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Attic Ventilation

This session explores attic ventilation, including its purpose, common misconceptions, ventilation needs by code, ventilation options, and safeguards for ensuring proper ventilation.

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Attic Ventilation

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  1. Attic Ventilation WEATHERIZATION ENERGY AUDITOR SINGLE FAMILY WEATHERIZATION ASSISTANCE PROGRAM STANDARDIZED CURRICULUM – December 2012

  2. Learning Objectives By attending this session, participants will be able to: Explain the purpose and principles of attic ventilation. Summarize misconceptions about attic ventilation. Determine ventilation needs by code and practical alternatives. List ventilation options. Propose specific attic ventilation. Describe safeguards for ensuring that attic ventilation does not cause problems. Attic Ventilation

  3. Purpose of Attic Ventilation Attic Ventilation • Removes solar heat during hot weather • Removes moisture vapor during cold weather Photo courtesy of US Department of Energy

  4. Common Misconceptions Attic Ventilation • Attic ventilation always removes moisture vapor during cold weather. • Attic ventilation will cure an attic moisture problem. • The more attic ventilation, the better. • Attic ventilation is not necessary.

  5. Post Wx Attic Moisture Problems Attic Ventilation Mold on Sheathing Frost on Sheathing Photo courtesy of US Department of Energy Both conditions occurred after attics were insulated and vented, not before.

  6. Passive Attic Venting: Convection Attic Ventilation Ridge vent How does it work? Outlet (Heated Air) • Natural Convection • Hot air rises, escapes high vents • Cooler air pulled through soffit vents • Heat rises,escapes ridge vents Inlet (Cool Air) Graphic developed for the US Department of Energy

  7. Passive Attic Venting: Wind Attic Ventilation WIND How does it work? • Bernoulli Principle • Positive pressure on windward side • Negative pressure on leeward (protected) side • Air sucked out leeward vents Graphic developed for the US Department of Energy

  8. Power Venting Attic Ventilation Fan-powered roof vent How does it work? • Attic fan mounted in central hallway used in non-air conditioned homes. • Roof vent (pictured) used in air conditioned homes. • Neither is recommended as weatherization measure. • Potential for moisture condensation is high. Graphic developed for the US Department of Energy

  9. Code Issues Attic Ventilation • Requirements are based on presence of ceiling vapor retarder, climate zone, and vent location.Options: • 1/150 • One square foot of net free vent area per 150 square feet of attic • 1/300 • One square foot of net free vent area per 300 square feet of attic Thermal boundary - Ceiling Graphic developed for the US Department of Energy

  10. Code Issues Attic Ventilation Thermal boundary – Roof deck Attic ventilation not required when the roof assembly is the thermal and pressure boundary. Graphic developed for the US Department of Energy

  11. Applied Ventilation Formula Attic Ventilation How many roof vents does it take? • Calculate the number of roof vents required. • Climate is Zone 7. • Ceiling is air sealed and insulated. • Ceiling has vapor retarder. • Ceiling area is 60’ x 30’. • Each vent has a net free area of 60 square inches. Photo courtesy of the US Department of Energy

  12. Applied Ventilation Formula Attic Ventilation • Ceiling = 60’ x 30’ = 1,800 square feet. • Formula for attic with vapor retarder = 1/300 • 1,800 ÷ 300 = 6 sq. ft. required NFA ventilation Convert to inches: • 6 sq. ft. x 144 sq. in./sq. ft. = 864 square inches • 864 ÷ 60 square inches NFA/vent = 14.4 15 vents!

  13. Ground Rules and Specifications Attic Ventilation Vented attics should not communicate with the conditioned space. • The ceiling air barrier must be continuous with no leakage. • Specify appropriate attic sealing as part of the work scope. • Specify that vents be placed low and high on roof. • Specify eave chutes when soffit vents are present and loose fill insulation is needed. • Specify that all mechanical ventilation ducts and plumbing stacks are vented directly to the outside.

  14. Venting Configurations Attic Ventilation In a perfect world... Continuous ridge vent Continuous eave vent (soffit vents) Graphic developed for the US Department of Energy

  15. Vent Types Attic Ventilation Gable vent Roof vent Eave vent Images courtesy of J&R Products Inc.

  16. Vent Types Attic Ventilation Turbine vent R-144 mushroom roof vent Images courtesy of J&R Products Inc.

  17. Vent Placement Attic Ventilation Roof vents placed high on roof Continuous soffit vent Photo courtesy of PA WTC

  18. Vent Installation Attic Ventilation • Cut hole to fit; test fit vent. • Caulk edges of hole where flange will sit. • Slide vent into place under shingles. • Seal and nail. Photo courtesy of the US Department of Energy

  19. Vent Placement Attic Ventilation Gable vents are attached similarly to roof vents. Photos courtesy of the U.S. Department of Energy

  20. Attic Window Vent Attic Ventilation Photo courtesy of the US Department of Energy

  21. Eave Chutes Attic Ventilation Eave Chute Photo courtesy of PA WTC; Product photo by J&R Products, Inc.

  22. Mechanical Vent Terminations Attic Ventilation Photo courtesy of PA WTC

  23. Summary Vent attics where possible and practical. Follow 1/150 and 1/300 code guidelines. Don’t specify vents to solve an attic moisture problem. Keep moisture out of the attic by air sealing. Use the right vent for the job. Place vents high and low. Avoid turbine or power vents. Attic Ventilation

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