How to Limit Water-Induced Damage to Buildings in the Design Stage?

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The damage to a building due to water/moisture is a significant factor that either shortens its useful life or necessitates costly repairs. 

The types of damages a building suffers due to moisture are listed below:

1. Decay of wood-based materials
2. Spalling of masonry caused by freeze-thaw cycles
3. Damage to gypsum plasters by dissolution
4. Corrosion of metals
5. Damage due to expansion of components or materials
6. Spalling and degradation caused by salt migration
7. Failure of finishes and creep deformation
8. Reduction in strength or stiffness.

Water-induced damage prevention must be considered at all phases of the construction process, including design, construction, and building commissioning. It must also be taken into account throughout building operation and maintenance and when the building is refurbished, rehabilitated, or undergoes a change in use.

This article discusses moisture sources and moisture migration, design evaluation tools, and design practices that enhance durability against the damages caused due to moisture.

Moisture Sources and Migration

The sources of moisture in the buildings can be broadly classified as follows:

1. Surface runoff of precipitation from land areas
2. Groundwater or wet soil
3. Precipitation or irrigation water that falls on the building
4. Indoor and outdoor humidity
5. Moisture from the use of wet building materials or construction under wet conditions
6. Errors, accidents, and maintenance problems associated with indoor plumbing. 

The moisture can migrate through various moisture transport mechanisms. The following mechanisms are most significant in building constructions and are listed in order of potential magnitude.

1. Liquid flow by gravity, air pressure, surface tension, momentum, and capillary suction.
2. Movement of water vapor by air movement.
3. Water vapor diffusion by vapor pressure differences.

These transport mechanisms can deliver moisture into the building or the building envelope, in which case they should be controlled. These transport mechanisms can also act to remove moisture from the building or building envelope, in which cases they may be used to promote drying.

Design Evaluation Tools

The means for evaluating the design of building envelopes from the perspective of moisture management can be classified as follows:

1. Conceptual Design Evaluation

This approach involves the following three-step procedure:

1. Determination of probable external and internal environmental loads (determine climate and interior design conditions).
2. Determination of the potential moisture transport mechanisms in each assembly.
3. Selection moisture control strategies. 

This method gives a qualitative picture of how a structure will function under the influence of all moisture loads to which it is expected to be subjected.

2. Computer Simulation Models

These models have been developed to quantitatively predict moisture and temperature conditions within proposed assemblies using boundary conditions representative of the climate and interior design conditions. These models mathematically model moisture and heat transfer mechanisms at the inner and outer surfaces of the assemblies and within the assemblies. 

3. Manual Design Tools

These provide quantitative estimates of moisture conditions within building envelopes like computer simulation models. They only account, however, for moisture transfer by vapor diffusion. Their focus is on predicting the occurrence of sustained condensation within building assemblies. A handheld calculator can easily perform the calculations for manual design tools. 

Methods to Limit Water-Induced Damage in Design

1. Drainage of Precipitation and Surface Runoff

The following solutions can be implemented to drain the water away from the building:

1. Surface grading- Ground should slope away from walls, so precipitation runoff from land areas does not pond near the foundation.
2. Building external drains- Discharge from ground-level drains should be carried away from the foundation and should flow away from it.
3. Below-grade drainage systems- In some cases, below-grade drainage systems may be required. In some cases, dissipation of collected water by pumping will be required.

2. Limiting Intrusion of Precipitation

Precipitation has the potential for delivering exceptionally large moisture loads to buildings and is usually the largest potential moisture source. The moisture from precipitation enters building envelopes almost exclusively in liquid form, either as rain or meltwater from ice or snow.

There are two broad strategies for controlling rainwater intrusion into walls.

1. Reduce the amount of rainwater deposited on building walls by providing roof overhangs, gutters, or other piped roof drainage systems to shelter walls from direct rain exposure or roof runoff.
2. Control rainwater deposited on building walls by installing adequate moisture seals between roof and walls.

3. Control of Indoor Humidity

Indoor humidity can be limited by controlling moisture sources or removing humidity by air exchange with the outdoors or by dehumidification. 

1. At normal rates for residential occupancy and moisture generation in mild humid climates, ventilation to a level of 0.35 air changes per hour (as recommended in ASHRAE Standard 62, Ventilation for Acceptable Air Quality) will usually be sufficient to prevent excessive indoor humidity. 
2. Mechanical dehumidification is more effective than ventilation for controlling indoor humidity in mild humid climates. 

Spot ventilation is recognized as a generally effective method to help control indoor humidity levels in buildings.

1. Venting of clothes dryers to the exterior is a form of spot ventilation and is recommended for all climates.
2. Spot ventilation in kitchens and bathrooms is common practice and is easily accomplished with exhaust fans.

4. Limiting Moisture Deposition within Assemblies by Air Movement

Air migration occurs from air pressure differentials. These are generally induced by mechanical systems, temperature differentials, wind, or the factors mentioned below:

1. Mechanically-induced pressure differentials- Air pressure differentials caused by mechanical air handlers is easier to control than the other pressure differentials. Control measures include the proper design of air handling systems, careful installation of ductwork to minimize duct leakage, and properly balancing the system when it is commissioned.
2. Thermally-induced pressure differentials- Thermally-induced pressure differentials generally cannot be prevented. Therefore, the strategy used to control air movements created by these differentials is proper construction practices that restrict air movement.
3. Wind-induced pressure differentials- It is recommended to limit wind-induced air movement through assemblies in the thermal envelope. This is generally done by designing and constructing such assemblies to restrict air leakage.

5. Limiting Moisture Uptake from Ground Water or Wet Soil

There are three broad strategies for limiting moisture uptake from groundwater or wet soil. These may be classified as follows:

1. Limit deposition of surface water onto the soil near the building.
2. Remove excessive soil moisture with below-grade drainage systems.
3. Isolate the building from soil moisture using vapor retarders and capillary breaks.

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