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Energy Assessment: Thermal imaging - interpreting external wall anomaly

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    Hi. I've borrowed a FLIR E-50 thermal camera, generously purchased by Dorset County Council for use by the local energy advice centre and community groups. I've visited about 10 homes so far, and in three cases (the only ones where I've scanned the base of the walls) I've found an unexpected warm zone at the foot of external walls, where the wall meets the garden soil or concrete yard. One of these homes has an unheated cellar, the others don't, so I don't think the cellar can be the explanation. The temp difference is about 0.5 - 1 degree between the warm area and the walls.

    Has anyone seen anything similar? Any ideas what causes it? (as you can guess, I'm no physicist :)

    Thanks,
    Sally
    • CommentAuthorMike George
    • CommentTimeFeb 13th 2013 edited
     
    Often cavity wall insulation starts at Ground floor level. In general construction, it's only recently that this thermal bridge has been considered seriously.

    Other than that it could simply be heat from the ground adjacent to the house.
    • CommentAuthorTimber
    • CommentTimeFeb 13th 2013
     
    Not an anomaly at all I would suggest.

    It depends entirely on the construction, but could be general geometric bridging, the fact that the ground is probably warmer than the air at this moment (so the ground is helping to keep the base of the wall warm), poor cavity wall insulation/cavity bridge by mortar at the base.
  4.  
    Could be a number of possibilites. Any of above. The difference is only 0.5 C and below zero. One explaination could be cold night at below zero. Ground at +5 to + 10 C at depth so it could just be conduction of heat up the outside wall. Also it is unusual for temperatures to be stable for any length of time. So it could just be the cooling down or warming up causing it. Could also be warm ground near the wall cooling down or heating up more slowly or quickly than the wall which will have less thermal mass than the ground.

    Richard
  5.  
    Mmm, thanks for your comments folks.

    I guess I hadn't considered thermal bridging at the base of the wall. Someone else I've spoken to since posting has also suggested that the foundations could be acting as a thermal store and emitting heat when the outside air and ground are colder. And thanks Richard for reminding me of the different thermal capacities of the ground and wall, of course I see now that there will be a difference in the speed of warming /cooling.

    Personally I am leaning towards the thermal bridge / foundations explanations, because the warm area appears as a band and doesn't continue up the wall. I will be attending a further training day in a couple of weeks and will ask the trainer if he has any further thoughts; if so I will post them up here.
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      CommentAuthorSteamyTea
    • CommentTimeFeb 14th 2013
     
    Can you take some more pictures of it early in the morning when some of the effects of the previous days solar gain may have dissipated?
    • CommentAuthortony
    • CommentTimeFeb 14th 2013
     
    it could also be that the cavity wall insulation, commonly batts only start at dpc leaving a thermal bridge underneath them.

    nice picture though, do some more please especially of a blobbed and dobbed wall in a newish house on a cold day.
  8.  
    Hi Sally,
    I have just monitored my house after a night below zero degrees. As you found seems to be a band around the house. Mine is cavity wall insulated and the recent extension I have seen all the insulation put in or have put it in myself. It has underslab and edge insulation around the slab. So it is up to current standards.

    Result @ 1 metre hight wall temperature = - 0.8 C
    Result @ 0.25 metre height wall temperature = - 0.4 C

    I think there are a number of reasons for this. As Tony said The cavity wall insulation does not usually go beneath the Damp Proof Course. Even if it does there is a heat conducion down the internal wall into the foundations and up the external wall. Also the foundations at 1 metre deep will be sitting in warmer soil (up to 10 C).

    Would be nice to have a time lapse film of a building over a couple of days and nights.

    Richard
  9.  
    Hi again everyone.

    Steamy Tea, the pic was taken at about 07.45 and the air temp was below zero.

    Richard, the temp at the warmest spot at the wall/ ground junction was -0.8, with -2.7 at the cool part of the wall about 400mm above, so a much bigger difference than you found. I guess your slab insulation must be reducing the cooling effect even if it hasn't eliminated it. (I can spot-read different parts of the picture with the FLIR software even though the figures don't appear on the pic that I posted.)

    Tony, thanks. Assuming it's OK with the various householders, I will post up some more pix on the project website http://greendor.wordpress.com/ in a few weeks when I have time, and will post a comment here when I've done it.

    Thanks again for your help everyone.

    Saly
    • CommentAuthorRobL
    • CommentTimeFeb 18th 2013
     
    I have a similar thermal picture of the corner of our house. It's a 1963 build, with fluff in the cavity down to the dpc, then rubble below that. Because of the thermal bridge to surrounding earth I fitted perimeter wall insulation from just below the dpc down 700mm to the foundations, and now you can see a sharp temperature change where this is fitted. The perimeter insulation is foamglass - you can only just see the top of it poking above the earth.
    • CommentAuthorRobL
    • CommentTimeFeb 18th 2013
     
    Another pic of our chimney stack attached too, showing how the cavity must be breached all the way up it, as you can see the hot vertical line up it.
    • CommentAuthortony
    • CommentTimeFeb 19th 2013
     
    Weep :cry::cry:

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