I've seen some T1-11 siding on a few sheds around here, but never knew what it called. Had to look it up.
All T1-11 is NOT Created Equal - YouTube
My guess is, you have sheet-rock, then fiberglass battens and then the T1-11.?.
If that's the case, then adding on an outside layer vinyl siding with insulation under it, would be easiest to do.
A light colored siding would be a bit cooler in the summer..
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