Dark lines or stains appearing in the corners of ceilings, along exterior walls or around windows form what is sometimes called ghosting. The pattern often follows the framing or the least insulated areas.
Common causes
these colder areas, thermal bridges at studs, corners or discontinuous insulation, promote condensation. Moisture settles there, and dust as well as mould spores accumulate. High indoor humidity, insufficient ventilation, or wood and candle heating amplify the phenomenon.
Signs to watch for
grey to black streaks or stains at wall-ceiling junctions and in corners, more pronounced in winter, sometimes arranged in regular bands that reveal the position of the framing elements, especially on walls facing outside.
Why you should not ignore it
beyond appearance, these marks signal two things: heat loss (deficient insulation, hence discomfort and higher bills) and the presence of moisture favourable to mould, which affects indoor air quality. The visible blackening is only the surface trace of an underlying thermal and humidity imbalance.
What a professional assessment reveals
These marks betray temperature differences in the envelope, and therefore areas where insulation is weak or interrupted. Thermography makes these thermal bridges visible and pinpoints the defects behind the phenomenon, something the eye alone cannot do.
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