- By Jane Martinsons for Post-Frame Advantage -
In describing the theory behind condensation to attendees of NFBA’s Frame Building Expo in February, Marcel Konijn, sales manager at Lantor BV, Veenendaal, The Netherlands, pointed out a basic truth: “Condensation is a fact of life,” he said, “and it simply can’t be avoided.” However, he added, condensation can be controlled in post-frame construction and metal roofing applications if one knows its effects and the products available to manage it.
As for its effects, condensation increases moisture, leads to corrosion, and deteriorates wood purlins in post-frame buildings. “Purlins can actually absorb condensation, and because the moisture won’t evaporate, the wood purlins stay wet and rot,” Konijn said. Once moisture is trapped, it will decrease the R-value of a building, rendering insulation “useless.”
Solutions for controlling condensation
Vapor barriers, which are frequently used in the industry, guard against condensation by preventing warm, moist air from coming into contact with cold, dry air, Konijn said. Likewise, depending on its R-value, insulation can prevent condensation from occurring by shifting the dew point, but it must be installed in a way that avoids trapping moisture or creating air leakage.
“It’s important that you use the right amount of R-value for your insulation,” Konijn stressed. “If the R-value is not high enough, you won’t have [a big] enough shift in dew point, and you still will have condensation and perhaps even bigger problems because moisture will be trapped inside your building.”
Ventilation is key to avoiding condensation, stressed Konijn and his co-presenter Michael Primavera, of H&H Metal Products, Inc., Charlestown, IN. “During the night when condensation occurs, the cold outside air conducts through the metal roof and comes into contact with saturated air. During the day, the condensation evaporates into the air,” Konijn explained. “You need ventilation in your building because, without it, there is a saturation point of the membrane. Once that is reached, there is no ventilation.”
Primavera emphasized the importance of completely turning the air in the building twice a day. “Charts from ventilation manufacturers show the ventilation per cubic foot you need in eaves, ridges, and soffits—wherever you need ventilation in the building. Without adequate ventilation, you are going to have condensation problems regardless of the R-value,” he said.
Available products
Primavera discussed several products used to minimize or manage condensation:
• Fanfold insulation: “Although an economical way to create a vapor barrier on your roof structure, when applied, it can leave gaps in the roof, underneath the metal, that can cause problems with thermal breakage and condensation,” he said.
• Rigid insulation: This is a “very good product,” although it can be difficult to install under adverse weather conditions, such as wind. Again, Primavera stressed the critical importance of having no thermal breaks.
• Vinyl-reinforced fiberglass insulation products: Although many of these products have a maximum R-6 value, Primavera suggested that R-15 or R-20 may be more suitable for regions such as the Midwest.
• Reflective insulation: Manufacturers suggest that, for reflective insulation to work properly, vapor barriers should be placed over trusses with purlins turned up on edge to create a 3.5-in. vapor gap to protect against condensation, said Primavera. “Many of us don’t put it over the trusses because it is a little more difficult to install,” he said. “Instead, we’re putting it over the purlins and slapping metal right on top, which works but can cause problems with condensation.”
• Moisture vapor barrier: “This is an inexpensive way to keep the condensation off whatever is inside the building by shedding water,” he said. “It doesn’t solve or prevent condensation, and it has no R-value. Actually, water is shed to eaves in a high-saturation condition.”
• Factory-applied condensation-control membranes: These membranes absorb condensation during the normal dew cycle of the day, Primavera stated. The drawback is that they don’t have an R-value and are thus more suitable for attics, where condensation is more of a concern than insulation.
[Originally posted by the National Frame Building Association “Post-Frame Advantage”. Re-posted with permission.]