Moisture Control Calculation in Roof Systems

Factors

• Dew point, ventilation, vapor retarders, condensation and the migration of moisture in a roofing system all are variables that factor in to the moisture control calculation for most roofing system types. The roof's exterior surface is a waterproof barrier that limits absorption from the outside, but moisture can collect inside. Climate, appliances used in the home and the way the house is built are all factors in this moisture accumulation. These factors are generally taken into account by builders and manufacturers, and as a homeowner you needn't worry too much about them. If you are building and roofing your own home, however, you should be aware of moisture control needs and recommendations.

Slope

• The slope of the roof will affect how much water vapor accumulates on its underside. This side of the underlayment typically is not protected by waterproofing materials, and therefore it is more prone to moisture absorption. High-sloped roofs tend to gather more moisture due to higher human occupancy loads than low-sloped roofs. You must take extra measures to ensure the roof is resistant to moisture absorption. The moisture accumulation threshold is usually calculated based on the type of insulation used. Each insulation type has a different absorbancy and resistance to moisture. In commercial roofing, the insulation is laid loosely over roofing trusses, or it is installed as a substrate attached to the underside of metal roofing materials. Residential insulation installed in the attic will generally help prevent moisture absorption by blocking warm, moist household air from rising into the attic during cold months and by blocking hot outside air from entering the living space below during warm months.

Ventilation

• Moisture accumulation is expressed in pounds per square foot. When a roof design will cause excess moisture accumulation, ventilation measures can eliminate the excess buildup. Ventilation is a primary consideration in the calculation of a roof's moisture accumulation failure threshold. Ventilation determines the air flow rate within the home, or the number of times per hour the inside air is replaced by the outside air. If you increase air flow, you change moisture control calculations for your roof. Roofing materials manufacturers design the materials to exceed the moisture control limits for a given geographic location. The threshold for moisture will vary based on the roofing insulation. Fiberboard reaches its failure threshold at 0.20 pounds per square foot, while foam insulation has a threshold of 0.012 pounds per square foot. Composite insulation's limit is 0.14 pounds.

Underlayment

• One of the best ways to protect against excessive moisture accumulation and retention is to use a vapor barrier in the form of a water-resistant underlayment. The National Roofing Contractors Association recommends you use a minimum of 30-pound asphalt-saturated underlayment to account for any potential moisture accumulation.

Ice Dams

• Ice dams can be serious roof problems in areas where extremely cold winters with excess snow and ice are common. Ice dams form when a roof is warmed by heated air inside the home. Snow on the roof is melted, and the melt water runs off and refreezes lower on the roof. Further runoff can pool and build up behind the ice dams formed in this way. The pooled water leaks into the attic and house, causing considerable interior damage. Adequate attic insulation and ventilation can help prevent the roof from warming to the point where ice dams can form.