Thermal and Moisture Control in Exterior Metal Walls
Achieving durable, economical, and sustainable metal wall systems
This course is no longer active
Peter J. Arsenault, AIA, NCARB, LEED-AP
7. Rainscreen Testing
As of 2005, there is no protocol for testing rainscreens
and multiple-component wall systems in the U.S., although
the American Architectural Manufacturers Association (AAMA)
is working on a solution. Nonetheless, two elements of testing
for water infiltration that leading consultants agree on is
a working test pressure of 15 psf and to replicate the fact
that air barriers are rarely installed without some perforations.
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Design Recommendations for Metal
Exterior Walls
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| Moisture control starts with the
selection of the outer wall element. Metal systems
and other non-porous materials do not absorb water.
Moisture Control Design
Recommendations
- When selecting porous materials, acknowledge
that water will penetrate the exterior skin
of buildings. Incorporate durable internal weather
barriers, flashings, and ventilation to prevent
water entry to wall cavities.
- Eliminate fungal food from surfaces that can
get wet so as to inhibit mold growth.
- Use materials that remain stable and functional
after exposure to transient moisture.
- Don't skimp on durability of concealed
moisture control components−replacement
is too costly.
(Source: "Mold, Moisture, Misery, and Money,"
Niklas W. Vigener, P.E. Simpson Gumpertz &
Heger, June, 2003)
Thermal Control Design
Recommendation
A design change that will improve the performance
of multi-component wall systems is frequently
referred to as the Massachusetts Energy Code Compliant
Design. Gypsum sheathing is applied on the exterior
side of the metal studs, then a combination vapor,
air and water barrier integral to the insulation
is applied covered with an outer rainscreen. Locating
rigid insulation outboard of the studs in this
manner has several key advantages including:
- A clearer separation of subcontractor responsibilities
- Better continuity of vapor barrier and retarder
- More efficient and continuous insulation
- The metal studs are on the warm side of the
wall system, reducing the possibility of corrosion
problems
Rainscreen Selection Recommendation
Pressure equalized rainscreens that minimize water
entry and provide ventilation to the cavity will
provide excellent performance. When designing
multi-component wall systems, use pressure equalized
rainscreens that provide compartmentalized back
ventilation.
Surface-Sealed systems
When designing the metal wall system, avoid using
surface-sealed metal wall systems types. In particular,
avoid face-sealed systems with frontal weeps.
These systems collect dirt, require expensive
sealant replacement, and do not provide adequate
ventilation. They offer a first cost advantage,
but higher life cycle costs and do not perform
as well.
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Originally published in Architectural Record.
Originally published in January 2006
