lrrowe wrote: replacing the steel vertical uprights help with the thermal bridging problem; because wood conducts less heat/cold then steel?
Correct. In home construction steel wall studs have an r-value of almost zero; whereas wood has an R-value of approx 1.0 per inch. Another way to look at it is to compare the overall r-value of two walls, one with 2x6 wood studs and the other with 2x6 steel studs; both with R19 in between the studs. The wood framed wall comes out with an overall r of about 17 and the steel framed wall about r 10 at best, IIRC.
lrrowe wrote: wouldn't the wood lend itself to being more prone to rot and mold as a result of moisture coming from internal wall panels then would if they were made from steel
Yes wood can rot. However the steel or aluminum is so much colder inside the wall, that it will cause condensation for certain. The wood OTOH will not be so ready, so cold, to have condensation occur. The big issue, in my view, is the aluminum skin on the exterior. It is cold no matter what the framing is. That is where condensation can still occur. The only way to prevent that is to keep the water vapor in the interior air from reaching the aluminum skin; or to keep the aluminum skin temperature above the dew point. Those are all difficult to achieve.
Cooking outdoors as much as possible reduces interior moisture levels. An A/C helps remove air borne moisture. Keeping roof vents open as much as possible can help.
I think the best way to prevent moisture condensation in a wall like this is the use of spray closed cell foam insulation. It's applied directly to the inside face of the aluminum skin. That totally seals air from the aluminum skin. With wood framing fill the entire space level. Then go from there. The wood framing is also nice as it breaks the metal screw to metal stud heat flow path as well.
For a metal framed CT apply the spray foam as above. That would be followed with 1/2 plywood strips screwed to the metal studs with a 1/2 layer of rigid foam between the wood. Over all that another layer, 1/2 to a full 1inch of rigid foam. Then the interior panels are screwed to the wood strips avoiding contact with the metal studs. The wood strip could also be applied and then the spray foam used to fill to the face level of the wood strips.
Just some thoughts on what I believe the ideal might be. I'm drawing on residential construction for that. I love spray on foam.
The idea is to keep interior moisture from making its way from the inside through the wall to the aluminum skin. Using a vapor barrier on the inside, under the interior wall panels, is a very bad idea if anyone was thinking of using poly sheet plastic as a vapor barrier on the inside. That creates a double vapor barrier, the poly and the aluminum skin. Moisture will make it in there somehow, and have a wonderful place to incubate all sorts of nasties.
Buildings with exterior wall steel studs can get around the heat conduction through the steel framing with the application of rigid foam sheets on the exterior. We can't do that when converting a CT.
