Mayor Robert Donchez, Senator Lisa Boscola and other dignitaries gather Wednesday, Nov. 29, 2017 for the dedication of a 626-spot garage in south Bethlehem. The garage at New and Third streets has been billed as jumpstarting redevelopment in that section of the South Side Business District. The $17.4 million is connected to a six-story office building under construction next to it. That office building will be home to offices for St. Luke’s University Health Network and Lehigh University. There will also be first-floor commercial space. Motorists can sign up for monthly parking passes for the garage. It costs $65 a spot. The parking authority leases 180 spots to the developer for his tenants.
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Buildings gain, lose, and store moisture along with heat energy through a variety of physical mechanisms. Operating efficiency, serviceability, and durability of the structure can be affected by moisture flow. Moisture is controlled through design, detailing, materials selection, and proper installation.
Modes of moisture flow into buildings include:
It many cases it is impractical to design a moisture-free assembly. Good design allows moisture to make its way through in a way that it is not retained.
To keep outside moisture out, designers can choose between rain barriers and rain screens. Rain barriers are face-sealed curtain walls and have sealed joints between the components. Drainage is generally not required in these assemblies for moisture originating from the exterior. For the wall to remain effective, the joints must be maintained. The most effective rain barriers will have the fewest joints.
Precast cladding can be considered a face-sealed curtain wall. The concrete, typically in excess of 5,000 psi in strength, absorbs and passes very little liquid water. Panel joints should have either two layers of sealant or sealant and a secondary method of defense against water penetration, and joints around openings should have primary and secondary seals.
Rain screens are not water-tight and must be ventilated. The screen relieves the driving pressure of water, which is drained or evaporated through an air space or cavity behind the exterior-facing surface. Pressure-equalized rain screens are a variant on the cavity wall design that attempts to prevent water penetration through detailing.
Moisture, both liquid and vapor, accumulates when the rate at which it is generated within a space, or the rate at which it enters the rain screen assembly, exceeds the rate of removal. Repeated wetting, followed by repeated drying, is OK if materials do not stay wet long enough to freeze or deteriorate.
Warm air can contain more water vapor than cold air. In general, water vapor, like heat, is cold-seeking; it migrates from the moist warm, high pressure side of a wall to the dry, cold, low pressure side of a wall and in the process creates what’s called vapor drive. Vapor drive requires humid or moisture-laden air, and positive air pressure such as the kind that exists in most commercial buildings.
The degree of vapor drive is controlled by the porosity of the wall, together with environmental factors, especially:
Moisture gradients – water vapor will naturally move from a high concentration to a lower concentration, until it is in balance. If the vapor pressure is high outside the wall and low inside the wall, vapor drive will be directed inward (and vice versa). The greater the difference of this vapor pressure or “concentration gradient,” the greater the vapor drive.
Temperature gradients – water vapor will naturally move from the warm side of a wall to the cooler side. If the temperature is high inside the building and lower outside the building, vapor drive will be directed outward(and vice versa). The greater the difference of this “temperature gradient,” the greater the vapor drive.
In other words, the movement of moisture via diffusion is a result of differences in vapor pressure that are related to the temperature and moisture content of the air.
Of these two, temperature is the greatest factor impacting vapor drive. In fact, when the temperature differences between indoors and out is great (say, 20 degrees or more), the vapor drive can be quite strong. Add a significant difference in humidity, and the vapor drive becomes even more vigorous.
Even in areas where moist-air vapor drive is not an issue, condensation in, or on, walls must be managed through proper detailing and insulation. The dew point is the temperature to which air must be cooled for the water vapor it contains to condense into liquid water. The condensation, or dew, occurs when the temperature of a surface drops below the dew point. Airborne water vapor – or moisture trapped in a wall – condenses in liquid form on the cool surface.
To prevent dew point temperature-driven condensation a wall must be designed so that it is free of thermal bridges, and so that the dew point temperature of the cross-section occurs WITHIN the insulation where free air is not present, or on some other surface designed to dry out. If it occurs anywhere else in the wall assembly, damage can result.