Building Science
411 on Moisture and Water Infiltration
07/25/2011 08:26 AM
Many homeowners have issues with interior moisture either from visible leaks or excess humidity. This can lead to a number of structural, cosmetic, and health-related issues. First, let’s focus on how water gets inside.
Water typically enters a home through one of four ways: bulk pathways, capillarity, air vapor, or vapor diffusion. Bulk water has far more volume than the other three with capillarity coming in second and on down the line; the order is important because this helps to determine management priorities.
Bulk Water: Rain, Runoff, and Wind-driven H20
Bulk water is driven by gravity along with wind and pressure differences. Think in terms of water on the building’s exterior, which is managed by moving it down and off the building; site features move the water away from the building. These can comprise a system of interconnected flashings, drainage planes or weather-resistive barriers, free-draining spaces, and claddings to manage exterior bulk water. Indoor solutions vary with bulk water being managed by preventing or containing plumbing leaks and condensation. Defenses include collection trays or pans, sensor-driven shut-offs, and routine maintenance.
Capillary Water
Capillary water moves in a tube-like manner under tension through porous building materials or narrow channels between these materials. When you consider the porous nature of many materials and the incredible cohesion and adhesion of water, you find that liquid water can move against the force of gravity quite effectively. Capillary breaks are a main defense in this realm. These non-porous materials such as sheet metal, impermeable membranes, closed-cell foams or plastics, or free-draining air spaces are placed in appropriate locations, such as between the foundation and moisture-sensitive materials sitting on it.
Air-transported Moisture
Air-transported moisture is the vapor content of air as it leaks out of or into a house. Air leakage is driven by a combination of holes through the home’s structure and one of three driving forces: wind, stack effect, or mechanically induced pressure differences (fans) between the interior and exterior. The main concern of moisture-laden leaking air occurs when this is accompanied by a temperature drop, increasing the potential for condensation. For example, warm, humid shower air during cold winter weather can leak around the bathroom light fixture into the attic, condensing on the roof sheathing and eventually leading to rot. We can manage air-transported moisture with a continuous air barrier in the building envelope, built with interconnected air-impermeable sheet goods, caulks, sealants, and spray foams. Most effective is when air barriers are in contact with insulation, which serves as a thermal barrier.
Vapor Diffusion
Quite often the vapor drive of water into homes is climate- and season-related: vapor drive is from the inside of heated buildings in the winter and from the outside of cooled buildings during summer. It’s key to balance the restriction of this climate- and season-based vapor movement into homes with the allowance for drying of the same structures. This is accomplished by conducting a vapor-profile analysis or hygrothermal (humidity plus temperature) modeling.
Other culprits
Here are additional possibilities to consider regarding water’s entry into a home:
Water Management and Insulation
All this info can help to understand these fundamentals when considering adding insulation to a home. Insulation restricts the flow of heat, which in turn reduces the ability of homes to dry out when wet. Most old buildings don't manage moisture very well; because they are so poorly insulated, they easily dry out. Adding insulation to older buildings is a good idea for many reasons, but remember to think about moisture at the same time.
We will further cover preventative measures and corrective procedures in an upcoming blog so stay tuned!
Writing collaborated by Duane Johns (Advanced Renovations, Inc.) and Ginger Sprinkle @ nine dots branding & marketing company (ginger@ninedotsbranding.com).
Water typically enters a home through one of four ways: bulk pathways, capillarity, air vapor, or vapor diffusion. Bulk water has far more volume than the other three with capillarity coming in second and on down the line; the order is important because this helps to determine management priorities.
Bulk Water: Rain, Runoff, and Wind-driven H20
Bulk water is driven by gravity along with wind and pressure differences. Think in terms of water on the building’s exterior, which is managed by moving it down and off the building; site features move the water away from the building. These can comprise a system of interconnected flashings, drainage planes or weather-resistive barriers, free-draining spaces, and claddings to manage exterior bulk water. Indoor solutions vary with bulk water being managed by preventing or containing plumbing leaks and condensation. Defenses include collection trays or pans, sensor-driven shut-offs, and routine maintenance.
Capillary Water
Capillary water moves in a tube-like manner under tension through porous building materials or narrow channels between these materials. When you consider the porous nature of many materials and the incredible cohesion and adhesion of water, you find that liquid water can move against the force of gravity quite effectively. Capillary breaks are a main defense in this realm. These non-porous materials such as sheet metal, impermeable membranes, closed-cell foams or plastics, or free-draining air spaces are placed in appropriate locations, such as between the foundation and moisture-sensitive materials sitting on it.
Air-transported Moisture
Air-transported moisture is the vapor content of air as it leaks out of or into a house. Air leakage is driven by a combination of holes through the home’s structure and one of three driving forces: wind, stack effect, or mechanically induced pressure differences (fans) between the interior and exterior. The main concern of moisture-laden leaking air occurs when this is accompanied by a temperature drop, increasing the potential for condensation. For example, warm, humid shower air during cold winter weather can leak around the bathroom light fixture into the attic, condensing on the roof sheathing and eventually leading to rot. We can manage air-transported moisture with a continuous air barrier in the building envelope, built with interconnected air-impermeable sheet goods, caulks, sealants, and spray foams. Most effective is when air barriers are in contact with insulation, which serves as a thermal barrier.
Vapor Diffusion
Quite often the vapor drive of water into homes is climate- and season-related: vapor drive is from the inside of heated buildings in the winter and from the outside of cooled buildings during summer. It’s key to balance the restriction of this climate- and season-based vapor movement into homes with the allowance for drying of the same structures. This is accomplished by conducting a vapor-profile analysis or hygrothermal (humidity plus temperature) modeling.
Other culprits
Here are additional possibilities to consider regarding water’s entry into a home:
- Interior leaks
- Water leaks into non-ventilated areas adjacent to windows
- Water intrusion
- Water through foundation wall
- Water in crawlspace
- Moisture migration through slabs
- Wet insulation
- Cracks, alligatoring, and checks leading to water intrusion to subgrade
- Water intrusion into the framing and living spaces resulting in dryrot in the wood-framing system; water-stained gypsum board; and damaged flooring materials, carpeting, and interior furnishings
Water Management and Insulation
All this info can help to understand these fundamentals when considering adding insulation to a home. Insulation restricts the flow of heat, which in turn reduces the ability of homes to dry out when wet. Most old buildings don't manage moisture very well; because they are so poorly insulated, they easily dry out. Adding insulation to older buildings is a good idea for many reasons, but remember to think about moisture at the same time.
We will further cover preventative measures and corrective procedures in an upcoming blog so stay tuned!
Writing collaborated by Duane Johns (Advanced Renovations, Inc.) and Ginger Sprinkle @ nine dots branding & marketing company (ginger@ninedotsbranding.com).