Basic Roof Ventilation Flashcards
What is the minimum required ventilation rate for attic spaces?
1 square foot of venting per 150 square feet of attic area.
Under what condition can the ventilation rate be reduced to 1 square foot per 300 square feet of attic space?
When most of the vents are high on the roof, such as with ridge and soffit vents, and air flow is induced from a lower point.
What is the primary function of attic ventilation?
To expel moisture-laden air and balance the temperature between the roof coverings and the outside air, improving the lifespan of the roof.
What is the “1-in-300 Rule” in attic ventilation?
A guideline that allows reducing the minimum ventilation requirement to 1 square foot per 300 square feet of attic space when a vapor barrier is installed on the warm side of the ceiling.
(It allows you to use less ventilation (1 square foot per 300 square feet) if you have a vapor barrier in the attic.)
What are some common types of roof ventilation?
- Gable vents: Provide cross-ventilation through screened openings.
- Turbine vents: Wind-powered to promote airflow out of the roof area.
- Passive vents: Allow airflow on flat or low-slope roofs.
- Soffit and ridge vents: Draw air from soffits and exhaust through ridge vents, providing efficient ventilation.
- Powered vents: Use thermostats or switches to activate fans when attic temperatures rise.
- Combination venting: Uses multiple methods, often including through-the-roof vents installed below the ridge.
What is the most common and efficient ventilation method used in new construction?
(What’s the best ventilation system for new roofs?)
Soffit and ridge vents, which allow air to flow from cooler air at the soffit and out through the ridge.
(Soffit and ridge vents are the most common and efficient, allowing air to enter from the bottom and exit at the top.)
Vent Problems
Vent Problems
What are the problems caused by a lack of roof ventilation?
- High temperatures break down roofing materials (asphalt shingles, flat, and tile roofs).
- High humidity leads to rot in sheathing and mold growth.
What issues arise from inadequate roof ventilation?
-Similar problems to lack of ventilation, but less severe.
- In cold climates: ice dams, frost, and rusting nails.
What happens when there’s excessive roof ventilation?
- Large, poorly screened vents can let rainwater into the attic.
- Too much ventilation is as harmful as too little, disrupting the balance between insulation, moisture barriers, and ventilation.
What are false roof vents?
- Vents that appear to be installed but are not actually connected through the structure.
- Example: Ridge vents not properly cut or installed to allow airflow.
How do damaged vents impact the roof system?
- Damaged or rusted vents compromise ventilation and may cause leaks.
- Flashing around vents should be checked for leaks and damage.
What is ice damming, and what causes it?
- Ice damming occurs when melting snow runs down the roof and refreezes at the eaves.
- Causes water to back up under shingles, damaging the sheathing and potentially entering the home.
What is an unvented roof system?
- A system without traditional attic vents, using airtight insulation like spray foam.
- Prevents moisture buildup on the underside of the roof and provides better temperature control.
What are the requirements for an unvented roof system?
- The attic must be within the building’s thermal envelope.
- No vapor barriers should be installed on the ceiling (attic floor).
- Air-impermeable insulation should be in direct contact with the roof deck.
