A recent article in a nationwide construction trades magazine expounded the mold-resistive virtues of light-gauge steel framing: “. . . wood framing is susceptible to damage from microbial growth and steel framing is not . . . By framing in cold-formed steel, builders can significantly reduce their liability to mold claims.”

We agree that numerous design, construction and performance benefits can result from the use of light-gauge steel framing, but it is also important for designers and builders to consider “the rest of the story.”

For example, many of the residential mold claims that we investigate involve mold growth upon the paper-faced gypsum drywall that is attached to the stud framing (whether wood or steel). In some of these claims, it could be argued that steel framing has only made the mold problem worse—because these walls lack the excess moisture storage capability afforded by wood studs.

In the February 2002 issue of the ASHRAE Journal, Dr. Joseph Lstiburek reports that the average wood-framed and wood-sheathed home can store 45 to 50 gallons of excess moisture within its wall assemblies. In contrast, he notes:

“Matters are considerably different when the exterior walls are constructed with steel studs and gypsum sheathing. Steel studs have no water storage capacity. Gypsum sheathing can store approximately 1 percent moisture content by weight before mold colonization occurs. Constructing the average home with steel studs and gypsum sheathing yields a hygric buffer capacity of 5 gallons.”

Dr. Lstiburek then adds the following warning:

  “Constructing highly insulated steel frame assemblies with gypsum sheathing are one of the significant challenges of moisture engineering. This assembly combines two perilous characteristics: low hygric buffer capacity (low safety margin) with slow drying times. Even small amounts of moisture will cause problems.”

In other words, without careful design review, it is possible that by framing in steel the designer and builder may actually increase their potential liability to mold claims. In this case, they may with to reduce these risks by specifying gypsum-based drywall and sheathing that are not paper-faced.

Plan for the Worst
Another potential problem with steel framing is seen in the photograph at right, which shows the exterior walls of Seattle’s Coe Elementary School on a cold November morning. The distinct pattern of the underlying steel framing is visible due to the contrast between the surfaces wet with typical morning dew and the drier surfaces of the wall warmed by thermal conduction from the heated interior.

This photograph shows the effects of thermal bridging (aka a ‘thermal short’) between the warm building interior and the wintertime exterior via the steel framing. Maybe some member of the project team failed to recognize the differing thermal performance properties of steel versus wood; at any rate, an effective thermal break to separate the walls from the steel framing appears to be missing. The bad news is that this condition represents a flagrant waste of energy; the good news is that this continual heating of the wall cavity during winter months may help prevent mold growth at the back side of the gypsum drywall if any moisture infiltration ever occurs at this wall.

However, let’s step back for a minute and assume the worst had happened. Imagine that due to unexpectedly high heating costs, the school officials decide to lower the interior temperature during an extended winter holiday when the building is not in use. Then let’s assume that at some point during this winter break the steel studs become the coldest surfaces within the wall cavity. Now imagine that a small amount of vapor condensation (from an unknown moisture source) occurs at the surface of these cold steel studs. This results in some water migration to the base of the wall cavity where a small area of mold growth then occurs at the bottom edge of the paper-faced gypsum drywall -- and is then discovered by maintenance personnel.

It’s not a far jump to imagine this leading to a spate of finger-pointing by alarmed parents, school officials, attorneys and dueling mold and moisture experts. At this point in this completely hypothetical example, let’s note again that this minor vapor condensation (and resulting mold growth) may not have occurred if the walls were framed with moisture-absorbing wood studs that remained warmer than steel studs.

Material Advantages
Does this mean that we think wood framing is superior to steel? Of course not; light-gauge steel framing exhibits many performance properties (e.g., resistance to rot) that clearly are superior, while wood studs also exhibit some superior properties (e.g., resistance to thermal transfer). Note, for example, that the free ‘whole wall r-value calculator’ provided online by the Oak Ridge National Laboratory at www.ornl.gov demonstrates that a simple replacement of 2 by 4 wood studs with 3.5-inch steel studs in a wall with R-11 batt insulation results in an approximate 35 percent reduction in the wall’s overall R-value. There are, of course, excellent code-approved design options for resolving this issue (e.g. adding an inch of rigid foam insulation under the exterior siding); however, the moral of the story is simply that the designers and builders who fail to recognize the key moisture and thermal performance differences between wood and steel stud framing run a great risk of eventually becoming embroiled in mold and moisture litigation.

Similarly, mold and moisture inspectors must understand these steel versus wood performance differences and adjust their investigative protocols accordingly. For example, for steel-framed walls, which can be more sensitive to vapor condensation problems, attention should be paid to the condition of any metal drywall screws that protrude into the wall cavity. Rusty-shanked screws may be evidence of performance deficiencies in the building envelope. In some cased, interior drywall will exhibit a shadow effect where it crosses steel studs that are experiencing some degree of vapor condensation.

Infrared thermometers and infrared cameras also can provide valuable information about the overall thermal and moisture-resistive performance of steel-framed walls. Prior to commencing any destructive testing, it may be a good idea to begin the investigation from the interior using an infrared camera to record exterior wall conditions.

January 2007
Issue One, Volume Two



This article by Colin Murphy and Lonnie Haughton.

Colin Murphy
Murphy is a founder and managing partner of Trinity | ERD.

Lonnie Haughton
Haughton is a construction codes and standards consultant with Richard Avelar & Associates.

Photos
The steel framing used to construct Coe Elementary School is highlighted by the damp spots on the exterior of the building.

 


Mold and Moisture Management Magazine Archives

 

American Society of Heating, Refrigerating and Air-Conditioning Engineers
http://www.ashrae.org/

 

Trinity | ERD
http://www.trinityerd.com

 


"Pushing the Envelope: A Monthly Journal of Issues Concerning Building Design, Science, and Litigation" is a monthly publication of Trinity | ERD. This newsletter is intended as a thoughtful look into the issues of building construction.

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