The specific definition of an exterior versus an interior door is not referenced in the IRS literature that I could find. I would assume it means a door that is on the outside of the house which is partially true with respects to a garage, as it is outside basically when the garage door is open. Also, like you said it is not insulated so is outside in that respect. If you get audited on the issue, you could at a minimum use similar arguments to support you taking the credit.
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