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Preventing wall deterioration.

A growing number of building owners and managers are being forced to repair and restore their facilities - not to upgrade amenities or performance, but to correct premature structural failure. In many cases, the culprit is deterioration within the cavity area of the exterior wall assembly.

Most of these buildings have been built since the 1960s and utilize thin wall construction. Guidelines published for decades by the American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) and other research-based groups have prescribed preventive methods of construction.

Too frequently, however, these principles have been ignored or misunderstood by building teams who may be constrained by tight budgets or who simply do not communicate.

More recently, the practices of fast-track building and value engineering may have pressured a new generation of design professionals to accept without inquiry methods of building that have often been tried, but not necessarily found true.

The decisions made a generation ago may have saved a few thousand dollars, but today's building owner pays the real bill-one representing perhaps millions in repair and restoration costs.

A building owner's first defense against deterioration is awareness, more so of the source than of the symptoms. The second remedy is the implementation of corrective methods of design. Contrary to widespread belief, these techniques are not too expensive when the expenditure is viewed as a long-term investment.

Causes of corrosion

A building's exterior wall assembly is virtually sandwiched by moisture. On the weather side, rainwater and relative humidity can permeate the exterior surface and attack the shelf angles, steel rebars, anchor brackets, and other structural members within. Vapor generated from the interior environment also passes through interior walls toward the exterior. Vapor is especially significant in residential and hotel facilities where multiple showers and kitchens create higher levels of moisture.

In 20 years of analyzing various types of exterior wall assemblies, I have found that deterioration and corrosion has been most likely to occur when one or more of the following conditions are present:

* Provisions have not been made to restrict the movement of air transferred through gaps in the wall assembly.

* The escape of moisture from within the cavity of the wall assembly has been inadvertently impeded or cut off.

* The thickness of insulation in the wall assembly is inappropriate for the thermal performance of the wall.

* The building does not have a vapor retarder, or the vapor retarder has been placed incorrectly or inadequately scaled within the wall assembly

* Materials within the wall assembly have been improperly chosen, inadequately protected from moisture, or not isolated from each other.

Rarely will an exterior wall provide a continuous envelope for the interior of a building. Windows, doors, roof penetrations, cracks, weepholes, and other junctures interrupt the surface. Moisture can also pass through building materials themselves, as vapor will through a concrete block wall. Unless these surfaces are thoroughly scaled and insulated, water deposit will likely damage steel and other building materials in the wall assembly.

Unplugged gaps in a wall assembly will also permit air leakage. Condensation forms when air currents of different temperatures collide within the wall assembly. In southern climates, this happens when mechanically-cooled air generated in interior environments meets warm, humid air from the outside. In northern climates, colder air penetrates from the outside during winter months.

Blocking through-wall air flow

According to the ASHRAE Handbook, six or seven times as much water deposit can result from air leakage as by vapor diffusion through a wall without a vapor retarder. Although it is impossible to completely block air movement, airtight layer construction provides the most effective means of reducing moisture present within the inner layer between the insulation and the exterior wall surfaces.

Airtight layer construction creates a sealed envelope within the wall assembly that renders the wall impermeable to through-wall air movement. For a majority of commercial buildings - those which rely on a brick/concrete block wall assembly - an application of a sealant on both sides of the wall construction is necessary. More difficult is the problem of those buildings whose structural steel is within the exterior wall construction, opening gaps wherever metal meets masonry (Figure 1).

ASHRAE recommends that the structural frame be inside and separate from the weather wall (Figure 2). The resulting positioning of the inner wall forms a more continuous air and water barrier. The control of air movement in the assembly provides the added benefit of reducing energy losses.

Weighing options

Not surprisingly, the most effective method of keeping moisture from penetrating the cavity assembly - the rain screen wall - is often the most expensive. A pure pressure-equalized wall allows air to circulate freely through the cavity and to reventilate, creating equal pressure on both sides of the wall. This negates the force that propels rainwater and moisture through the wall from the outside.

However, the presence of air in the cavity threatens any structural connecting system not fabricated from stainless steel. A second wall inboard of the exterior wall also must serve as a weather barrier. Most commercial building owners cannot justify such a costly and sophisticated solution.

Similar results have been attained with a modified rain screen, or two-stage sealant approach for precast concrete panel walls. A second seal added inboard of the traditional face seal on a precast panel-to-panel joint creates an internal air pressure cavity. A second set of weeps, specifically located to ventilate this cavity, assists in equalizing pressure.

Thermal performance

and vapor retarders

Water vapor behaves differently from air. It exerts a specific pressure, moving from areas of higher vapor pressure to lower pressure.

The common use of the term "vapor barrier" exaggerates the ability of such materials to completely block vapor exfiltration. ASHRAE has instead adopted the term "vapor retarder." Processes for controlling vapor diffusion also are effective at preventing through-wall air leakage.

In residential, hotel, and laboratory facilities, whose interior vapor and humidity levels tend to be high, vapor retarders are essential in order to keep moisture from entering the exterior wall assembly. Vapor retarders are less commonly found in low-rise commercial buildings, where interior vapor levels are lower and where construction materials tend to better withstand moisture and decay.

The modest expense of installing a vapor retarder will certainly pay off in long-term savings on corrosion-caused repairs. However, in cases where the designers have not taken into account the following important variables, the wall assembly has been placed in greater risk of structural failure for one of several reasons.

* The assembly is incorrectly sequenced. Moisture is trapped inside the exterior wall assembly when the wrong material acts as the vapor retarder. Consider a building in a northern climate with high relative indoor humidity or sources which produce vapor, with a wall assembly of an epoxy-painted vapor retarder, concrete block, and a metal panel facade. Vapor will permeate the interior block construction and be trapped by the sealed exterior metal facade, which has a higher permeability resistance value than the inner layer of the wall assembly.

Condensation will form on the surface of the supporting material acting as vapor retarder on the wrong side of the insulation; the outer layer acts as a second vapor retarder, thus trapping moisture within the sandwiched construction. Discoloration of the facade and corrosion within the cavity construction may result. Freeze/thaw thermal cycles will also affect moisture trapped in building materials, causing the expansion of materials, spalling, and delamination.

In northern climates (where heating is the predominant environmental conditioner), a vapor retarder on the interior surface must have a greater resistance to the transfer of moisture than each of the succeeding outboard materials (Figure 3).

The vapor retarder must be placed on the warm side of the properly designed thickness of insulation to prevent the collision of warm and cool air causing condensation on the interior side. The opposite is true in southern climates, where the building-to-outside-air relationship is reversed.

* Insulation is inadequate Hot and cold air colliding within the wall assembly produces condensation at a precise location, known as the dew-point, which has a temperature lower than that of vapor.

By calculating the thermal performance of the wall assembly - how each layer transmits hot and cold air - the mechanical engineer can predict exactly where the dewpoint will occur, within a range of temperatures that depend on the climate of the building's location. Because the dewpoint will also be in flux as temperatures change, the engineer will require an appropriate thickness of insulation so that the dew-point occurs within the insulation and not on the surfaces of the materials.

Building owners must make sure that their architect requests this thermal performance information from the consulting engineer and that the engineer has completely evaluated all exterior building conditions. When too thin a layer of insulation is placed within the wall assembly, a dewpoint in flux may occur on the surface of the vapor retarder, introducing condensation to the surface members nearby.

* Wrong materials are used. Plain kitchen aluminum foil has proven to be the best vapor-retarder in building walls. Hot asphaltic rubberized membrane materials are equally effective in plazas and decks. PVC membrane is a widely used vapor retarder, though it is less effective than aluminum foll. PVC is less impervious to vapor, does not seal as tightly at the joints, and has the further disadvantage of being combustible.

Many builders and owners of commercial buildings still specify kraft paper mopped with an asphaltic application. This method may not even be adequate for small residential construction because the succeeding layers of materials may be more resistant to vapor transmission. Deterioration of the kraft paper, which happens often, only exacerbates the situation.

* The retarders have gaps. A vapor retarder will not adequately perform unless all of its holes and cracks have been sealed. The more complex the design, the more likely that there are gaps at locations where the vapor retarder should be continuous.

In steel-framed buildings, these gaps may occur at framing joints, slab edges, and above-floor beams where fluted deck has been placed. Perimeter joints at punch-through windows, ports for through-wall air conditioning units, pipes, and holes surrounding electrical outlets also interrupt the vapor retarder, increasing the likelihood of breaches in the design.

Continuous vapor retarders, such as those made of aluminum, also risk puncture during installation. As added protection, a layer of wall board is recommended for floor-to-floor construction.

Acoustical sealant is most effective for filling in gaps in steel-stud framing and gypsum panel board construction. Insulation and a smoke-seal sealant provides one alternative for plugging flutes of metal decks.

Moisture is almost always present within the building sandwich of the exterior wall. Thus, it is better to have a less expensive facade and a proper wall design than a poorly designed and constructed assembly that violates thermal performance principles.

Protecting metal components

The protection of building steel is the most fundamental defense against corrosion. Yet take a walk around any older city today and you will see many building owners replacing corroded steel shelf angles, and consequently the surrounding building materials.

Corroded steel flakes, expands, and loses its structural strength as it deteriorates. The expanding steel also further compresses the building units around it, resulting in fractures, chipping, and excessive wall movement.

Building professionals continue to place supporting steel members that have not been properly safeguarded from moisture within the cavity wall assembly. Budget-conscious building owners and builders routinely specify A36 steel angles and channel supporting members, and, on occasion, longer lasting, low-alloy, content-grade 50 steel tubing to attach building systems to the structure.

In the 1950s the Steel Structures Painting Council (SSPC), when it responded to the surge in highrise building, recommended the application of red lead oxide primer to building steel as a temporary (six months, at most) protection from weather exposure during the erection of the primary structural columns and beams, Then as now, fundamental design practice required that steel structural members exposed to moisture in the final assembly receive a top coat of moisture-protective paint.

Gradually, builders began to install A36 steel secondary supporting members that had been treated with only the primer, and without the necessary top coat or galvanized protection. More recently, designers have begun to place primer-only-coated steel shelf angles within the cavity of the weather barrier - a trend that invites disastrous results. The steel attachments in many high - rise building assemblies - whether block and brick, stone on truss, or concrete precast panel - are corroding due to the moisture present in the cavity walls.

Red lead oxide primer is actually just that - a primer coating used as a corrosion inhibitor. At the very least, it requires a second coating as protection from moisture. Before any coating is applied, however, the surface of the steel must be prepared to bond with the protective coating.

First, the steel must be blast cleaned to remove its mill scale, or rough spots - much like scraping removes loose paint from wood surfaces prior to repainting. For maximum protection, a zinc-rich or epoxy primer must then be applied, followed by a second coat of epoxy.

In brick construction, the most widely specified anchoring components are galvanized. Though in itself an effective rust inhibitor, galvanized metal also tends to corrode or react chemically when in contact with dissimilar metals, mortar, and concrete. Where galvanized anchors have been connected with stainless steel, an electrolytic reaction takes place, and the galvanized metal may suffer considerable damage in the presence of concrete substrates.

Short of using stainless steel embedments in concrete, steel attachment members must be treated with an epoxy or acrylic coating specifically designed for embedment in concrete. This protection will also retard reaction with other connecting steel members.

By following some basic guidelines, building owners can prolong the effective performance of the materials they specify.

* If budget permits, use non-magnetic stainless steel (300 series).

* Always properly coat steel situated where moisture will be present, including rebars in reinforced concrete panels and corresponding metal attachments. A low-alloy steel (grade 50, for example) need not require surface preparation; a layer of epoxy mastic paint (one with a high ratio of solid to solvent) will suffice.

* Use compatible metals for connecting in the cavity wall assembly. If aluminum panels have been specified for the building exterior, aluminum connectors should fasten the wall to the building's structural columns and beams. Be sure to isolate dissimilar materials by coating them or by installing isolation pads or other proven methods of separation.


When rehabilitating or contemplating new construction, owners and managers must weigh their expenditure for water and vapor barriers and component protection against the performance standards they expect of their buildings.

Building owners must also make certain that the materials and construction specified by the building team conform with guidelines established by ASHRAE, the Precast/prestressed Concrete Institute, the Corrosion Institute, the Brick Institute of America, and other recognized industry advisory bodies. Only in this way will long-term structural integrity of a building be preserved.

George L. Maness, R.A., is the exterior systems consultant and a senior project manager for the New York firm of Helpern Architects. A specialist in the design and the laboratory and field testing of building assembly systems, Mr. Maness has contributed to the design of large-scale commercial buildings across the United States. He holds a degree in architecture from the University of Idaho.
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Title Annotation:Operating Techniques & Products Bulletin 410
Author:Maness, George L.
Publication:Journal of Property Management
Date:Sep 1, 1991
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