Mould! The silent killer... If you're concerned about indoor air quality then you need to think about mould risk. In this instance I was checking the house for possible thermal bridges and so began with the corners which were hitting 13°c so at risk of condensation. Spotted an external corner with loads of stuff.. stuffed into it and thought I'd better have a look. Wow! It was hairy.
Corners are nearly always going to be risky areas (this was traditional solid wall, 9 inch) so you must make sure airflow is good.
You also need to think about where airflow is coming from in your house - 'natural ventilation' if pulling damp air and mould spores from crawl spaces and disused chimneys is NOT good ventilation!