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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthortony
    • CommentTimeNov 20th 2008
     
    I am about to build a new house and am seeking opinions about where to put temperature sensor probes.

    The house will have a basement -- it is very easy to build in probes/thermocouples but quite difficult to retro fit them.
    • CommentAuthorCWatters
    • CommentTimeNov 20th 2008
     
    I think you should try to install them in pairs either side of the insulation so the thermal resistance between them is known. Would help settle the issue of how much power is flowing through the floor.
    • CommentAuthorEd Davies
    • CommentTimeNov 20th 2008
     
    CWatters, as the multifoil discussion has already pointed out, knowing the temperature on each side of or even inside layers of insulation doesn't actually help much with knowing the rate of heat flow.
    • CommentAuthorCWatters
    • CommentTimeNov 22nd 2008
     
    Seems to work for heatsinks in electronics.
    •  
      CommentAuthordjh
    • CommentTimeNov 22nd 2008
     
    Because there's no argument about the conductivity of a heatsink :)
    • CommentAuthorJackyR
    • CommentTimeNov 22nd 2008
     
    Posted By: CWattersSeems to work for heatsinks in electronics.

    Not sure what you're saying works, but two possible contexts:
    1) system maintains constant temperature difference between two points, power varies (any thermostatically controlled system, eg hotplate, heater when room at steady state);
    2) system maintains constant power, temp diff varies (eg electronics, heater applied to slab which hasn't yet reached steady state).

    To evaluate insulative properties, identify one quantity to be constant and then measure the other...
    • CommentAuthorTimber
    • CommentTimeNov 22nd 2008
     
    If you are fitting thermo couples, you could also fit some thermal conductivity sensors (can't remember the exact name).

    They can be used to directly work out the thermal conductivity of a material, and the heat flow as well as temerature.

    We use them at work for some of out 'reasearch' projects. I will find the specs and exactly what they do if you are interested.

    Timber
    • CommentAuthorEd Davies
    • CommentTimeNov 22nd 2008
     
    I've seen heat flow sensors advertised somewhere quite recently but can't remember where. Peltier effect used backwards sort of thing. Just looked on the Maplin and Rapid Electronics sites and couldn't find them immediately. The bad news is that they were expensive, about £250 each IIRC.
    • CommentAuthorTimber
    • CommentTimeNov 22nd 2008
     
    Yeah that is the one, looks like a little petler jobby.

    I didn't realise they were that much though. Still, if you are sticking them into a new build and plan to do some serious research, they may be worth it depending on how much extra info can be extrapolated.
  1.  
    If you can measure two of the heat loss variables then you can calculate the third.

    If Q = UA delta T

    Heat loss [kWh] = u-value x temp difference between probes.

    Therefore you need to be able to measure the energy used in the test to be able to calculet the u-value and hence the R-value
    • CommentAuthorCWatters
    • CommentTimeNov 23rd 2008
     
    Thats exactly what you do when measuring the thermal resistance of a heat sink. You apply a known amount of power and measure the temperature gradient. Calculate the thermal resistance.

    I accept there is an issue of how you best model insulation so that the results are representative of the real world walls and roofs (Hot box, in-sutu etc) but I feel the situation for a floor slab should be much simpler and give results close to what you get in the lab. You have a known tickness of insulation with known thermal resistance. You can measure the temperature either side of the _insulation_ (just the insulation). Then work the numbers. Do this near the center of the floor and at a few places around the edge and it should be possible to calculate the power loss.

    To improve accuracy you could even insert the probes a known depth into the insulation.
    • CommentAuthorJackyR
    • CommentTimeNov 23rd 2008 edited
     
    Posted By: CWattersI think you should try to install them in pairs either side of the insulation so the thermal resistance between them is known. Would help settle the issue of how much power is flowing through the floor.


    Posted By: Ed DaviesCWatters, as the multifoil discussion has already pointed out, knowing the temperature on each side of or even inside layers of insulation doesn't actually help much with knowing the rate of heat flow.


    Aaah, CWatters, you mean "either side of the insulation because the thermal resistance between them is already known". Some of us read that as "so that resistance between them can be found". *light dawns*
    • CommentAuthorCWatters
    • CommentTimeNov 23rd 2008
     
    Exactly. Sorry if I wasn't clear.
    • CommentAuthorMike George
    • CommentTimeNov 23rd 2008 edited
     
    The point is though that it is the thermal resistance is what is questioned [at least in the case of multifoil] Using the hot box derived value does little to settle the argument.

    In the case of floors, the P/A method of calculating u-values based on 'known' steady state conductivities is also questioned by some [a minority including me] Once again using the said conductivities [of say PUR] as a basis for calculating energy use is not going to establish whether it is better to put the PUR in or not.
    • CommentAuthorCWatters
    • CommentTimeNov 23rd 2008
     
    Posted By: Mike GeorgeThe point is though that it is the thermal resistance is what is questioned [at least in the case of multifoil] Using the hot box derived value does little to settle the argument..


    Yes all of my commens above were with a floor in mind. Im assuming Tony would be using a rigid insulation like Celotex. I have more confidence that the figures quoted for solid insulation materials could be applied to the situation found under a concrete basement slab.

    In the case of floors, the P/A method of calculating u-values based on 'known' steady state conductivities is also questioned by some .


    That's kind of why it would be nice to have the temperature gradient at several different places on a floor, unfortunately the fact that it's a basement probably means we would see a much more uniform loss distribution. The standard P/A calulation wouldn't apply to basement floor.

    Once again using the said conductivities [of say PUR] as a basis for calculating energy use is not going to establish whether it is better to put the PUR in or not.


    No that wasn't my intention really (I assumed tony wouldn't be happy to insulate one half only) but it would just be interesting though to get a feel for the scale of losses through the floor. Almost a shame it's a basement.
    • CommentAuthorJackyR
    • CommentTimeNov 23rd 2008
     
    So Tony, how do you feel about packing your basement with earth for a year, for benefit of science?
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