I tend to caulk only areas where water is likely to penetrate, as if you do
indiscriminate caulking, you can cause water to be trapped where it would
normally drain out after a rain, such as the seam/crack just below a lower
window sill. The upper drip edge should be a Z shaped galv. strip. If it is,
then water should not leak in there. If there are no stains or swelling of
wallboard on the inside of the window frame, from prior storms, you are
probably ok without caulk on the outside.
Does the metal part start under the siding then over the top of the
window? I suggest not caulking IF the metal extends up under the siding.
That allows any water behind the siding to drip out over the metal, caulking
it would trap it behind the siding.
Windows should be caulked for air infiltration, and caulked on the
inside. It is usually inadvisable to caulk windows on the outside,
because it traps moisture around the frame and causes rot. The
exception is if wind drives rain directly against the house, like the
windward side next to the ocean.
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