Did an energy assessment today and captured this image of an exterior wall. I believe the wall is framed, with sheet rock and a brick facade. It is a 1929 vintage home, but I believe there have been renovations. This image is taken on the second floor and is on an exterior wall with a SW exposure. Any ideas as to this unusual pattern? I have seen a strange pattern like this before on walls that have 1970's vintage degraded foam insulation.

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Mold?

Shoot I have a picture of a smiley face on a wall - reason it was sprayed with something & then painted over, but it was enough to create a thermal difference that can be caught via camera only

SW  exposure.  What is outside?  Was it sunny?  Trees?  Large Shrubs?   I have had the shade pattern show on an interior wall.

You have a house with a Brick Facade.  That means you must consider the thermal capacity of the Brick and Mortar.  You also have to consider the drainage plane.  On that basis I would seriously doubt any thing coming through those two components of your wall assembly.  Do you have any experience with that period construction in your area or know someone who does?  How did they construct the drainage plane.  I've seen chicken wire or hail screen used to create the space.  I've also seen multiple layers of building felt and the outer layer rots away and voila.

I would be thinking water otherwise.  Break out your moisture meter. Sean's example of a smiley face is appropriate. Until you know how the wall is assembled and can account for those foibles, you have to assume the worst.  Sometimes you never know. All you can do is rule bad stuff out.

I think Sean is onto the answer. The blotchyness makes me think they may have used a spray can primer like "Killz" in a can to cover water damage or mold. If they shot it on there thick enough it could create a thermal difference.

How did the surface texture of the drywall look? You mentioned it was a 1929 build...it isn't lath and plaster is it?

Is this a close up or broad view?

Infrared imaging is only one of many tools to be used for assessment, and is very much subject to interpretation. In order to interpret as accurately as possible, one must know a variety of factors. What was the span of the image and what was the temperature inside and outside the home? Has the temperature changed by more than a few degrees inside in the previous few hours? Was the wind blowing outside at more than about 10 mph, or was a blower door running inside? Are there other factors which could affect the image, such as moisture in or on the wall? What was the dewpoint of the air inside? Has the weather outside been raining a lot lately?

I doubt that the image is from paint, because the paint would have to be still wet and it does not look like any pattern of painting. It could be moisture, but that is a wild guess, not knowing what there is for insulation. As John N. noted, you have to know the wall assembly, particularly what is in the wall for insulation.

Without knowing any of the above, my wild-ass guess would be that the wall has UFF (urea formaldehyde foam) in the wall that was improperly installed and now had shrunk, creating voids. With a brick exterior and common moisture problems associated with poor installation, this could also be an indication of moisture in the wall.

The pattern doesn't look like the UFFI foam shrinkage I've seen. UFFI foam shrinkage  tends to show horizontal cracks and gaps along the top and vertical edges of the stud space. A small test hole can confirm if it's UFFI. The pattern suggest a moisture-related problem and moisture readings would be a good idea. 

You are correct, Richard. This pattern is not typical of UFF, but that does not mean that it cannot be UFF. What we don't know are the conditions: what is the span of the image? The difference in shading is relatively minor if the span is about 4 degF, but significant if it is more like 20 degF or more. Was the temperature between inside and outside changing significantly in the previous 12 hours or so?

Moisture will show on IR by one of two mechanisms, thermal capacitance or latent heat of vaporization. The former requires a recent temperature change where the wall materials with the moisture heat up slower than the drier parts of the wall. The latter requires a source of moisture dampening the surface of the wall so that moisture is evaporating from the wall. If the moisture is coming from outside, it is a strange pattern either way, and yes, a moisture meter can help in that determination, especially if insulated probes are used to measure the moisture in the wall. Knowing the composition of the wall is very important.

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