Moisture Control
By Kenton Shepard
Moisture in homes can cause serious problems in a number of ways. The type of moisture problem most commonly found in a particular home depends on the type of climate in which the home is located. Homes located in hot/humid, cold/dry and hot/dry climates will experience moisture in different forms.
Four Forms of Water
Water changes forms depending on the temperature and materials in which it finds itself.
- Liquid- Falls from the sky, bounces upward from the ground, moves over and through soil and gets blown by wind.
- Solid- As ice, it gets a grip on roofing materials and holds moisture next to them and gets into cracks to pry materials apart. As snow, it adds weight to roofs and holds moisture against the home.
- Gas- Water vapor can move through solid materials such as wall assemblies and can collect in surprising large quantities by condensing on cold surfaces.
- Adsorbed- Adsorbed moisture can move through foundations and slabs, sometimes for surprising distances, through capillary action.
What Makes Moisture Move
The Gradients:
- Gravity Gradiant: Moisture moves downhill!
- Concentration Gradient: Moisture moves from wet areas (high moisture concentration) toward dry (low moisture concentration) areas.
- Thermal Gradient: Moisture moves from warm areas toward cold areas.
- Pressure Gradient: Carried as water vapor in air, moisture moves from areas of high air pressure to areas of low air pressure.
Moisture in soil
Gravity is always working to move moisture in its liquid form downward. Runoff from snowmelt and rain will seep into the soil at different rates, depending on the permeability of the soil. It will continue to seep downward until it reaches an impermeable layer.
This impermeable layer may form the bottom of an aquifer, in which case the water will stop flowing and will contribute to recharging the aquifer, or water may reach an underground, sloped, impermeable material such as rock or clay and may change direction and flow nearly horizontally, sometimes forming underground streams or springs. This condition can undermine foundations, causing them to sag or crack. It can also add considerably to the weight of the soil and can increase lateral (sideways) pressure on foundations causing them to bow or crack. This pressure is called hydrostatic pressure.
A rising water table may cause problems as groundwater levels rise.
Some types of soil, called expansive soil, expand as they absorb moisture. Expansive soils are powerful, easily cracking concrete, and can do considerable damage to foundations unless the foundation is designed to allow for soil expansion. Allowance for expansion often means supporting foundation walls on concrete columns which rest on bedrock, while using sacrificial cardboard void forms beneath walls. Void forms are designed to crush if the ground heaves. The presence of void forms cannot be visually confirmed after walls are poured.
Source: Kenton Shepard
Outside wall forms are in place. The cardboard void forms are visible along the base of the foundation wall.
Vertical rebar at corners is for columns which will rest on bedrock.
Moisture and Masonry
Efflorescence
Masonry, including concrete, is porous and moisture is able to travel through it. As moisture passes through masonry, for example a foundation wall, it encounters and dissolves salt crystals. When this moisture reaches the inner surface of the foundation wall the moisture evaporates, leaving behind the white salt crystals. This is a common condition and the white deposits are called “efflorescence”.
Undermining
Due to poor surface or roof drainage practices water may be routed to a foundation in volumes great enough to wash soil from under the foundation. Undermining comprimises the ability of the foundation wall to support the structure above.
Source: Kenton Shepard
Efflorescence and undermining visible in this foundation wall (no footing) are the result of water flowing under and through the wall.
Hygric Buffering
Hygric buffers are materials which absorb and release water easily. Brick absorbs moisture like a sponge. Straw bale homes have a huge hygric buffering capacity. Materials with a large hygric buffering capacity are a concern to inspectors because depending on the direction in which stored moisture travels as the material dries, drying can be a problem. When inspecting a home clad with a material with high hygric buffering capacity, the inspector needs to bear in mind the forces, the gradiants which move water. If forces exist which will cause moisture to dry to the inside of a structure, significant moisture-related problems may develop.
Protecting the Home Interior
Roof and exterior wall coverings are a building’s first line of defense against moisture intrusion. Both areas use a two-part system to provide protection…
1. Drainage Planes
The first part consists of an underlying vapor permeable membrane. These membranes are also known as drainage planes. They act as a barrier to water in its liquid form, but allow water vapor to pass through. This means that some of the moisture created in the home from cooking, bathing, clothes washing and respiration can dry to the home exterior by allowing water vapor to diffuse through the walls.
Perm ratings
The permeability of a material is its ability to allow moisture vapor to pass through. Materials are rated in perms. High ratings mean moisture passes through more easily. Low perm materials have a rating below 15. High perm materials may be rated as high as 80.
Drainage planes come in four forms:
Also known as felt paper, tar paper, roofing paper or roofing underlayment. It’s a popular for roofing applications and has been widely used on walls. Building paper is vapor-permeable.
House wrap is a thin, vapor-permeable plastic which is much lighter than felt paper and more expensive. It’s available from a variety of manufacturers in different perms.
- Thin structural sheathing
Thin structural sheathing can provide both structural sheathing and a drainage plane for the walls. Approximately 1/8 inch thick, it’s constructed from recycled wood fiber.
Rigid foam sheathing provides insulation in addition to sheathing and a drainage plane. Its insulative value is typically ranges from R-3 to R-5.
2. Roof and Wall Coverings, Flashings and Sealants
Exterior Roof and Wall Coverings
Exterior roof and wall coverings are seldom waterproof, but are designed to work together with an underlying membrane to shed the majority of water. Coverings vary in quality of material and installation and in design lifespan.
Exterior Wall Flashing
Exterior walls should be flashed where joints are not protected from weather by overlapping or where dissimilar materials meet.
Sealing of Penetrations and Terminations
Penetrations are any area in the walls or roof in which a hole has been cut. Doors and windows are the most common wall penetrations along with holes cut to allow pipes and wires to pass through. Chimneys, vents and mounting brackets are also common roof penetrations.
Doors and windows should have a head flashing installed above them and should be properly flashed and/or caulked.
Holes cut through exterior walls for plumbing pipes and electrical conduit should be caulked, and all caulking must be maintained, since over time it will dry, shrink and crack.
All penetrations in Exterior Insulated Finish Systems (EIFS) walls must be caulked. This is a common omission and is not true with hard-coat stucco.
The proper method for terminating an exterior wall covering will vary with the type of wall covering and with the location of the termination.
Generally, an exterior wall covering must be able to drain any water which gets behind it and sooner or later, water will get behind it. The bottom of the wall must allow water to drain, but wall framing must also protected from contact with wind-driven rain and snowdrift. Typically, some type of flashing is installed. Metal or vinyl flashing will have holes to allow water to drain, but will keep out most water from snowmelt or rain.
- Covered Entry, Porch Roof or Awning
Protecting entry doors from direct assault by wind and rain will help prevent heat loss and moisture intrusion.
Interior Sources of Moisture
Significant amounts of moisture are generated by human activities inside homes. Cooking, bathing, doing laundry, watering plants (over 99% of water used to water plants enters indoor air). Through respiration and perspiration alone, a family of four adds about 3 gallons a day to home air.
Indoor Air Quality
Poor indoor air quality may result if air inside the home is not changed often enough. The optimum rate is 3.5 air changes per hour.
Sources for More Information
For a more in-depth view of the Physics of Moisture Control , Water Vapor Control Basics and Rain penetration control, visit the website of the Building Science Corporation
Understanding Vapor Barriers- Building Science Corporation |
