Controlling Moisture & Humidity

Controlling Moisture & Humidity in the Home

Moisture generally appears in two forms, liquid and gas (vapor). Some amount of moisture is always present in the air. This moisture presence is referred to as Relative Humidity (RH). 0% RH would mean there was absolutely no moisture in the air and is largely theoretical. Even a desert has a little bit of moisture in the air. 100% RH means that the air is completely full of moisture, and can’t hold anymore. Once air reaches 100% RH, water vapor becomes liquid water and starts falling out of the air as precipitation, or forming on surfaces as condensation. Generally, air inside a home should contain about 40% to 60% RH.

Indirect Health Effects of Relative Humidity in Indoor Environments

Relative Humidity Image Courtesy of AHRI

As the chart shows, an environment with RH between 40% and 60% inhibits the reproduction of mold, fungus, bacteria, and other microbes that can cause illness to the occupants of a home. In addition, this range is also the range at which humans feel most comfortable. Drier air causes our skin, nasal passages, and sinuses to become irritated, and wetter air causes our skin and furniture to feel clammy, making us sweat in the summer and freeze in the winter.

In addition, 60% RH is about the limit of the humidity content that the structural and mechanical components of a home can stand over the long haul. When higher humidity is present in a crawlspace, basement, attic, or bathroom over a long period of time, the components start to decline:

  • Drywall becomes clammy to the touch and softens; paint peels.
  • Wood framing becomes softer, to the point where a screwdriver tip can plunge into a piece of lumber instead of glancing off the surface.
  • Hardwood floors can buckle, linoleum starts to peel back from the edges of the room.
  • Furnaces and water heaters corrode.

In order to make the best decision about how to address moisture problems in a particular home, we need to understand how moisture moves. Moisture is governed by the laws of thermodynamics:

  1. Energy cannot be created or destroyed, but energy can move from place to place and change forms
  2. Energy will always move from a high concentration to a low concentration

Understanding How Moisture Moves

Moisture moves from wetter places to drier places via a number of mechanisms:

Bulk Moisture

Bulk Moisture refers to the direct entry of liquid into areas of the home. Bulk moisture can come from many sources:

  • Roof leaks
  • Poor drainage
  • Plumbing leaks
  • Flooding

Bulk moisture is the most obvious form of moisture to discover, but bulk moisture problems can vary greatly in the amount of work required to remedy them. Roof leaks and plumbing leaks typically get noticed and fixed quickly and relatively inexpensively. Drainage and groundwater problems can be much more vexing to deal with. I once did an energy audit for a homeowner in Oakland, CA, who had a creek running through the dirt floor of his crawlspace. An actual creek! With clear, flowing water up to a foot deep and two feet wide. The source was a spring, which was disrupted when the foundation was dug. Correcting this issue would require digging out the foundation on two sides of the home to divert the water from the spring through a drain pipe around the home.

Capillary Action

Capillary action (wicking) refers to a liquid’s ability to move through solid vapor-permeable materials. Moisture wicks from the ground, into, and through building materials such as wood and concrete block, in the same manner that a towel wicks up spilled water. Capillary breaks, such as metal or polyethylene, are moisture-impermeable materials that are often installed between the ground and a home’s foundation, and where the masonry foundation meets the wood framing for the floor of a home. However, capillary breaks were not required by building codes until recently, and they are difficult or impossible to effectively retrofit into homes, without undertaking major foundation work.

Vapor Diffusion

Vapor diffusion occurs when humidity in air (but not the actual air!) moves through vapor-permeable materials, such as drywall. Over time, building codes have incentivized vapor diffusion retarders to slow the process of vapor diffusion from the inside of homes into the attic, walls, and floor. The most common vapor diffusion retarder is the paper facing on fiberglass insulation. Vapor diffusion, by nature, is a relatively weak force, and contrary to popular belief, is not the main source of moisture accumulation in building cavities.


Evaporation occurs when moisture moves from a liquid or moisture-saturated solid into less humid air. Remember, moisture will always move from wet to dry, so damp soil that is 90% RH will still move water vapor into the air in a crawl space, even if the air in the crawl space is already pretty moist – say, 75% RH.

Air Movement

Mean Annual Relative Humidity

Image courtesy of

Air movement is often the largest force driving moisture movement in a home. Air contains moisture, but as air warms, the air molecules move farther apart reducing the overall humidity in the air. Conversely, when air gets colder, molecules move closer together, limiting the air’s ability to hold moisture and increasing its relative humidity. In many climates, air leakage is a leading cause of roof failure. Hot air leaks from the warm inside of the home into the cold attic through unseen penetrations and gaps in the ceiling. When this hot air cools, and then hits the cold underside of the roof, the air can’t hold the moisture any more, and condensation forms on the underside of the roof, eventually rotting the roof deck.

Since most climates in the US have an average annual relative humidity of 40% or higher, we almost never have to worry about adding moisture to a home.

Home Retrofits for Moisture Control

Rather, homes should be built or retrofitted to limit the ability of moisture to accumulate in any part of the structure, and to be able to remove moisture from the home. The best solution for a home depends on the problem.

Drainage Improvements

Drainage improvements are required to solve most bulk moisture issues that are not related to simple leaks. Since bulk moisture around the outside of the home wicks through the foundation and structure of the home, drainage improvements also often alleviate symptoms associated with moisture transfer through capillary action. Many drainage problems stem from grading around the home. The building site should be graded so that moisture moves away from the perimeter of the building. Drainage problems also frequently result from a lack of drainage, or inadequate drainage. Gutters are a frequent culprit. When they clog, rainwater simply spills over the top of the gutter and accumulates right around the perimeter of the home. Also, most gutters don’t drain away from the home – the gutter downspout simply deposits water adjacent to the corners of a home. Most drainage problems can be alleviated by installing or improving gutters or a french drain. Sump pumps are also commonly used to remove moisture that accumulates in crawl spaces due to bulk moisture or water table problem. While sump pumps are effective at removing bulk moisture, they do not address the source of the problem.

Drill & Fill Insulation

Drill and fill insulation can solve common problems with condensation or mold building up on exterior walls, especially north-facing exterior walls. Without wall insulation, the plaster or drywall of an exterior wall is usually much colder than the inside of the home during winter – 50 degrees or colder. However, the interior temperature might be over 70 degrees, so when the warm air hits the colder surface, it increases in relative humidity. Given enough of a temperature difference, the cooling warm air sheds moisture in the form of condensation on the wall, eventually leading to mildew and mold growth. This phenomenon is especially common in bathrooms, where evaporation from hot water creates very humid, warm air inside the bathroom.

Air Sealing

Air sealing is typically the most important project a homeowner can do to reduce moisture problems. Sealing air leaks in the home prevents moist air from crawlspaces and basements from leaking into floor systems, wall cavities, and the inside of the home. Just as importantly, air sealing prevents warm air from leaking into the attic or vaulted ceiling and condensing on the underside of the roof, the insulation, and other surfaces.

Vapor Barriers

Vapor barriers in crawl spaces prevent moisture in the soil from evaporating into the crawl space air. Over the long term, high humidity in a crawl space leads to mold and mildew growth as well as structural rot, and evaporation from soil is a leading source of humidity.

Closed Crawl Spaces

Closed crawl spaces can be even more effective at controlling crawl space moisture than vapor barriers, especially in humid climates, where humid, warm air enters the crawl space through vents designed to provide airflow and prevent moisture accumulation. When this warm, humid air enters the crawl space, it quickly cools, which causes it to become more humid. 85 degree air at 70% RH reaches dew point at 73 degrees – a common summer temperature in a below grade crawl space that is shaded from the sun. It is common to find crawl spaces in these climates dripping condensation from insulation and ducts, with moist framing taking on more and more moisture.

Closed crawl spaces essentially turn the crawl space into a conditioned part of the home. The foundation walls are insulated, the vents to outside are blocked, and a vapor barrier is installed to cover the dirt floor and the below grade foundation walls. However, closed crawl spaces must be installed flawlessly, with humidity sensors and alarms, so that if there ever is a burst pipe or other moisture intrusion, the occupants can find out about the problem and correct it before large amounts of moisture build up and cause mold, mildew, and structural problems.


Ventilation should be installed in every home, not only to remove moisture from inside the home, but to provide fresh air for occupants.

  • Exhaust fans should be installed in all bathrooms and kitchens, to remove evaporative moisture from cooking and showering, among other activities.
  • Heat Recovery Ventilation systems, if used, should be installed to exhaust air from bathrooms, kitchens, and other potentially moist areas, while supply air should be installed in bedrooms and common areas.
  • Energy Recovery Ventilation systems exchange moisture and heat between incoming and exhaust air, and are a good ventilation strategy in humid climates.

Gas Appliance Venting

Gas appliance venting can be a huge source of moisture problems. Every gallon of gas burned in a combustion appliance, like a furnace or a range, produces about a gallon of moisture in flue gas. In vented appliances, such as water heaters and furnaces, the flue venting system is designed to carry this moisture out. However, modern gas ranges and ovens do not have venting systems, so it is important to run and install a kitchen exhaust fan that is ducted to the outside of the home. The fan will pull moisture both from cooking and combustion, as well as combustion byproducts like carbon monoxide.

Fix the Source of the Problem for a Long-term Solution

There are so many ways that moisture can interact with different parts of the home. When moisture problems do develop, it’s critical to track down the source of the problem, rather than to just cure the symptom. Many homeowners are sold central dehumidification systems that can cost upwards of $3,000. These systems can improve comfort, but waste energy to operate, and do nothing to address the underlying problem. As a result, these systems often delay, rather than prevent, the moisture from reaching a point where action is required, and the longer a homeowner waits to address a moisture problem, the worse the problem gets. To get to the bottom of the problem, it is best to work with a contractor who specializes in home performance or moisture remediation.


RH chart: AHRI

Annual Humidity Map:


Copyright September 2014 by Advanced Home Energy

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