Wet house, dry house?

We’re looking for a new house at the moment.  We’ve decided to be a lot more environmentally friendly and shift out of Cambridge and move to Hamilton to cut down the pesky commute in the mornings and evenings. We haven’t made much progress, though, with finding a house – what we’re looking for is in short supply in the areas of Hamilton that we want to live in.  What there is seems all to be for auction.  The search is made all the more pressing because we’ve now sold our Cambridge house. We had it on the market just five days. 

Anyway, getting to the physics part, we got a house inspection done of something that we might bid on at auction in a couple of weeks time. That’s the trouble with auction, you’ve got to get your research done before you bid, and that costs you money for a house that you might be outbid on. Doesn’t seem fair to me.  The house inspection people must love it though. One of the bits of technology that is becoming fairly common for house inspections is thermal imaging.

 I know a bit about infra-red physics and thermal imaging, having used it in a previous life, but in a rather different context to the housing people. Mostly we think of thermal imaging as detecting hot things  (hot things give off more infra-red radiation than cold things – other things being equal) – you’ve probably seen imagery of fleeing criminals at night on those police TV shows – or this sort of thing.  However, that’s not the only source of contrast on an infra-red image.

The amount of radiation emitted by an object depends not only on its temperature but on its emissivity. A blackbody is 100% emissive and completely unreflective, but you can also get objects that are very reflective and not very emissive.  N.B. Don’t think that just because something’s white that it is non-emissive in infra-red – it could be very reflective to visible light but very un-reflective (i.e. emissive) to infra-red. Emissivity depends on wavelength.  Emissivity and absorption are related too – a good emitter of infra-red is also a good absorber of infra-red. 

Water is a pretty good emitter and absorber of IR. Although I"ve never carried out a systematic experiment on it, I would image it’s a lot more emissive than dry gib board and wallpaper, as used in houses.   Therefore a damp piece of house is likely to look different to an infra-red imager than a dry piece. And we can get a contrast between the two, even though the two areas may well be at the same temperature. In the visible band, of course, we can’t see water content, since it doesn’t change the reflectivity of the object.

IR will also help detect coldspots, of course, e.g. patches of ceiling that aren’t insulated properly. The systems aren’t cheap, but give you some confidence that the house you might bid on will be warm and dry, as turned out to be the case with this house.  And that’s always a good thing in Waikato.




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