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Roof Ventilation Basics

In your area, shed and ridge vents are likely to be covered with snow when you need them the most. Heat will accumulate in the roof slope and attics and eventually melt them open, but some icing will occur during the “closed” condition. The length of time it takes for them to open is related to outdoor temperatures and snow depth. The deeper the snow (and the more insulation it provides), the more that melt will occur before ventilation takes over to remove heat from the attic.

Inlet and outlet ventilation areas should be matched to work effectively. Valley areas will have significantly more outlet area than net free inlet area. The hip roof will have very little outlet compared to the available inlet area, especially when the ridge is covered by snow. Typical soffits and ridge vents are designed to be matched, i.e., one foot of ridge or shed vent for one foot of soffit vent. Complex roof configurations require careful planning to maintain this match (see distribution below). Hip and valley conditions often require more than just the area provided by conventional ridge and soffit details.

Ventilation should be evenly distributed. Areas without adequate inlet or outlet will create dead spots in the airflow pattern with little cooling effect from the available ventilation, especially if it is marginal or unmatched. Comparing the gross inlet area and gross outlet area to check balance is a common error when designing ventilation. Each area of roof or attic must be balanced for cooling to be uniform over the entire roof area.

Roof and attic ventilation provisions can actually promote roof warming if they are open to the interior conditioned space(s). Exterior (wind) and interior (stack effect) building pressures tend to force warm interior air into the ventilation spaces, exacerbating roof warming. Closing or sealing the sources of roof warming from warm inside air leakage must be eliminated before increased ventilation will be effective.

Comment - The original ventilation provisions, with any improvements listed above, are likely to be adequate to provide the necessary cooling and drying once the “connections” between the interior space and the attic and cathedral roof slope vent spaces have been significantly reduced or eliminated.

Prioritizing these problems (in order of severity)

Note: When prioritizing the problems within each category, the heat loss related to the problem you are trying to solve should be rated as the highest priority.  Thus, in the case of the ice dam problems, the focus should be on “causes” that are in the “watershed areas” above each problematic “Ice Dam Area”.  In the case of the moisture problem in the Hot Tub room and roof above it, the focus should be on eliminating the source of the moisture, or eliminating the possibility of the moisture reaching a condensing (cold) surface.

Keeping this in mind will allow you to put your money into only the work that will eliminate the problems.

(Note: The codes following each item are provided as my best estimate of the importance of the problem. The meaning of each code is described below.

  1. Air leakage from pressurized mechanical systems.

  2. Air leakage from spaces that are normally pressurized by mechanical systems.

  3. Air leakage from penetrations (ceiling or mechanical system) in the tallest sections of the building.

  4. Air leakage from penetrations (ceiling or mechanical system) in the shorter sections of the building.

  5. Radiant heat sources.

  6. Un-insulated surfaces on mechanical systems or interior mechanical spaces at higher than indoor ambient temperatures.

  7. Un-insulated surfaces on spaces at normal indoor ambient temperatures.
    Partially insulated surfaces.

“L{ A large area or a large number of these types of areas that combine to make a significant source of attic warming.

“S” A small area or small number of bad leaks that are not the most important source of attic warming relative to other sources.

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