Preventing or controlling the change from water vapor (gas) into moisture (liquid) must be managed in any building. This change occurs based on the process of convection - movement of air will always be from warmer areas to cooler areas. This means that in winter, the warm, moist air inside is constantly trying to escape to the colder air outside.

Warm air can hold more water vapor than cool air. The amount of water vapor in air is always relative to the temperature. When the relative humidity is 100%, the air is saturated and cannot hold any more water vapor. This is called the dewpoint. If this 100% saturated air is cooled, the extra moisture condenses the vapor into liquid water or moisture. If the temperature is cold enough, the water vapor will turn directly into a solid in the form of ice or frost.

Inside a building, water vapor will condense whenever it comes into contact with a surface that is cold enough. The air will lose heat to the cold surface through conduction. As the air cools, the dewpoint drops and relative humidity increases until the cooler air is saturated, at which point condensation occurs.

Adding traditional fiberglass batt insulation to slow the loss of inside heat from a steel building creates a problem. Instead of warm air rising and striking the metal walls and roof directly causing condensation and dripping, the insulation will make the air cool more gradually. The warm, moist indoor air will cool and reach the dewpoint either somewhere inside the insulation or when it reaches the roof or siding trapping moisture. Moisture trapped within the insulation will cause its R-value and effectiveness to drop dramatically as water is a very good conductor of heat. Even though the metal in a quality steel building will be treated, water trapped between the insulation and the building exterior will, over time, cause rust, oxidation, mold and fungus growth - conditions that will degrade the steel walls and shorten the service life of the building.

A vapor retarder installed on the warm side of the insulation will act as a physical shield to repel moisture, keeping it inside in the form of water vapor. A vapor retarder like aluminum foil or polyethylene sheeting will slow the movement of water vapor. Some small amount will still get through and proper ventilation at the roof is necessary for this to escape. Air infiltration through the building exterior in humid areas will also allow some moisture in below the roof decking and into the insulation. As any amount of moisture seriously degrades the insulation capabilities of fiberglass batt insulation, fiberglass is not recommended for use in steel buildings in humid areas. Reflective foil insulation is recommended instead as it both insulates and creates a vapor barrier that protects the building exterior.

Reflective insulation installed under the roof decking

Human beings and animals give off a significant amount of water through breathing and perspiring. Any other processes in the building that use water, such as steam cleaning or food processing, will add to the moisture content of the air. Gas, oil and propane space heaters give off considerable moisture through the process of combustion as well.

Condensation in an un-insulated steel storage building can damage the contents inside through dripping. The primary cause of condensation in un-insulated steel storage sheds is moisture wicking up from the ground and evaporating inside the building. The building needs to go onto a base which includes a vapor retarder like aluminum foil sheeting to stop this rising dampness.

Excavated earth will also give off a significant quantity of moisture as the soil is exposed to air. If the building is closed over a dirt or gravel floor, this moisture will stay inside the building. Freshly poured concrete is another significant source of large amounts of moisture. If concrete is poured after the building is closed, the building will have to be ventilated to remove all this extra moisture.

In steel buildings, visible condensation occurs on exposed surfaces while concealed condensation occurs within the insulation or roof and wall cavities.

Visible condensation

Visible condensation shows up when moisture appears on the inside surface. This is the nuisance type of condensation you see on windowpanes and purlins that results in dripping. Visible condensation can be controlled with ventilation and the addition of reflective insulation. The insulation will reduce the cold surface area where condensation is likely to occur. Running the insulation over the inside of steel framing members will reduce this even further.

Ventilation to exchange indoor air with high moisture content for outdoor air of lower moisture content will even further reduce visible condensation. Keeping fans running to circulate the air will decrease the excess humidity in the air as well. Installing a vapor barrier under the floor during construction will also help prevent the problem.

Concealed condensation

Concealed condensation is a more difficult problem and can cause more damage than visible condensation. Concealed condensation occurs when vapor has passed through the insulation or the vapor barrier to condense either inside the insulation or on the metal surface of the roof or exterior wall. It is best controlled during construction by having proper ventilation in the roof and a tight vapor barrier on the warm side of the wall.

Fiberglass batts will hold moisture inside as either water vapor or condensed moisture both of which negate its effectiveness as an insulating material. Removing moisture from fiberglass insulation once it gets inside is difficult and replacement is usually called for.

Preventing concealed condensation is the most effective means of control. Proper use of a vapor barrier is critical. As vapor barrier will significantly reduce the movement of vapor transmission but not stop it completely, it is more appropriately called a vapor retarder. The ability of a material to resist water vapor is called permeance. A material must have a permeance of less than 1 grain of vapor transmitted per hour per square foot per inch of mercury vapor pressure difference in order to be considered suitable for use as a vapor retarder.

Aluminum foil reflective insulation has a perm rating of 0.05 making it an excellent vapor barrier. When properly installed, waterproof reflective insulation can reduce or eliminate condensation. As reflective insulation is non-absorbent, it will not mildew or promote fungus growth. It can be stapled, nailed, or glued in place. As the Foil-Foam-Foil and Foil-Bubble-Foil types of reflective insulation products carry R-values of up to 14.5 (Details), they can double as wall/roofing insulation and vapor barrier in one.

Other advantages of using reflective insulation over fiberglass and polytheylene sheeting are that reflective insulation provides a superior aesthetic finish and reflects light inside the building thereby reducing indoor lighting requirements.

Click here for comprehensive information the different types of reflective insulation.

Water vapor moves through a vapor retarder based on pressure. Warm humid air has greater vapor pressure than cool dry air. This means higher vapor pressure exists on the warm side of the vapor retarder. As water vapor always moves from an area of higher pressure to an area of lower pressure, the vapor retarder must be placed on the warm side of the wall - usually the inside except for refrigerated buildings.

The vapor barrier must be continuous to work - no holes, leaks, un-taped seams or edges. Leakage will negate the effectiveness of the barrier. Any tears, openings, or punctures that may occur during construction must be completely sealed with aluminum tape. Ending the vapor barrier at the floor or ceiling junction has the same effect as a large hole. To create the vapor barrier, the seam between the ceiling and wall must be sealed well. Think of your building as a large balloon with the vapor barrier as the skin of the balloon. A leaky barrier is not much better than no barrier.

Electrical boxes and plumbing lines will compromise the barrier if the edges are not sealed completely with tape.

Reflective insulation vapor barrier in a steel storage building

Click here for comprehensive installation instructions using reflective insulation as an Insulating Vapor Barrier.

National Insulation Assoiciation
Condensation Fact Sheet for Metal Buildings
http://www.insulation.org/metalbuilding/pages/design/condensationfacts.html

Reflective Insulation Manufacturers Association
Metal Buildings and Condensation
www.rima.net