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Housing - New Build: Foundations for passiv h (alternatives)

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  1.  
    • CommentAuthoralant
    • CommentTimeNov 9th 2021
     
    Any thoughts on alternatives to concrete and foam / EPS type foundations? Trying to find a solution for a passiv design that also uses materials that have less embodied energy / low CO2 / lower plastic content. eg foamglass and limecrete?
    Thoughts, personal experience very welcome.
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2021
     
    Posted By: alantAny thoughts on alternatives to concrete and foam / EPS type foundations? Trying to find a solution for a passiv design that also uses materials that have less embodied energy / low CO2 / lower plastic content. eg foamglass and limecrete?

    Well, you can use a suspended timber floor - needs to be a box filled with insulation of your choice, and it needs to be supported on something. Perhaps piles or perhaps a beam and block floor.

    You could conceivably build a limecrete/LECA floor but I've no idea how thick it would need to be to reach PH levels of insulation nor whether they're strong enough to support the walls and roof or if you'd need to engineer some other thermal bridge free detail for those. Ditto for foamglass.

    You'll also need to meet the level access requirements so plan carefully how to achieve that - that's one of the main reasons inorganic insulants are used underfloor.
    • CommentAuthorJonti
    • CommentTimeNov 10th 2021
     
    I wonder how much heat is actually lost through the earth/ground given that when it is put on the roof it is considered a good form of insulation.
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2021
     
    Posted By: JontiI wonder how much heat is actually lost through the earth/ground given that when it is put on the roof it is considered a good form of insulation.

    PHPP or whatever the BRE standard is will give you the answer for any particular design.
    • CommentAuthorGreenPaddy
    • CommentTimeNov 11th 2021
     
    Jonti, I'm not sure that earth is considered a "good" form of insulation on a roof, especially as it will spend much of its life wet. Perhaps as a shield to extreme solar gain?

    Do you have any links to tested insulative properties of earth in its various forms? I've looked previously, but never found anything I felt I could trust, so had to make a guess at values for previous projects, or ignore it in Uval calcs.
  6.  
    I think there's more mileage (in carbon terms), in trying to reduce the amount of concrete in the slab and foundations rather than skimping on the insulation.

    If the building is relatively small scale then it's worth looking at some of the alternatives here and I suspect you'll save way more carbon in the rebar and concrete than you would with EPS.
    • CommentAuthoralant
    • CommentTimeNov 12th 2021
     
    Thanks for your comments folks.

    djh - you are absolutely right about the access being more of an issue with a cassette floor.

    I guess the temperature differential between the underside of a cassette floor and the internals would be much higher than a slab resting on the ground. The air temp could drop to -10 or more in winter whereas the ground temperature should not fluctuate as much o not drop as low as the air temperature.

    An advantage of the cassette flooring would be avoiding EPS and when you read the passive catalogue for ecologically rated construction materials it is surprising just how many nasties are involved in EPS.

    Trying to do some form of carbon comparison and cost comparison makes things difficult especially when most alternatives come from mainland Europe and have to be transported to the North of Scotland.
    •  
      CommentAuthordjh
    • CommentTimeNov 12th 2021
     
    Posted By: alantI guess the temperature differential between the underside of a cassette floor and the internals would be much higher than a slab resting on the ground. The air temp could drop to -10 or more in winter whereas the ground temperature should not fluctuate as much o not drop as low as the air temperature.
    That's exactly the issue. It could be 20°C colder, so you need a lot more insulation to keep the same loss. Plus the access hassle. And when I investigated it was surprisingly expensive (I don't remember the details, sorry). [it was piles plus timber cassette]

    An advantage of the cassette flooring would be avoiding EPS and when you read the passive catalogue for ecologically rated construction materials it is surprising just how many nasties are involved in EPS.
    Well, yes, but it's surprising how depressing the reading is for most materials.

    Trying to do some form of carbon comparison and cost comparison makes things difficult especially when most alternatives come from mainland Europe and have to be transported to the North of Scotland.
    I kind of gave up on the foundation and slab and accepted they were going to involve nasties.:sad:
  9.  
    AIUI the underside of a suspended floor is thermally coupled with the subsoil down below it, heat is radiated to and fro across the airspace in the void, so the subsoil helps to keep the suspended floor warmer in winter than it would be if it were an external surface such as a wall. This is counteracted by cold air moving through the void and cooling both the floor/subsoil surfaces to some extent, depending how much of a gale blows through the void.

    However, as discussed above, subsoil is not great insulation anyway, irrespective whether the floor sits on it or up above it. If you are building a passivhouse then the soil contributes a fairly irrelevant fraction of the insulation that you are going to need. Typically soil R=~ 1 or 2 m².K/W according to the BRE formula and you’ll want 10, so it doesn't matter much either way.

    The data I came across suggested that mineral or glass wool have the lowest embodied CO2 of the non-organic insulation materials per unit of insulation value, substantially less than EPS or XPS and (perhaps surprisingly) better than woodfibre boards. Embodied CO2 is becoming more important than it was even a few years ago, as electric heat is decarbonising (operational CO2 declining).

    The embodied CO2 of a limecrete or concrete slab will be high compared to a suspended timber/wool floor, but the thermal mass might be useful. There seem to be mixed data about embodied CO2 of lime vs cement, if it's the rapid setting 'NHL' stuff then it doesn't reabsorb so much CO2 as air-lime, and you'd probably use more of it than cement for the same strength.
    •  
      CommentAuthordjh
    • CommentTimeNov 12th 2021
     
    Posted By: WillInAberdeenHowever, as discussed above, subsoil is not great insulation anyway, irrespective whether the floor sits on it or up above it. If you are building a passivhouse then the soil contributes a fairly irrelevant fraction of the insulation that you are going to need. Typically soil R=~ 1 or 2 m².K/W according to the BRE formula and you’ll want 10, so it doesn't matter much either way.

    The point is the temperature of the outer face of the insulation rather than the thermal resistance of whatever it is. For a slab on ground, it is the soil temperature which might be something like 10°C while for a suspended floor it's somewhere between the soil and the air temperature, which might be -10°C or even lower.

    The data I came across suggested that mineral or glass wool have the lowest embodied CO2 of the non-organic insulation materials per unit of insulation value, substantially less than EPS or XPS and (perhaps surprisingly) better than woodfibre boards. Embodied CO2 is becoming more important than it was even a few years ago, as electric heat is decarbonising (operational CO2 declining).

    I don't pretend to be an expert but there's a big difference between the carbon in a woodfibre board and that in EPS. That in carbon was captured from the air quite recently, and hopefully is being captured again by a new tree after this one was felled. So making and using woodfibre boards is part of an ongoing active carbon capture system. The carbon in EPS was captured millions of years ago and was quite stably held before being mined. Hopefully it will still stay embodied but ...

    The embodied CO2 of a limecrete or concrete slab will be high compared to a suspended timber/wool floor, but the thermal mass might be useful. There seem to be mixed data about embodied CO2 of lime vs cement, if it's the rapid setting 'NHL' stuff then it doesn't reabsorb so much CO2 as air-lime, and you'd probably use more of it than cement for the same strength.

    Indeed, there are problems with evaluating lime it seems, and the numbers for concrete vary a lot for the many different types of concrete. According to ICE 3, lime is a bit better than average concrete and much worse than GGBS concrete, but they say they're not sure about the lime. My own view was that it's more important to choose the materials for other properties.
  11.  
    The free air temperature in N Scotland is sometimes well below -10degC, sometimes +25 away from the coast, more often 0-15degC. The temperature in an underfloor void is similar, but sheltered from the highest and lowest extremes. The average year-round ground temperature is very similar to the average year-round air temperature. The average year-round temperature gradient for heat loss to ground, is similar to that for heat loss to air.


    The embodied carbon in a woodfibre board is apparently from the fuel used to dry it after it has been wet-formed, and the fuel used to truck heavy boards over here from Central Europe where they seem to be made. The biological carbon 'captured' in the woodfibre doesn't count as it isn't captured for geological time, despite what the retailers may claim!

    (Edit for clarity)
  12.  
    Posted By: WillInAberdeenThe average ground temperature is very similar to the average air temperature.

    Only at the surface. Despite sustained winter temps here around -15 on occasions I have not seen frost damage below 30cm.

    Posted By: WillInAberdeenThe average year-round temperature gradient for heat loss to ground, is similar to that for heat loss to air.

    But a different story if you consider the centre of a room with an uninsulated slab, (and in this thread we are talking about foundations) whilst the periphery will be cold (but not as cold as the outside) the centre be considerably warmer. When considering large buildings I understand that it is now acceptable to insulate the periphery but leave the centre on the bare soil.
    Given the choice I would always go for a slab rather than a suspended floor and I would use Granulated blast-furnace slag (GBFS) for the concrete if available locally otherwise ordinary concrete and I believe it is difficult to better the performance of EPS for below ground insulation or anywhere which could the subject to damp.
    •  
      CommentAuthordjh
    • CommentTimeNov 13th 2021
     
  14.  
    For the purposes of thermal modeling boundary conditions is it appropriate to use this delta T with a resistivity coefficient suitable for the soil? Say 0.66 W/m2K?

    Posted By: WillInAberdeenThe free air temperature in N Scotland is sometimes well below -10degC, sometimes +25 away from the coast, more often 0-15degC. The temperature in an underfloor void is similar, but sheltered from the highest and lowest extremes. The average year-round ground temperature is very similar to the average year-round air temperature. The average year-round temperature gradient for heat loss to ground, is similar to that for heat loss to air.


    The embodied carbon in a woodfibre board is apparently from the fuel used to dry it after it has been wet-formed, and the fuel used to truck heavy boards over here from Central Europe where they seem to be made. The biological carbon 'captured' in the woodfibre doesn't count as it isn't captured for geological time, despite what the retailers may claim!

    (Edit for clarity)
    • CommentAuthorEd Davies
    • CommentTimeJan 9th 2022
     
    Posted By: MarcusTait79: “…T with a resistivity coefficient suitable for the soil? Say 0.66 W/m2K?”

    Combining this with your other post [¹] leaves me even more confused. W/m²K are not suitable units for resistivity which is usually expressed in m·K/W. W/m²K are used for conductance per unit area (U-value).

    [¹] http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11107&page=2#Comment_294052
  16.  
    Apologies, this is my grasping attempt to find a reasonable U-value to apply in the form of a coefficient to a boundary condition in a ground bearing slab scenario!

    Posted By: Ed DaviesPosted By: MarcusTait79: “…T with a resistivity coefficient suitable for the soil? Say 0.66 W/m2K?”

    Combining this with your other post [¹] leaves me even more confused. W/m²K are not suitable units for resistivity which is usually expressed in m·K/W. W/m²K are used for conductance per unit area (U-value).

    [¹]http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11107&page=2#Comment_294052" rel="nofollow" >http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11107&page=2#Comment_294052
  17.  
    Realise this isn't exactly an answer to the question you posed but it got me thinking creatively about tackling the problem without resorting to the expense of a proprietary solution.

    https://youtu.be/A5tVWDFYvBI


    Posted By: alantAny thoughts on alternatives to concrete and foam / EPS type foundations? Trying to find a solution for a passiv design that also uses materials that have less embodied energy / low CO2 / lower plastic content. eg foamglass and limecrete?
    Thoughts, personal experience very welcome.
  18.  
    Hi Marcus, that depends a lot on what you are modelling, and for whom!

    If you look in SAP there's an appendix with temperature data for different regions of the UK, or you can use degreedays.net .

    If you are modelling a slab on/in the ground then the size and shape (when seen in plan view) are important, if it has a wiggly shape then it loses more heat (such as a L or U shape floor plan). Large slabs loose relatively less heat than small ones. It's due to the average path length from each point of the floor to the outside air. BRE published a rough formula based on Perimeter P in metres and Area A in m²
    U = 0.05 + 1.65(P/A) - 0.6(P/A)²

    The origin of this is unclear and it doesn't include soil resistivity anywhere, but that might not matter because the soil resistance is never enough to make up a significant part of the insulation value you'd need for a new building.

    Your final building warrant sign off requires a more highly polished SAP model than your first exploratory design doodles, so just make it good enough for the stage you are at.
  19.  
    Many thanks Will

    That’s interesting information regarding SAP I’ll have a dig into it!

    These models are just me attempting to dial in design elements for our own house before handing the details over to be modeled by someone competent!🥴

    I’ve been using THERM 6.3 and seem to be getting on ok with walls, roof etc but software uses film coefficients in the boundary condition library and AIUI these don’t exist when the elements in question aren’t immersed in air. Entering a zero value for a bespoke boundary condition coefficient doesn’t work as the software appears to treat this as being wide open irrespective of the thermal resistance of the materials forming the boundary.

    Posted By: WillInAberdeenHi Marcus, that depends a lot on what you are modelling, and for whom!

    If you look in SAP there's an appendix with temperature data for different regions of the UK, or you can use degreedays.net .

    If you are modelling a slab on/in the ground then the size and shape (when seen in plan view) are important, if it has a wiggly shape then it loses more heat (such as a L or U shape floor plan). Large slabs loose relatively less heat than small ones. It's due to the average path length from each point of the floor to the outside air. BRE published a rough formula based on Perimeter P in metres and Area A in m²
    U = 0.05 + 1.65(P/A) - 0.6(P/A)²

    The origin of this is unclear and it doesn't include soil resistivity anywhere, but that might not matter because the soil resistance is never enough to make up a significant part of the insulation value you'd need for a new building.

    Your final building warrant sign off requires a more highly polished SAP model than your first exploratory design doodles, so just make it good enough for the stage you are at.
    •  
      CommentAuthordjh
    • CommentTimeJan 9th 2022 edited
     
    Posted By: MarcusTait79wide open irrespective of the thermal resistance of the materials forming the boundary.

    Posted By: WillInAberdeenHi Marcus, that depends a lot on what you are modelling, and for whom!

    Possibly a silly question, but why do you post your answers above the text you are quoting? It must be obvious that the convention here is to post responses below. i.e. bottom-posting.
  21.  
    Posted By: djh
    Posted By: MarcusTait79wide open irrespective of the thermal resistance of the materials forming the boundary.

    Posted By: WillInAberdeenHi Marcus, that depends a lot on what you are modelling, and for whom!

    Possibly a silly question, but why do you post your answers above the text you are quoting? It must be obvious that the convention here is to post responses below. i.e. bottom-posting.


    Roger that!🤦‍♂️
  22.  
    Ok, understand now, I think the heat flow through the short path of soil from the floor next to the foundations to external soil surface, is going to be more significant, compared (say) to the very long heat flow path from the centre of the room running deep under the foundations to the surface. I'm not sure you can use the same insulation value for both those.

    A) as a worst case scenario, you could model the soil as being air, IE assume no credit for the insulation value of the soil. You need a total resistance of about 10m²K/W so if the soil is only giving you <1m²K/W then you could ignore it.

    B) insert deep blocks of soil in/under/around your model floor and foundations. Set the boundary conditions beneath them to some very high resistance (no heat loss to the centre of the Earth, only to the outside air).

    Remember Therm is 2D and heat loss through soil is 3D (diverging outwards from corners of the building when seen from above) so the model is going to be approx at best and not worth too much headache!

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