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Moisture Management Design Rule 1: Control Liquid Water

May 08, 2015

Rain is the biggest water threat to a house, but there are some slow-and-steady mechanisms that can snowball on you, too

Capillary water flow in a brick that is in contact with water at the bottom. The time elapsed after the first contact with water is indicated. The brick height is 225 mm (about 9 inches). From the weight increase, the estimated porosity is 25%. Photo: Hankwang via wikicommons

Think about how tall a redwood tree is: that's how high water can climb in wood.

Below is an excerpt from EPA's Moisture Control Guidance for Building Design, Construction and Maintenance

The first principle of moisture control is to keep liquid water out of the building. Sheltering occupants from water is a primary purpose of building assemblies including roofs, walls and foundations.

Among the sources of water from outside a building are:

  • Rain and melting snow, ice or frost.
  • Groundwater and surface runoff.
  • Water brought into the building by plumbing.
  • Wet materials enclosed in building assemblies during construction.

Problem: Building Assemblies and Materials Get Wet

Moisture problems are common. By their very nature, buildings and the construction process are almost certain to encounter moisture problems that could lead to poor indoor air quality and other negative impacts.

The most common liquid water problems include:

Rain and snow get inside. Rainwater, surface water and groundwater, including snowmelt, may enter a building through leaks in roofs, walls, windows, doors or foundations. In most climates, rain is the largest source of water in buildings. Rainwater intrusion can cause great damage to the building itself and to its contents.

Plumbing leaks. We intentionally bring water into buildings for cleaning, bathing, and cooking, and we intentionally drain wastewater out of buildings. Any water brought in and drained out is contained in pipes, vessels, and fixtures that can tolerate being wet all or most of the time.

However, leaks in plumbing supply lines, drain lines, sinks, showers, and tubs may cause problems. Although model plumbing codes require both the supply side and drain/vent side of plumbing systems to be tested for leaks, these tests are sometimes performed poorly or not at all.

Large plumbing leaks are immediately obvious, but small leaks inside walls and ceiling cavities may continue unnoticed for some time.

Water during construction causes problems. Some materials are installed wet because they were exposed to rain or plumbing leaks during construction. Wet concrete masonry units (CMUs), poured or pre-cast concrete, lumber and the exposed earth of a crawl space floor have all been sources of problems in new buildings.

Some materials are installed wet because water is part of the process. Poured concrete, floor levelers, wet-spray insulation and water-based finishes all contain water.

Porous materials that appear dry may contain enough water to cause problems if they come in contact with moisture-sensitive materials or if they humidify a cavity after they are enclosed. Flooring, wall coverings, and coatings will fail if they are applied before surfaces are dry enough. Water from these materials may indirectly cause problems by raising the humidity indoors during a building’s first year of use, leading to condensation problems.

Solution: Control Liquid Water Movement

Effectively controlling liquid water intrusion requires all of the following:

Drain rain, irrigation water, and snowmelt away from the building. The first step in water control is to locate the building in dry or well-drained soil and use or change the landscape to divert water away from the structure. In other words, drain the site.

This includes sloping the grade away from the building to divert surface water and keep subsurface water away from the foundation below grade. After the site is prepared to effectively drain water away from the building, the building needs a stormwater runoff system to divert rain from the roof into the site drainage system.

Keep rain and irrigation water from leaking into the walls and roof. Leaking rainwater can cause great damage to a building and to the materials inside. In successful systems, rainwater that falls on the building is controlled by: yy Exterior cladding, roofing, and storm-water management systems to intercept most of the rain and drain it away from the building. yy Capillary breaks, which keep rainwater from wicking through porous building materials or through cracks between materials. A capillary break is either an air gap between adjacent layers or a material such as rubber sheeting that does not absorb or pass liquid water. A few rain control systems consist of a single moisture-impermeable material, sealed at the seams, that both intercepts rainwater and provides a capillary break. Membrane roofing and some glass panel claddings for walls work in this way.

Keep water from wicking into the building by using capillary breaks in the building enclosure. Moisture migration by capillary action can be interrupted using an air space or water-impermeable material. • Prevent plumbing leaks by locating plumbing lines and components where they are easy to inspect and repair, are unlikely to freeze, and are not in contact with porous cavity insulation.

Avoid enclosing wet materials in new construction by protecting moisture-sensitive and porous materials during transport and on-site storage and by drying wet materials before they are enclosed inside building assemblies or covered by finish materials.

Drained Roofing and Wall Cladding Roofing and cladding systems are frequently backed by an air gap and a moisture-resistant material that forms the drainage plane. Most of the water that seeps, wicks or is blown past the cladding will drain out of the assembly.

The drainage plane prevents any water that might bridge the air gap from wetting the inner portions of the assembly.

Some examples of drained cladding systems:

  • Roofs. Asphalt or wooden shingles, metal panels and elastomeric membranes are common outer layers for roofs.
  • Walls. Wooden and vinyl siding, stucco, concrete panels, brick, concrete masonry units and stone veneers are common outer layers for walls.
  • Drainage planes. Building felt tar paper and water-resistant barriers are commonly used as drainage planes beneath roofing and wall cladding systems. Single- and multi-ply roofing combines the drainage plane with the outer layers of the roof—there is no inner drainage plane material.

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