Transcript Attic Ventilation

WEATHERIZATION ENERGY AUDITOR SINGLE FAMILY
Attic Ventilation
WEATHERIZATION ASSISTANCE PROGRAM STANDARDIZED CURRICULUM – December 2012
1 | WEATHERIZATION
ASSISTANCE PROGRAM STANDARDIZED CURRICULUM – December 2012
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Learning Objectives
ATTIC VENTILATION
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.
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Purpose of Attic Ventilation
ATTIC VENTILATION
Photo courtesy of US Department of Energy
• Removes solar heat during hot weather
• Removes moisture vapor during cold weather
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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.
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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.
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Passive Attic Venting: Convection
ATTIC VENTILATION
How does it work?
Ridge vent
Outlet
(Heated Air)
Natural Convection
• Hot air rises,
escapes high vents
• Cooler air pulled
through soffit vents
Inlet
(Cool Air)
• Heat rises,
escapes ridge vents
Graphic developed for the US Department of Energy
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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
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Power Venting
ATTIC VENTILATION
How does it work?
Fan-powered
roof vent
• 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
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Code Issues
ATTIC VENTILATION
Requirements are based on
presence of ceiling vapor
retarder, climate zone, and
vent location.
Thermal boundary - Ceiling
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
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Graphic developed for the US Department of Energy
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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
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Applied Ventilation Formula
ATTIC VENTILATION
• Calculate the number
of roof vents required.
How many roof vents does it take?
• 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.
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Photo courtesy of the US Department of Energy
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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!
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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.
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Venting Configurations
ATTIC VENTILATION
In a perfect world...
Continuous ridge vent
Continuous eave vent (soffit vents)
Graphic developed for the US Department of Energy
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Vent Types
ATTIC VENTILATION
Gable vent
Roof vent
Eave vent
Images courtesy of J&R Products Inc.
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Vent Types
ATTIC VENTILATION
Turbine vent
R-144 mushroom roof
vent
Images courtesy of J&R Products Inc.
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Vent Placement
ATTIC VENTILATION
Roof vents placed high on roof
Continuous soffit vent
Photo courtesy of PA WTC
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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
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Vent Placement
ATTIC VENTILATION
Gable vents are attached similarly to roof vents.
Photos courtesy of the U.S. Department of Energy
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Attic Window Vent
ATTIC VENTILATION
Photo courtesy of the US Department of Energy
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Eave Chutes
ATTIC VENTILATION
Eave Chute
Photo courtesy of PA WTC; Product photo by J&R Products, Inc.
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Mechanical Vent Terminations
ATTIC VENTILATION
Photo courtesy of PA WTC
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Summary
ATTIC VENTILATION
• 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.
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