Energy, Moisture, and Building Durability

Home | Insulation | Conserving Energy

Heating | Books | Links

One key issue in the field of building science is the relationship between moisture and energy; another is the effect of moisture on building durability and indoor air quality We’ve all heard horror stories about brand new buildings that are rotting because moisture gets trapped inside them, or houses that are “built too tight” so that the air inside becomes dangerously polluted with combustion gas. These stories make good headlines and have some basis in fact, but they typically oversimplify situations and perpetuate myths about the relationships among energy, moisture, air quality and building durability.

So, let’s clear up some of the misconceptions about energy efficiency and moisture damage. One way to do that is to compare the moisture performance of new, energy-efficient homes to the performance of old, inefficient ones.


Most common insulation materials do not stop air movement. Fiberglass batts act like an air filter, stopping only the dust.

Trick of the Trade

Fiberglass does not stop air under pressure, but it does stop dust. This photo shows a piece of fiberglass that has been sitting on top of a plumbing chase where warm air constantly escaped into the attic during the winter. The black color isn’t mold but particles from the air that were trapped by the insulation as the air passed through it.

FIG 16-0: Older homes “breathed” because they were so full of leaks and lacked insulation. While moisture didn’t get trapped by the walls, neither did much heat, making these homes energy hogs.

Fig 16-1: Now that buildings are constructed with continuous sheathing materials and insulation, moisture that makes its way into a wall cavity takes much longer to dry, potentially causing structural and health problems.


Some of the biggest air leaks are at the top of the house, but people don’t tend to notice them because there is no cold air leaking in. Those leaks actually draw warm air to that part of the house on its way out, so they are not noticed. They also cause more cold air to be drawn in from the bottom, increasing discomfort on the lower levels.

Older homes “breathed” because they were so full of leaks and lacked insulation. While moisture didn’t get trapped by the walls, neither did much heat, making these homes energy hogs.

Energy and moisture: myth and reality

Older homes usually had little or no insulation and were quite leaky. They are the basis for that pearl of wisdom “a house has to breathe” (more on that later).These homes were basically flow-through systems; heat, air, and water vapor could move easily through walls and roofs. If the exterior wall sheathing got wet, heat loss through the wall cavity would dry it out quickly. These homes were quite forgiving of moisture, but they were also uncomfortable energy hogs.

The way we construct buildings has changed dramatically in the past 50 years. The sheet materials we use as exterior sheathing—plywood and oriented strand board (OSB)—slow air movement and act as a condensing surface for water vapor in cold weather. Unfortunately, insulated wall cavities are much slower to dry out when they do get wet. Although energy efficiency by itself does not cause moisture problems, air—sealing and insulation can change the moisture dynamics in a building and may make it less forgiving of moisture that is already being generated inside or leaking from outside the house.

One approach to dealing with these issues is a head-in-the-sand stance: Let not make this house “too efficient” in the hope that it will “breathe” and stay healthy. A much better approach is to control moisture intelligently and avoid those problems deliberately.

FIG 17-0: Damp basements have more effect on a house than just damaging the walls; they can also be a major source of moisture that causes damage in the rest of the building. “Moisture-resistant” greenboard didn’t help much here.

FIG 17-1: Water in the basement is not only annoying, but it can also add a lot of moisture to the air in a house.

FIG 17-2: These photos show the difference between water resulting from a roof leak (top) and water caused by interior water vapor that condenses on the roof sheathing in cold weather (bottom). A roof leak, besides being more likely to show on the ceiling below, often does not have the broad impact that moisture rising through an air leak has. Moisture from in doors often has a more damaging impact over time, partly because it’s less likely to be detected until it’s too late.

According to Code:

Most states have adopted some version of the International Energy Conservation Code (IECC) or its predecessor, the Model Energy Code (MEC). Focused on new residential construction, these codes also apply to additions to existing homes in most states. Although the energy-efficiency requirements of state or local codes may be less stringent for remodeling, check before starting an energy project to see whether there are any minimum requirements and whether you need a building permit for the work.

The importance of managing water

Unwanted water can be a real scourge in any house. We need water in our homes; plumbing systems are designed to bring water into the house and remove it in a controlled way. But too much water in the wrong places contributes to mold growth, which is not only annoying but also potentially hazardous, Water can contribute to structural and furnishing damage; damp basements are renowned for slowly turning everything that’s left there into a soggy; stinky mess. To understand how to control the movement of water, we must under stand where it comes from and how it moves.

FIG 18-0: How Much Water Vapor?: Typical Family of Four (Usage in Gallons per day):

  • Dish washing
  • Cooking
  • Showers and bathing
  • Perspiration and respiration


Although roof leaks are usually pretty obvious (if not always easy to fix), water that enters sidewalls may go undetected for years.


Even though your basement may appear to be dry, it can be a significant source of moisture. To find out whether this is true, tape a piece of 2- to 3-ft.-sq. clear polyethylene to your basement walls or floor. Stretch it tightly; if the surface is damp, you may need to brace some pieces of wood around the edges to keep it in place. Leave it for a few days; if water droplets form on the side you can touch, then the moisture is coming from humid inside air condensing on the cool concrete. If water collects under the poly, then moisture is migrating through the concrete.


When you change any part of your house, it affects the entire system. Moisture, indoor pollution, combustion safety, radon, and fire safety can all be improved by applying the techniques in this book. Careless, incomplete, or misguided treatments can also aggravate structural or health problems. If you think the situations shown in this book don’t apply to your house, or if you aren’t sure what to do, enlist the help of a building performance professional to assess your house and recommend the best approach for you.

Where does unwanted water come from?

Unless you live in the desert, one of the largest sources of water is from outside the house: rain and groundwater. Rain can enter through roofing, siding, or flashing defects and soak into roof, ceiling, and exterior wall materials. Although roof leaks are usually pretty obvious (if not always easy to fix), water that enters sidewalls may go undetected for years .Water that enters foundations and crawl spaces may also be obvious. However, groundwater can also enter a house through evaporation, and it can be difficult to detect in its vapor form.

Water in the ground under a crawl space, in a full basement, or even in a slab foundation can evaporate before it reaches the surface. Concrete is capable of wicking up moisture from an under ground water source and “pumping” it into a house. Even though the floor and walls may appear dry, they can actually be the source of moderate-to-large moisture loads in a building. In hot, humid weather, water vapor also comes from outdoors, when humid air seeps into the house through air leaks (this is called a “latent load” in air-conditioning).

In cold weather, the other common source of water vapor is the people and activities in the house. People generate water vapor through perspiration and respiration. Everyday activities, such as showering, bathing, and cooking, also add vapor to the air. If you have a gas or propane cookstove, the combustion process adds water vapor to the air as well.

Many damaging moisture problems have at their root large or unusual sources of water vapor. This moisture may find its way into exterior walls or roof framing in cold weather, then condense on cold surfaces, causing rot, deterioration, or mold growth. These conditions may go undetected for years and have the potential to cause major structural damage or health problems.

If you are insulating and sealing your home, you will probably be crawling through the attic, basement, and crawl space, which will give you the opportunity to identify existing moisture problems in the house. In all cases, you’ll want to be sure to identify the source of any water or vapor and correct it before continuing your weatherization project.

Managing water vapor

The first step in managing water vapor in your house is to eliminate large sources that can be removed, then deal with sources you cannot remove with proper spot-ventilation (more on ventilation in upcoming section).The next step is to control vapor movement. Traditionally, water- vapor control has focused on diffusion. Water vapor is made up of individual molecules suspended in the air, and they actually move through solid materials; this movement is called diffusion.

In cold climates, vapor primarily moves from indoors (warm and humid) to outdoors (cold and dry). In hot, humid climates, vapor moves from hot, humid outdoors to cooler, dryer indoors, especially with the use of air-conditioning. Trouble is avoided as long as the rate of vapor diffusion is low, but problems crop up when the concentration of moisture is large enough -- or vapor movement is fast enough—that condensation occurs in walls or roof structures.

The most common method for reducing vapor transmission is to use vapor retarders, materials that slow the rate of vapor diffusion to very low levels, generally low enough to prevent condensation. In cold climates, vapor retarders are traditionally installed on the inside of the thermal envelope to keep water vapor from entering the wall cavity and reaching the cold exterior surfaces of the building.

FIG 19-0: Air Leaks Are the Main Cause of Attic Moisture: Many small air leaks allow moist air to escape into the attic, where it can condense and cause frost, mildew, or structural damage to rafters and sheathing. If a fan isn’t vented to the outside, even more moisture will end up in the attic. Double-wall plumbing chase. Openings around electric fixtures. Insulation does not stop air leakage.

FIG 19-1: The kraft-paper facing found on some brands of fiberglass insulation is an effective vapor barrier and should be installed facing the inside of a house in mixed and cold climates. In hot, humid climates, the vapor barrier must face the outside. Polyethylene sheeting should be used only in cold climates. In mixed or cold climates with humid summers, an air-conditioned home is better off with a kraft-paper vapor barrier rather than poly.

In all but the coldest climates, it is helpful to provide drying ability in both directions. Drying ability does not depend on air leaks, and it is necessary to use only materials that are relatively permeable to water vapor (these include drywall, polystyrene foam board, solid wood, felt papers, and housewraps).The presence of a vapor retarder on the cold side can result in moisture condensing in a wall, roof, or floor, leading to mold, moisture damage, and structural decay. Unfortunately, the driving force is reversed in hot, humid weather in any home that has air-conditioning. Fortunately, kraft paper (typically used on kraft-paper-faced insulation) is much more forgiving than poly ethylene and allows drying to the interior under summer conditions. Hence, it is the best interior vapor barrier to use in mixed humid climates (roughly from Kentucky and North Carolina to central Texas) and in cold climates in homes that have central air-conditioning.

Water vapor moves on air

Although the focus on water-vapor control has traditionally been vapor retarders, a much more important effect is air leakage. This cannot be overemphasized! Although vapor retarders are important, they do no good if warm, humid air can leak into the attic or an exterior wall in the winter. Building scientists have found that, over the course of a winter, small air leaks actually move far more water into cold building assemblies than vapor diffusion does. This air movement is the result of the same convection forces we have already discussed, with the stack effect, duct leakage, and other mechanical systems typically being the largest factors. In fact, the lack of a vapor retarder in an insulated wall or a ceiling is secondary—in all but the most extreme climates—if air leakage into those areas can be controlled. This should come as a relief to anyone who wants to insulate a poorly insulated house to which it is difficult to add a vapor retarder.


To manage water vapor, eliminate the large sources that can be removed, ventilate the ones that can’t, and control the movement of remaining water vapor.


Large sources of water vapor from inside buildings are called moisture loads. These can include exposed dirt floors in crawl spaces or basements, indoor pools or spas without proper ventilation or dehumidification, foundation drain access pipes and sump pits, unvented or improperly vented combustion appliances, firewood drying indoors, fish tanks, attached greenhouses, and even small plumbing leaks.

The force that pushes water vapor through solid objects is called vapor pressure. Water molecules move from high concentrations (humidity) to low concentrations (dryness).Vapor also moves from warm areas to cold ones. One example of such vapor diffusion is a loaf of bread, tightly wrapped in a paper bag, left out for a couple of days. Even if the bag is tightly wrapped, water molecules will escape right through the bag (from a higher concentration to a lower one). A waxed bag will keep the bread fresh longer, and plastic will slow the moisture loss the best.

Prev.: Defining the Thermal Boundary
Next: Air Barriers and Indoor Air Quality

Other resources:

Information and Resources for Improving Indoor Air

Top of page