• Applying solvent-based paint over a damp or wet surface.
• Moisture seeping into the paint film through the exterior walls (less likely with water–based paint)
• Exposure of water-based paint film to high humidity or moisture shortly after paint has dried , especially if there was inadequate surface preparation

Remedy:

• If blister do not go all the way down to the substrate:
  Remove blisters by scraping, sanding and repaint with a quality acrylic water –based interior paint.
• If blisters go down to the substrate: Remove the source of moisture , if possible. Repair loose sealants, consider installing vents or exhaust fans. Remove blisters as above, remembering to prime before applying the top coat.