Look what we found behind all of the window and door trim on a recent project:
One of the previous contractors used steel wool as backing material for spray foam. The wool rusts, and tannic acid stains form as a result. The spray foam breaks down in a very short amount of time and can cause more harm over time.
One more step towards the proper sealing of this home is to remove it from behind the trim, clean away the small pieces left behind, install backer material, and seal around the widows properly with textured caulk. Steel wool caulking is definitely not recommended.