane too? CHEERS!
      pencoedfoundations.jpg
  1.  
    Welcome to the Forum!
    • CommentAuthorbardo
    • CommentTimeJan 20th 2019
     
    <blockquote><cite>Posted By: Nick Parsons</cite>Welcome to the Forum!</blockquote>:bigsmile:
    •  
      CommentAuthordjh
    • CommentTimeJan 20th 2019 edited
     
    Posted By: bardoFew questions.

    As Nick says, welcome.

    (1) Any potential issues with rendered finish direct onto the revealed concrete blockwork? I'm presuming cement or lime is ok?

    I'd be interested to see/hear more about how you're planning to build on top of your dwarf wall, and what surface treatment you will use. In the absence of other information I'm assuming you will use lime over the bales, in which case I'd use the same over the dwarf wall. The main problem will be potential cracks along the top of the wall where different materials are joined. Make sure your plasterer understands the issues and solutions.

    (2) Will I need drainage pipe within the gravel on the outer side of the blockwork as suggested by engineer? I can't see how drainage at this level will help as it will only drain if and when water rises from below meaining water will remain in trench

    Yes, you want the drainage pipe on the outside, and it must drain into a surface-water drain or open ditch/water, which may be what determines its level. It's a 'French drain' - you can read up about them. But as long as it drains the surface water, it's doing its job.

    You have a clay heave board on the inside. If your clay is expansive enough to need that, why is there not one on the outside as well?

    IMHO, I'd make the outside French drain wider. 600 mm or so is a good width. The aim is to reduce splashback onto the wall and helpp keep rodents away from your walls.

    (3) The plasticity of the soil is low to medium however the engineer suggested using 50mm strip of vertical polystyrene heave board. This is specified to sit directly above the concrete base against the inside soil wall with gravel infill separating it from the blockwork. Is this necessary? Conventional builders round here don't use it.

    It's the expansiveness rather than plasticity that matters for heave? As above, why inside and not outside? You can read up on the engineering specs to second guess your engineer if you want, but you're paying him for his expertise. Ask him for his reasons if you don't understand. You aren't paying the conventional builders so don't pay too much attention to them.

    (4) Building regs require that we put a weed barrier underneath the building. Was thinking a blinding using sub-base.Do I need a woven plastic membrane too? CHEERS!

    It's a question about the building regs, which I think requires a weed barrier and suggest a concrete slab is deemed to satisfy, whereas hardcore isn't. How much growth is likely in the dark under a building anyway? The question didn't arise in my case, since we have a concrete passive slab so I'm not sure of the answer, but I think some of the more eco-conscious builders (e.g. Barbara Jones) have other solutions that have been accepted. What floor structure are you planning?

    edit: Just being nosy, but where are you building?
  2.  
    The land drain pipe and the weed barrier membrane could be useful in some circumstances. Probably included as part of the 'cover my ar*e' approach adopted by most professionals these days. On the other hand they might have some benefit and given the minimal cost of both why not include them.

    With regard to the land drain I would put it just above the base not 2/3 way up as shown in the diagram, however IMO the gravel should have no fines in it to ensure free draining and most important the resultant land drain should have somewhere to drain out otherwise instead of a drain you will create a moat and no amount of drain pipe will solve that.

    Just IMO
    • CommentAuthortony
    • CommentTimeJan 21st 2019 edited
     
    There are several things I don’t like, g/l same inside and out, no need to compact lean misx and top slopes outwards, what are the air bricks for?

    The sub structure is very cold and causing a monster thermal bridge under the sole plates, I would like to see 100 eps from dpc to top of foundation to mitigate this and hang Straw bales out beyond it with a drip bead to master the step in the render.

    Mixing sway bale and timber frame seems overkill as straw bales can form the structure.

    Where is inside finished floor level?
  3.  
    Posted By: tonyMixing sway bale and timber frame seems overkill as straw bales can form the structure.

    Structural straw bales can be difficult to get past the SE and BC because they are an unknown quality and density - apart from the problem of getting the quality straw bales needed. A timber frame with straw bale infill gives a much easier path through the building regs and the quality of the bales (density) is not so important
    • CommentAuthorbardo
    • CommentTimeJan 21st 2019 edited
     
    Posted By: tonyThere are several things I don’t like, g/l same inside and out, no need to compact lean misx and top slopes outwards, what are the air bricks for?

    The sub structure is very cold and causing a monster thermal bridge under the sole plates, I would like to see 100 eps from dpc to top of foundation to mitigate this and hang Straw bales out beyond it with a drip bead to master the step in the render.

    Mixing sway bale and timber frame seems overkill as straw bales can form the structure.

    Where is inside finished floor leve?


    Thanks Tony.

    Lean mix not compressed, just poured. Error on diagram. The plinth wall will go all the way round to provide decent circulation underneath building.

    Please clarify what you mean by 100mm eps from dpc to top of foundation? I should have said that the soleplate make-up is an insulated box that alleviates the cold bridging and is not your standard soleplate arrangement. The engineer also mentioned that I could transition to insulation concrete blocks above ground level to reduce the cold coming up the stem.

    We explored the options in depth. Load bearing works well though in end we decided on lightweight frame. This will help provide structure for timber cladding too.
    • CommentAuthorbardo
    • CommentTimeJan 21st 2019
     
    <blockquote><cite>Posted By: Peter_in_Hungary</cite>The land drain pipe and the weed barrier membrane could be useful in some circumstances. Probably included as part of the 'cover my ar*e' approach adopted by most professionals these days. On the other hand they might have some benefit and given the minimal cost of both why not include them.

    With regard to the land drain I would put it just above the base not 2/3 way up as shown in the diagram, however IMO the gravel should have no fines in it to ensure free draining and most important the resultant land drain should have somewhere to drain out otherwise instead of a drain you will create a moat and no amount of drain pipe will solve that.

    I agree with that concern Peter, thanks.

    Just IMO</blockquote>
    •  
      CommentAuthordjh
    • CommentTimeJan 21st 2019
     
    Stefan, if you edit your posts and select Html instead of Text format, they will have the blue boxes that make them mor readable.
    • CommentAuthorbardo
    • CommentTimeJan 21st 2019
     
    Sorry all, if my response isn't clearly formatted. To make sure your responses are clearly distinguishable from my reply do I simply click quote and then html?
    •  
      CommentAuthordjh
    • CommentTimeJan 21st 2019
     
    Posted By: bardoSorry all, if my response isn't clearly formatted. To make sure your responses are clearly distinguishable from my reply do I simply click quote and then html?

    Yes. If you want to intersperse your answers then you need to cut and paste < / blockquote > and < blockquote > tags to balance the quoted parts against the new parts. You can always go back and edit your posts again and again until it looks right.

    If you Whisper then only you and the person you whispered to can see the post. Useful for private messages.
    • CommentAuthorbardo
    • CommentTimeJan 21st 2019
     
    Posted By: djh
    Posted By: bardoSorry all, if my response isn't clearly formatted. To make sure your responses are clearly distinguishable from my reply do I simply click quote and then html?

    Yes. If you want to intersperse your answers then you need to cut and paste < / blockquote > and < blockquote > tags to balance the quoted parts against the new parts. You can always go back and edit your posts again and again until it looks right.

    If you Whisper then only you and the person you whispered to can see the post. Useful for private messages.


    Thanks, noted. I'll tidy up my replies later for coherence.
    • CommentAuthorbardo
    • CommentTimeJan 21st 2019 edited
     
    how do I delete an earlier duplicate on this thread?
    •  
      CommentAuthordjh
    • CommentTimeJan 21st 2019
     
    Stefan, I'm not sure why you whispered your answer to my questions. It's generally considered bad mannered to respond privately in a public conversation like this. I'll respond if/when it's public.
    • CommentAuthorbardo
    • CommentTimeJan 21st 2019
     
    Posted By: djhStefan, I'm not sure why you whispered your answer to my questions. It's generally considered bad mannered to respond privately in a public conversation like this. I'll respond if/when it's public.


    djh, thanks for pointing this out. Still getting the hang of using this type of formatting.
    • CommentAuthorbardo
    • CommentTimeJan 21st 2019
     
    I spoke with the engineer today based on this discussion. Seems like I misunderstood his spec when putting the diagram together. The heave board goes on inside of gravel next to wall (apparently one is sufficient given the expansiveness of the local clay). The drainage pipe sits lower down trench which makes more sense. Revised diagram attached. Would still like to hear from Tony about the stem wall acting as a cold thermal bridge. As far as I'm aware if the soleplate above is completely insulated will that not close the bridge to the above structure? Or advisable to add more insulation below? Tony, please clarify what you mean by 100mm eps from dpc to top of foundation. Thanks
      pencoedfoundations1a.jpg
    • CommentAuthortony
    • CommentTimeJan 21st 2019
     
    On your section essentially it would go replace the outside block skin which would move inboard 100mm
    • CommentAuthorCWatters
    • CommentTimeJan 24th 2019
     
    Posted By: bardoThe plasticity of the soil is low to medium however the engineer suggested using 50mm strip of vertical polystyrene heave board. This is specified to sit directly above the concrete base against the inside soil wall with gravel infill separating it from the blockwork. Is this necessary?


    Some clay shrinks and expands a lot. Trees complicate the situation. We were advised to put boards both sides of our conventional founds on clay and did so.
    • CommentAuthorCWatters
    • CommentTimeJan 24th 2019
     
    Posted By: bardoAny potential issues with rendered finish direct onto the revealed concrete blockwork?


    Normally a drip bead/bell cast and gap is formed at the DPC level so the DPC isn't bridged by the render. Render manufacturers recommend a waterproof render below the DPC.
    • CommentAuthortony
    • CommentTimeJan 24th 2019
     
    Any potential issues with rendered finish direct onto the revealed concrete blockwork?

    no but no insulation, probably best nor rendered as it could fall off, lime-wash it?
    • CommentAuthorbardo
    • CommentTimeJan 25th 2019
     
    Posted By: tonyOn your section essentially it would go replace the outside block skin which would move inboard 100mm


    Sorry Tony, struggling to picture this. Do you mean replace the outer layer of blocks with insulated blocks?
    • CommentAuthorbardo
    • CommentTimeJan 25th 2019
     
    Posted By: tonyAny potential issues with rendered finish direct onto the revealed concrete blockwork?

    no but no insulation, probably best nor rendered as it could fall off, lime-wash it?


    lime wash sounds interesting. Would it need re-doing now and then. What about cladding it with facing stone?
    • CommentAuthorbardo
    • CommentTimeJan 25th 2019
     
    Posted By: CWatters
    Posted By: bardoThe plasticity of the soil is low to medium however the engineer suggested using 50mm strip of vertical polystyrene heave board. This is specified to sit directly above the concrete base against the inside soil wall with gravel infill separating it from the blockwork. Is this necessary?


    Some clay shrinks and expands a lot. Trees complicate the situation. We were advised to put boards both sides of our conventional founds on clay and did so.


    The engineer said one is fine considering the relatively low expansivity of the clay here.
    • CommentAuthorbardo
    • CommentTimeJan 25th 2019
     
    Posted By: CWattersdrip bead/bell cast and gap


    Posted By: CWatters
    Posted By: bardoAny potential issues with rendered finish direct onto the revealed concrete blockwork?


    Normally a drip bead/bell cast and gap is formed at the DPC level so the DPC isn't bridged by the render. Render manufacturers recommend a waterproof render below the DPC.


    Thanks, I'll look into this.
    • CommentAuthortony
    • CommentTimeJan 26th 2019 edited
     
    "Render manufacturers recommend a waterproof render below the DPC."

    But they sell it! and don't like you using lime, I wouldn't do it as a) no insulation, b) it will fall off, stone cladding, even paving slabs over insulation would work

    I have no problems with EPS bridging over a dpc
    • CommentAuthorbardo
    • CommentTimeJan 27th 2019 edited
     
    Posted By: tony"Render manufacturers recommend a waterproof render below the DPC."

    But they sell it! and don't like you using lime, I wouldn't do it as a) no insulation, b it will fall off, stone cladding, wven pavind slabs over insulation would work

    I have no problems with EPS bridging over a dpc


    Thanks Tony. I'm struggling to find EPS blocks which can fix to an existing concrete block wall. The Quinn Lite Super seem like they are capable of eliminating the thermal bridge, though they are not made from EPS. Your thoughts?
  4.  
    The Quinn Lite Super blocks that you quote are building blocks whereas EPS is insulation - two different things. If you want to attach insulation to an existing concrete block wall I would fix EPS batts to the concrete blocks with the standard EPS adhesive and depending upon the situation also use mechanical fixings (plastic plugs and nails made for the purpose) and then use the thin film render, that is standard external wall insulation.
    • CommentAuthortony
    • CommentTimeJan 28th 2019
     
    +1
    • CommentAuthorRex
    • CommentTimeJan 29th 2019
     
    Why the heave guard and pea gravel on the inside? On my build, also in a clay area, the ground workers removed the soil around the inside so I effectively have a trench; nothing touching the blockwork
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