Typically, internal doors are used in bathrooms, bedrooms, laundry rooms, and other areas within the interior. Even though our internal doors do not have security door characteristics, they have soundproofing features. On the other hand, exterior doors are heavier, more durable, and more expensive than interior doors because they are built specifically for "security". Exterior doors need to be properly insulated because they protect our homes against external elements. Exterior doors must be able to withstand corrosive weather.You could possibly install an exterior door inside your house by enlarging the doorway frame, however you wouldn't want to use an interior door as the front door.