[wynne jones]

Does someone know how this works or has a theory.

Went to look at some draft marks on a light wool carpet- the usual. I lifted the carpet and the lines were where the underlay butted together. It's been down two years and when looked at the underlay and the base of the pile fibre it's as clean as a whistle.

Why does it just stick on top?

[John Kelly]

Probably because its air exiting the room via the carpet rather than coming in to it. Therefore the carpet pile is filtering the dirt as it touchs it. Draughts work both ways not just incoming.

[wynne jones]

Probably because its air exiting the room via the carpet rather than coming in to it. Therefore the carpet pile is filtering the dirt as it touchs it. Draughts work both ways not just incoming.

That makes sense John, its still weird though.