Design Roof Ventilation to Control Moisture and Ice Dams

When designing and constructing a vented roof system, adequate ventilation is important for a number of reasons:

• Condensation control
• Temperature control of roof surface and attic space
• Energy efficiency
• Prevention of chronic ice damming at eaves

Ventilating unconditioned attic spaces beneath steep-slope roofs and roof cavities within cathedral roofs or below low-slope roofing systems (without adequate above deck insulation) is intended to prevent damaging levels of moisture in materials as a result of condensation or exposure to high humidity. Ventilation is also intended to reduce the temperature of the attic or space below the roof deck. This effect can reduce summertime cooling energy use and also prevent the formation of ice dams in the winter (addressed later in this publication). It also helps reduce the temperature of the roof deck and roofing material during hot periods, improving the durability of materials with volatile organic compounds (like composition asphalt shingles). In fact, some composition shingle warranties may require installation over a ventilated roof deck. Finally, roof ventilation can help remove indoor moisture which makes its way into the attic space as a result of air-leakage through the ceiling caused by an absence of or inadequate air barrier practices (refer to Section 4.3.2). This latter concern has been the cause of some amazing attic “rain storms” due to excessive condensation on the underside of roof decks in homes with high indoor humidity during the winter.

When designing and constructing a vented roof system, adequate ventilation is important for a number of reasons:

• Condensation control
• Temperature control of roof surface and attic space
• Energy efficiency
• Prevention of chronic ice damming at eaves

Ventilating unconditioned attic spaces beneath steep-slope roofs and roof cavities within cathedral roofs or below low-slope roofing systems (without adequate above deck insulation) is intended to prevent damaging levels of moisture in materials as a result of condensation or exposure to high humidity. Ventilation is also intended to reduce the temperature of the attic or space below the roof deck. This effect can reduce summertime cooling energy use and also prevent the formation of ice dams in the winter (addressed later in this publication). It also helps reduce the temperature of the roof deck and roofing material during hot periods, improving the durability of materials with volatile organic compounds (like composition asphalt shingles). In fact, some composition shingle warranties may require installation over a ventilated roof deck. Finally, roof ventilation can help remove indoor moisture which makes its way into the attic space as a result of air-leakage through the ceiling caused by an absence of or inadequate air barrier practices (refer to Section 4.3.2). This latter concern has been the cause of some amazing attic “rain storms” due to excessive condensation on the underside of roof decks in homes with high indoor humidity during the winter.

Minimum Roof Ventilation—Attic spaces and roof cavities should be ventilated at least in accordance with minimum local building code requirements as represented in Table 4–3, and a greater amount of ventilation is advisable for reasons previously stated. In addition, for cathedral ceilings with long slender vent pathways between rafters, the minimum depths of the vent pathway (air pathway from inlet vent to outlet vent) should be 2” for roof slopes of 3:12 to 5:12 or 1.5” for roof pitches greater than 5:12 to minimize resistance to air flow. Typical building codes only require 1” vent pathway depth which may be inadequate for many applications.

Sample roof ventilation configurations are shown in Figure 4–4 and apply to cases with reasonably balanced distribution of high and low vents and a recommended vertical separation of at least 3’ between high and low vents. However, if there is any imbalance in high and low vent amounts, it is better to place slightly more ventilation low than high in cold climates, to avoid creating low pressure in the roof space. Low pressure of this type can draw moist air from the house into the attic space.

Frequently, the required ventilation amounts are not properly calculated, not calculated at all, or not enforced. Again, this is a matter of proper design details and quality control during construction. Such mistakes coupled with other errors such as exhausting bathroom fans into an attic space and not air-sealing the ceiling can cause major moisture durability problems (condensation) in roof systems.

Example: How to determine the required vent amounts

Consider the roof eave and ridge vent scenario shown in Figure 4–4 and assume that the roof ceiling area is 1,200 ft2 and that the eave and ridge vents are separated by more than 3’ vertically. Thus, Table 4–3 requires a vent ratio of 1:300 or 1 ft2 net free vent area (NFVA) for every 300 ft2 of ceiling area. Consider also that this building is in a cold climate such that the exception in footnote ‘b’ of Table 4–3 applies.

The calculations follow:

Consider the following split of vent area for the eaves (low) and ridge (high):

Determine the vent length required which depends on the vent product’s rated NFVA per foot length of vent (obtain this value from the vent manufacturer). Assume the product selected for both the eaves and ridge has a NFVA per foot value of 8 in2/foot length.

The soffit vent should be continuous for the entire soffit and the ridge vent should also extend most of the roof length. Thus, the calculated lengths may need to be increased to accommodate the actual building roof dimensions to avoid creating stagnant areas. However, calculations as shown above should be redone to verify a balanced vent area or one that is slightly skewed with more eave (low) vent area than ridge (high) vent area if the building is in a cold climate.

Ventilation to Prevent Ice Dams—The formation of ice dams at roof eaves is a common cause of roof water intrusion in cold climates where snow may accumulate on roofs during the winter. Elevated attic and roof temperatures during the winter cause snow on the roof to melt. Elevated attic temperatures may be caused by inadequate roof ventilation, poor ceiling insulation, indoor air leakage through the ceiling into the attic, leaky or uninsulated ductwork in the attic, heating equipment in the attic, or a combination of these factors. Consequently, snow melting on the upper portion of the roof will drain underneath the snow toward the colder roof eave where it may refreeze, creating an ice dam that causes the roof melt-water to pond and seep through water-shedding roof coverings as illustrated in Figure 4–5. A particularly severe ice dam is shown in Figure 4–6. In such cases, ice dams are not only a moisture durability concern, but also cause damage to roof eaves and guttering while presenting a life-safety hazard (e.g., deaths have occurred from falling icicles or masses of ice).

For enhanced protection against the formation of ice dams, use Table 4–4 to determine roof vent area ratios. The ventilation ratios in Table 4–4 are a function of the venting layout and ceiling (attic floor) insulation levels. These recommendations should be employed in areas with a ground snow load greater than 30 pounds per square foot (psf) and strongly considered in other areas where below freezing winter temperatures and roof snow accumulation are expected. The following additional practices also apply:

• A balanced placement of high (outlet) and low (inlet) vents as shown in Figure 4–4 and Table 4–3 above;
• Use of air barrier practices in the ceiling to prevent warm, moist indoor air leakage into the attic space (refer to Section 4.3.2);
• Adequate insulation and sealing of ductwork and HVAC equipment located in the attic space; and
• Venting of all exhaust fans to outdoors (not the attic or attic eave).

Where the roof ventilation recommendations in Table 4–4 exceed code-minimum roof ventilation requirements (have more net vent open area than is required with the result of the 1:300 calculation), additional benefits will also be realized in the control of water vapor diffusion and condensation in a roof system. Also, use of ice dam flashing in accordance with Figure 4–7 is still strongly recommended as a means of providing “back-up” protection.