Condensation occurs when there's more moisture than the air can hold. Warm air can hold more moisture, but as it cools it reaches saturation and water droplets form. Ceiling drips and surface moisture are caused when warm moist air comes in contact with the cooler roof-line or walls of your metal building.
High interior humidity is a common cause of condensation. It can come from your heating and air conditioning system, the way you use your building, improper construction techniques, human respiration, or oil or gas-fired heaters. All give off moisture vapor.
Condensation tends to occur more often in climates where the temperature often falls below 35 degrees F over an extended period.
Types of condensation
Two types of condensation exist: visible and concealed.
- Visible condensation occurs on surfaces as water, frost, and ice. You may see it on insulation vapor retarders, skylights, cold water pipes, and cooling ducts.
- Concealed condensation is more damaging and difficult to deal with. It occurs when moisture is passed into the building’s interior and condenses on a surface that has a temperature at or below the dew point.
There is a wide variety of ways moisture can be introduced into the building’s envelope:
- Improper vapor retarder selection
- Improper installation of the vapor retarder
- Improper installation of insulation
- Roof leaks
- Water infiltration through penetrations
- Lack of a structural air barrier
- Incorrect vapor pressure differential between the inside and outside of the building
Concealed condensation can be seen as damp spots, stains, mold, and mildew on walls or ceilings. It may show up as delamination of laminated surfaces, bubbles or blisters in asphalt surfaces, peeling paint, or damp insulation.
Problems from unwanted condensation
What is so bad about condensation in metal buildings? Here are a few issues that could make a building operator cry:
- Corrosion, even on surface-treated metal. Metal walls and fasteners exposed to moisture can oxidize and weaken, shortening the life of your building
- Reduced effectiveness of insulation
- Mold or mildew growth, which causes unpleasant odors and health risks, especially for those with allergies or asthma.
- Insect infestations
Reducing and eliminating condensation
Your goal is to eliminate moisture and maintain a relative humidity indoors between 30-50%.
- Insulate your metal building. Using a good vinyl-backed insulation on the roof and walls of your metal building prevents the contact between the warm, humid air and the cool, dry metal panels
- Ensure air can circulate. If necessary, use fans to encourage evaporation.
- Ensure proper ventilation. Vent gas or oil heaters directly to the outside, and add ceiling vents that enable warm moist air to escape, especially if there are many people or a lot of equipment working inside
- Review lighting fixtures, too, as they can have varying effects on indoor moisture levels
- Use a dehumidifier or run your AC compressor
What is so bad about wet insulation?
Damp insulation is an expensive source of condensation damage. Fiberglass, the most common metal building insulation, loses performance when it gets wet. You can dry and re-fluff it, but that doesn't address potential side-effects such as mold or mildew. Wet insulation holds moisture against the metal panel, increasing the chances of early corrosion.
Within the insulation
Make sure your insulation is sealed at the ends by rolling the vinyl-backing over the ends or taping the side-laps wit vinyl tape. All joints and seams must be lapped, sealed, and secured to prevent moisture from reaching the fiberglass batt.
On a metal building, insulation has to be continuous, not just inserted between the framing members. Otherwise, the exposed metal areas are still vulnerable to condensation as well as unwanted heat transfer.
With vapor retarders (facings)
It's crucial to use a vapor retarder in conjunction with insulation. Most metal building insulation already comes with a vapor retarder over the fiberglass batt.
Vapor retarders inhibit the passage of warm, moist air into the inner areas of the roof or wall systems. They are rated by the amount of moisture that can pass through them, called perm ratings. The lower the perm rating, the more effective the vapor retarder is at keeping moisture out.
Types of vapor retarders include:
- Membranes of polyethylene film, aluminum foil, paint, asphalt laminate, and saturated building paper
- Structural membranes of rigid steel sheets or other impermeable materials such as zinc alloy, aluminum sheets with caulked and formed standing seam edges, vinyl siding, tilt up concrete panels sealed at the edges, and foam insulated metal panels
- Coating membranes made of paint, troweled-on bituminous coatings, epoxy, and urethane foams
If you are using fiberglass batts, laminated facings can keep the blanket from sagging as well as provide impact resistance. Laminated facings can improve reflectivity and emissivity, helping to save energy.
Vapor retarders are installed on the warm side of all insulating material. They are also used in joints of crawl spaces, under-slab ductwork, attic openings, ceiling fixtures, and other penetrations. You can also place them over interior exposed ground surfaces, between the sub-flooring and the ground slab, and on both sides of the insulation in buildings used for cold storage. A clear retarder can be installed over a skylight.
Ventilation dilutes a moist interior air mass with drier outside air and lowers the relative humidity of the air mass. Without proper ventilation, the building will have elevated heat levels, higher humidity, and the air will be stale.
Ventilation is measured by the number of air changes per hour, and the appropriate number depends on building use.
When properly designed, insulation systems raise the surface temperatures within the building envelope to above the dew point. Heat loss is controlled at the exposed or exterior sides of the building.
Insulation prevents contact between the warm, humid air and the cool, dry metal panels. Additional insulation may be needed in ceiling or wall cavities. Other areas to insulate are doors, cold pipes and ductwork, and floor slab edges. If you have surfaces where condensation tends to occur, paint them with moisture absorbing paint.
Controlling condensation at the source
A well-drained base course of crushed rock or washed gravel under the grade level slabs will help control condensation from ground level. Make sure there is adequate drainage and divert any rain or melt water away from the foundation. Vent all mechanical systems by exhausting them to the outside and reduce any supplemental humidification inside.
Condensation is a natural occurrence when warm moist air meets a cold, dry surface. But if it occurs in the wrong place, such as within your insulation or as a prolonged accumulation against even a coated metal surface, corrosion, mold, and mildew will decrease the effective life of your metal building.
Provide appropriate ventilation, insulation, and vapor retardation to mitigate the perils of condensation and preserve the long-term durability of your steel structures.