Found moisture/mildew behind exterior window trim/repair or replace everything with Hardie?
We started removing deteriorated exterior window trim and found paper-faced exterior gypsum behind the siding with some moisture/mildew damage. We’re also finding questionable/missing flashing and failed caulking in some areas.
My contractor is going to remove more of the deteriorated trim so we can see the extent of the problem. One option is replacing all of the window trim with Hardie, but that’s significantly more expensive.
If the remaining trim is still sound, is there any reason we couldn’t replace only the damaged trim with matching material, correct the flashing/waterproofing, address the affected gypsum, and properly seal everything back up?
I’m also curious how you would handle the moisture/mildew on the paper-faced gypsum, if it’s still structurally sound, can it be cleaned and thoroughly dried, or should affected sections be removed and replaced?
And how do you determine how far to investigate? If mildew is visible on the gypsum around the window openings, how far beyond those areas would you typically open things up before deciding you’ve found the extent of it? I obviously don’t want to remove a huge amount of siding unnecessarily, but I also don’t want to close everything back up without knowing whether the problem continues behind it.
Photos attached. Would really appreciate opinions from anyone experienced with siding, flashing, moisture remediation or building envelopes.