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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Building Regulations and Standards: Heat loss through concrete

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  1.  
    • CommentAuthortony
    • CommentTimeJun 15th 2008
     
    If I could draw a picture and load it I would. Say we have a concrete 150mm thick with 100mm of insulation and then solid material like soil on the other side of it. We keep the top of the concrete at 17 C what temperature will the opposite side of the insulation attain in steady state? Remember that as heat is lost slowly through the insulation it will warm the soil. After a long time -- steady state -- the soil will warm to almost 17 C so long as heat is not lost too quickly through the soil.

    Certainly the soil will warm up to some extent in all cases.

    How do these facts impact on floor heat loss calculations?
  2.  
    No expert, but isn't the dissipation of heat through the soil likely to be faster than the heat transfer through the insulation (if the insulation is any good!)?
  3.  
    The soil/earth will act as an infinate heat sink. presumably the steady state temp of the soil will be the average annual airtemp. so the calc will be 17 deg internaly moving towards something like 12 degrees on the soil side depending on the location of the building.
    I think any warming of the soil from energy passing from the insulation will be balanced by the soil chilling the insulation in return.

    The earth is a 6,000,000,000,000,000,000,000,000 kilogram heat sink
    • CommentAuthortony
    • CommentTimeJun 20th 2008
     
    Yes but the soil in contact with the insulation will get to at least the average temperature of the house :smile:

    Then taking a point under the middle of the floor there will be a temperature gradient slowly going down to your 11 or 12 C some meters away.

    The soil is an insulator so to assume that it is not is a wrong assumption. Also to assume 11 or 12 C immediately under the house is also incorrect.

    We are talking steady state aren't we? It may take several years for steady state to be reached!
    • CommentAuthorCWatters
    • CommentTimeJun 20th 2008
     
    I don't have a subscription but this looks like it would help...

    http://jen.sagepub.com/cgi/content/abstract/29/1/51

    I almost wish I'd put a sensor under there when we built ours.
    • CommentAuthorSimonH
    • CommentTimeJun 20th 2008
     
    You identified a problem when using U values - heat loss calcs typically don't take into account what the temperature on the outside of the wall when they're based on the average seasonal air temperature and degree days to work out yor heat load. Now if only SAP was about to be changed to reflect underground sheletered walls in the heat loss calcs...doh! they already do it!

    http://projects.bre.co.uk/sap2005/pdf/SAP2005-2008-update.pdf Section 3.1 page 13.

    Apparently you have to use ISO 13370, in section A3 of the CIBSE Guide A or in the Approved Document 'Basements for
    dwellings' . to work out the equivalent U value. Well that's ok if you didn't have to buy them.

    I did come across this older version of the building regs which shows you how the deeper go the less insualtion you need to get the same target U value. Around page 43.

    http://www.basement-living.co.uk/downloads/basements_for_dwellings.pdf

    Simon.
    • CommentAuthorCWatters
    • CommentTimeJun 20th 2008
     
    The free heat loss calculator I looked at two years ago used the outside temperature you entered.

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