|Jerome J. Lazar, RA, CCS, CSI, SCIP|
Post Number: 156
|Posted on Tuesday, September 13, 2005 - 12:58 pm: |
Stupid question number 324 (multiple parts) - for commercial construction using 25 ga steel studs is the standard galvanized coating G40 or G60? Does this 'standard' change based on region of the country? How do you prevent the cut edges of steel studs from corroding? In Coastal regions why would interior use galvanized steel studs rust? I know these ? are dumb, but my mind is fried today, so someone educate me.
Post Number: 32
|Posted on Tuesday, September 13, 2005 - 01:23 pm: |
ASTM C 955 says G60 is the minimum. There is no option for no coating.
The coating on the two surfaces provides some sacrificial protection to the cut edges, however, the thicker the material the less protection provided. Also, the cut surface could be butted up to a galvanized surface, unless it's a movement joint.
Is there high humidity inside the walls?
|Doug Brinley AIA CSI CDT CCS|
Post Number: 111
|Posted on Tuesday, September 13, 2005 - 03:47 pm: |
From what I've seen, all it takes for light gage metal studs to rust inside interior walls is presence of air. I don't think high humidity is necessary. Some surface corrosion is just a fact. Cut edges aren't really a problem because the galvanizing is usually sufficient to self-heal UNLESS the cut surfaces are repeatedly damaged.
Whether 'studs' rust enough to be a stability/structural problem is another thing entirely.
For exterior metal stud walls we've moved to a fluid applied vapor barrier/rainscreen approach at a much higher cost. We don't try to insulate inside the metal stud cavities any longer.
For residential applications, corrosion of light gage metal studs in interior walls can be an extreme problem. I've seen them corroding even before the walls are closed up. Why? Because multi-family structures are really complex; with complicated floor plates (especially concrete floors). This yields a wet work environment. Even good quality work is often wet and is too far removed from ordinary sunlight to dry out sufficiently. So, the studs are stored outside, and stood up in damp areas with really no environmental control.
Light gage metal studs aren't 'weatherproof' and sometimes I think we forget that fact.
Post Number: 58
|Posted on Tuesday, September 13, 2005 - 04:56 pm: |
ASTM C 645, "Standard Specification for Nonstructural Steel Framing Members" allows G40 as the minimum. As a practical matter G40 is stocked in the supply houses around here. G60 requires a special order.
|David Axt, AIA, CCS, CSI|
Post Number: 539
|Posted on Tuesday, September 13, 2005 - 05:49 pm: |
Remember: Industry standard (SSMA - Steel Stud Manufacturer's Association) has switched over to mils for indicating thickness. Therefore, 25 gauge is referred to as 18 mils.
BTW, 25 gauge is the thinnest stud that is made. Our office specifies 20 gauge steel studs. (Some locations have 22 gauge studs available.)
For more info:
Post Number: 65
|Posted on Tuesday, September 13, 2005 - 06:32 pm: |
You are a wise man Doug Brinley.
The much higher cost you mention will be much much much higher when the exterior building enclosure fails prematurely do to condensation traped in the stud cavities and deteriorating studs, fasteners, thermal batt insulation, and paper faced gypsum board (mold). But of course, out of sight, out of mind. The end result is like a cancer eating the walls from the concealed spaces outward. Pay me now or pay me a lot more later.
We have witnessed projects with thermal insulation on the exterior side of the air/vapor barrier, supplemented by batt insulation in stud cavities. Go figure.
If you must split the r-value between exterior and exterior, place minimum 2/3 of the r-value exterior (cold in winter side) of air/vapor barrier products such as the Henry AirBloc family of products.
If you are using air tight poly, place all the insulation on the cold in winter side of the poly air/vapor barrier.
For others interested, I have a good article on "KEEPING STUDS WARM AND DRY" that I will share. E-mail me at firstname.lastname@example.org and I will send a pdf file.
There is a noteworthy distinction between 20 gage studs and 20 gage studs. SSMA lists 30 mils (20 gage) - drywall studs and 33 mils (2 gage) - structural studs, with minimum design thickness (exclusive of coating) of 0.312 inch and 0.346 inch respectively.
At CollinsWoerman we standardized on minimum 27 mils minimum thickness. The only 18 mils thickness products were hat channels used as furring.
Our research also revealed 3.5 inch steel studs are not readily available in all regions. Dietrich comes to mind.
|John Regener, AIA, CCS, CCCA, CSI, SCIP|
Post Number: 228
|Posted on Wednesday, September 14, 2005 - 02:31 am: |
Isn't there a fundamental misunderstanding here about how galvanizing (zinc coating) works? The cut edge is not a problem because the zinc coating works in a sacrificial way to protect the steel. Galvanic action causes deterioration of the zinc rather than the steel.
This is how steel ships survive in salt water. Put a piece of unprotected steel in salt water and it will corrode away in a relatively short time. Attach a zinc bar to the hull of a ship and it will last indefinitely, as long as the zinc is periodically replaced and the continuity (i.e., "electrical" connection between the zinc and the steel) is sufficient.
The Navy protects its port facilities and structures with cathodic protection for just this reason --- but cathodic protection is a whole 'nuther subject.
|Doug Brinley AIA CSI CDT CCS|
Post Number: 113
|Posted on Wednesday, September 14, 2005 - 11:47 am: |
I'm tired and maybe I gave an incomplete picture. We've got lots and lots of galvanic protected steel; coated steel; etc. here in Seattle. Also 8 years saltwater sailing experience.
What I've seen is that once you cut an edge, that's a localized damage in the coating. Yes, the zinc acts superficially, sacrificially, to protect the steel. But in a macro scale, a larger underlying piece of steel is still capable of corroding although zinc remains. And it is possible to overwhelm a sacrificial zinc coating.
Zincs on a boat have to be correctly sized, or galvanic action will take overwhelm the available protection. That's also sound basis for the G60/G90 methodology.
I don't have time now for 'stray current' in the water, like happened at our Shilshole Bay Marina.