[Formulabruce]

So I have a 15 year old garage insulated walls with plywood walls.
Exterior is T-111. Some of it got dry and rough. Other areas discolored as determined by direction it faces. It has had a Lot of the stain/ " water proofer" which is now well over 50 bucks a gallon. The T-111 drinks this stuff and my 3.0 nozzle lays it down, seals those creases.
To reduce time working on it, would like to put the metal building wall ( whats on my roof) , on the T-111. Should I " wrap" the garage with TYVEX first? Thanks for your thoughts.

[3X24SPD]

My bare T-111 sided shop was about 15 years old & looking pretty sad & weathered- probably about the same as what you're describing.

About 5 years ago, I had my buddy Jeff who is professional painter get after it.
I hit it with a power washer first, and it brought it back amazingly. Looked almost new again.

Think it still looks pretty good IMHO, 5 years later.
Might have him put another coat on it next year or two.

Jeff's got a sweet looking barrier blue Tripower 66 GTO.
You might know him if you're in our area of the NH seacoast.
I'm sure he'd give you a good deal as one of our local pontiac brethren.

[TAKerry]

The tyvek or type of material will act as a vapor barrier. I would see no problem with putting the metal siding on and since your facing a heated space would probably use it just because. If I were doing so, I would run 1x3's on a horizontal pattern 2' o.c. to attach the siding to. This will leave an air space and not trap moisture between the siding and the plywood.

[Nobuddy]

Some kind of underlayment is required or you risk rot in the T-111 from condensation. But probably not an actual vapor barrier like polyethylene sheet. Tyvek isn't a vapor barrier, it blocks air and liquid water penetration but is still permeable to vapor. So it does not trap moisture and I expect that is what TAKerry meant.

I suggest you check with your metal supplier to ask what they recommend for your locale. They likely have guidelines for warranty purposes if nothing else. The details of the existing wall construction may matter, like what kind of insulation or is there already a vapor barrier on inside or outside the insulation or tar paper under the T-111 etc....

[jww]

You can also buy metal roofing that has “flock” already on the backside if that works for you.

I have some on my last garage addition and no sweating on the inside in winter.

[TAKerry]

Yes, I stand corrected. Plastic is a no no as it will indeed trap moisture. You need to let the moisture out of the building.