Black strips on wallboard mounted on metal studs
 

I have developed dark vertical strips on wall board attached to metal studs. I have been told this is due to incomplete combustion of my HVAC system but it only occurs on outside walls. Has any one expereinced this and is there a solution. Repainting only helps for a year or so.

Possibly due to the excessive heat flow through the steel studs creating a temperature differential than can attract or allow air borne particles to stick to the wall. In an extreme situation, you can even get moisture patterns on the walls or ceilings. - The concept is referred to as "thermal short-circuiting".

It is even more pronounced in walls or ceilings where insulation such as fiberglass is used. The effective R-value of the bulk insulation can be reduced by 20-40% in steel stud walls. The same effect occurs in wood frame walls, but not to as much a degree. Walls with a 1-2" layer of extruded foam do not exhibit the "thermal short circuiting".

WOW....I did not know that....:thumbsup:

Black strips on wallboard mounted on metal studs
 

The "R-value" of a bulk insulation is the value of the insulation itself in a laboratory. Then, it goes into the wall the wall construction and properties of the other materials can significantly change the real R-value of the wall. - In other words, R19 insulation in a wall does not give you an R19 wall (you get something less).

Different materials or closer spacing of studs have an effect. A prime example is 6" metal studs @ 12" O.C. - Sort of reminds me of a radiator.

It even get more complicated when you can get big insulation credit for heavier walls that increase the effective insulating value of a wall in real life subjected to annual and/or daily temperature cycles.

Concretemasonry has it nailed here.The stripes are a direct result of moisture from condensation caused by the thermal bridging of the metal framing members.