One of the common defects home inspectors find with thermal / double pane windows, are broken thermal seals. When the partial vacuum seal between the two panes of glass is lost, condensation can occur between the glass panes. When this condensation goes through a drying period, it leaves a foggy type stain between the glass panes that cannot be cleaned. The only way to correct the situation is to have the double window pane replaced. In this broken thermal seal case at a home in British Columbia, it appears some type of fungi has taken up residence between the two panes of glass.

[broken thermal seal with fungi]