You didn't mention if the grout was colored. Be very careful to NOT use any bleach based products on any colored grouts. Something that works good on all types of grout is bon-ami mixed with vinegar. ... read more
Verify that all window and door seals/weatherstriping is in good shape.
Check to be sure doors and windows are caulked properly (no gaps, cracks or missing).
If you have an attic access, treat it like ... read more