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Housing - New Build: thermal absorption of floor

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    We are currently designing a passivhaus with heat recovery ventilation. The site is not well orientated but we have as large a window on the south elevation as planning will permit. I am planning an insulated concrete raft. Does the floor finish affect the solar absorption? If I have a carpet or a wood finish would this affect the heat absorbed by the slab?
  2.  
    Yes, the thermal resistance between the illuminated surface & the concrete slab affects the ability of the slab to regulate the temperature in the room. A ceramic tile finish is best, i.e. leads to the minimum swings in room temperature.

    David
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      CommentAuthorfostertom
    • CommentTimeMay 4th 2012 edited
     
  4.  
    fostertom

    Have you done any simulation of uninsulated slabs?

    I would always insulate a ground floor slab for a new build because its easy & because it gives you a known minimum thermal resistance between floor & outside air. Thermal mass is useful, but it can't compensate for heat loss.

    David
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      CommentAuthorfostertom
    • CommentTimeMay 4th 2012 edited
     
    Posted By: davidfreeboroughHave you done any simulation of uninsulated slabs?
    That's still inaccurate AFAIK because tho Therm has rule-of-thumb workarounds, no one's written software that really models infinite-depth subsoil with its 2yr time-to-equilibrium. I'd love to be told that's wrong!

    BTW, I'm claiming excellent heat-loss performance, as well as thermal mass. Certainly, you can throw enough downstand and/or wing insulation at it, to get whatever heat-loss you want. And it completely sorts the leaky wall-floor insulation discontinuity problem.

    About the usual rule of thumb, that only the exposed 100mm or so of any thermal mass is really useful for solar/diurnal absorbtion (buffering solar gain via windows). I have a feeling that I can't quite express, that that doesn't apply to a deep subsoil situation, and that lots of downward thickness pays dividends, once the long charge-up has equilibriated.
    • CommentAuthortony
    • CommentTimeMay 4th 2012
     
    Also only applies to poorly insulated buildings

    Deep thermal mass = nice stable temperatures
    • CommentAuthortony
    • CommentTimeMay 4th 2012
     
    With or without insulation above the slab (carpet) it will warm to average room temperature.
    • CommentAuthorNovice1
    • CommentTimeMay 5th 2012
     
    I'm planning a large glazed southern space to capture as much solar gain. No shading. I plan to deal with the overheating by fanned vents and UFH pipes in the walls and floors to cool the space. This heat wil be redirected to colder parts of the house and an interseasonal store (VH type coils in sand). Still no idea how I will control it all!

    Would that mean that the 300mm of EPS under the 100mm slab would be counter productive. I have limited wing insulation. (EPS 150 against foundation blocks to the top of footings (500mm below FGL)+
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      CommentAuthorSteamyTea
    • CommentTimeMay 5th 2012
     
    Posted By: fostertomno one's written software that really models infinite-depth subsoil with its 2yr time-to-equilibrium

    Many have tried but it never gives the desired result:wink:
    •  
      CommentAuthorjoe90
    • CommentTimeMay 5th 2012
     
    Posted By: SteamyTea
    Posted By: fostertomno one's written software that really models infinite-depth subsoil with its 2yr time-to-equilibrium

    Many have tried but it never gives the desired result:wink:" aria-posinset="0" aria-setsize="0" hspace="0" alt=":wink:" vspace="0" loop="1" src="http://www.greenbuildingforum.co.uk/newforum/extensions/Vanillacons/smilies/standard/wink.gif" ismap="false" width="15" start="fileopen" height="15" >


    ST, does that mean it does not work compared to underslab insulation or that the results are not quantifiable ?.
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      CommentAuthorJSHarris
    • CommentTimeMay 5th 2012
     
    I think that the main problem is the very wide degree of variability, caused by differing soil types, moisture levels, sub-surface water flow patterns etc, all of which make modelling pretty difficult.

    I've looked (briefly) at the sort of impact that water can have at just a metre or two down, and it's massive. Even a very slow water flow rate through the soil will make a massive difference to the rate of heat flow through the soil.
  12.  
    Posted By: Novice1Would that mean that the 300mm of EPS under the 100mm slab would be counter productive. I have limited wing insulation. (EPS 150 against foundation blocks to the top of footings (500mm below FGL)
    Insulating the slab gives you control over where the heat goes. Without it you're relying on thermal mass which can limit temperature swings, but never compensate for heat loss.

    David
    • CommentAuthorCWatters
    • CommentTimeMay 5th 2012
     
    A stone or tile floor at room temperature feels cold to the touch so wouldn't be my choice.
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      CommentAuthorjoe90
    • CommentTimeMay 5th 2012
     
    Posted By: CWattersA stone or tile floor at room temperature feels cold to the touch so wouldn't be my choice.


    Conversely I have read on this forum someone said "a tile floor at 20' feels warm in the winter and cool in the summer" :confused:
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      CommentAuthorfostertom
    • CommentTimeMay 5th 2012
     
    Posted By: SteamyTea
    Posted By: fostertomno one's written software that really models infinite-depth subsoil with its 2yr time-to-equilibrium

    Many have tried but it never gives the desired result:wink:" alt=":wink:" src="http:///newforum/extensions/Vanillacons/smilies/standard/wink.gif" >
    Am I right in remembering that you ST did your postgrad thing on this topic, and proved it didn't work in the way that some do 'desire'?
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      CommentAuthorSteamyTea
    • CommentTimeMay 5th 2012
     
    This morning I made a small model that can calculate the energy movements caused by differing energy inputs. It uses a column of fixed size, mass, SHC and conductance (really should get a girlfriend). I am currently running the model 8760 times to simulate 2 years worth of solar input (only does daylight hours but could add in 8790 hours worth of no input).
    From that I can look at the temperature gradient for each energy input group. Can tell you now though that it does not vary hardly at all, about 0.009 K.m^-1.W^-1.
    •  
      CommentAuthorfostertom
    • CommentTimeMay 5th 2012
     
    •  
      CommentAuthorfostertom
    • CommentTimeMay 5th 2012 edited
     
    Posted By: CWattersA stone or tile floor at room temperature feels cold to the touch so wouldn't be my choice.
    I'm told that just a little bit of insulation - say 25 EPS - close beneath such a solid surface modifies the temp gradient so the hard surface stays closer to room air temp under all circumstances.
    • CommentAuthorEd Davies
    • CommentTimeMay 5th 2012
     
    Yes, but CWatters's comment was that a stone or tile floor feels cold to the touch even when it's at room temperature. In general, a highly conductive material will feel cool unless it's close to body temperature (i.e., very warm for a room).

    It's a bit of a conundrum - you want good thermal contact between the room and the slab to stabilise the temperature but at the same time you don't want too much conduction when sitting on the floor or walking in bare feet, etc. Tiles with a few rugs or just a thin layer of insulation seems right to me. Personally, I like cork tiles.
  20.  
    Posted By: CWattersA stone or tile floor at room temperature feels cold to the touch so wouldn't be my choice.


    Natural terracotta tiles feel much warmer to walk on than ceramics, don't know what their insulation properties are in terms of installing over UFH.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 5th 2012 edited
     
    Right, run the model, popped the results into energy forcing bins and plotted temperature against column depth.
    Bit hard to tell what is happening at the individual forcing level, the the trend is clear, once you you are a metre down you are not going to get more than about 2.5°C temperature increase. This is without insulation and assumes that the surrounding earth is an infinite heat sink. I did not weight the energy forcing to the normal distribution of sunlight (lots of small amounts, few large ones). If I do get around to it, it will lower the temperature range I think.
      Temp Gradient.jpg
  22.  
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      CommentAuthorfostertom
    • CommentTimeMay 8th 2012 edited
     
    Nevertheless, AGS in US, and BESA members' houses in UK http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9030&page=1#Item_5 do seem to absorb and store solar heat more deeply than rafts of prima facie calcs 'prove'. Experience with such prototypes suggests that other factors not yet understood, are in play.
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      CommentAuthorSteamyTea
    • CommentTimeMay 8th 2012 edited
     
    Posted By: davidfreeboroughDid you mean to post this on the followign thread?

    Might have done :shamed:

    Actually no, as it was a responce to Tom's question about writing code for doing this.

    Probably relevant to both threads though, and any other that is about storing heat in the ground under a house.

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