All that lovely (expensive) heat that used to escape through your loft was helping to keep it dry. Now that you've insulated the loft the damper conditions up there are encouraging fungal growths (which is almost certainly where the white powdery stuff is coming from).
Many moulds are reasonably harmless but dry rot (which is far from harmless) sometimes first appears as a white powder:
http://www.spectrumpe...com/Images/fungus.jpg
It would be wisest to get it checked out by an expert.
In the meantime check carefully to see whether there's any possible route for water ingress into your loft. (Use binoculars to look at your roof tiles). Despite its name, dry rot likes damp conditions. If you can get rid of the moisture you'll have won a major part of the battle against almost any type of fungal growth.
Chris